GTA
All Springer/NP/PCP Air Gun Discussion General => Projectiles => Boolit and Pellet Casting => Topic started by: rsterne on March 07, 2019, 07:00:58 PM
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I have a hot .25 cal Disco that is a genuine 100+ FPE gun, so I pulled it off the wall today to try some velocity experiments with different size bullets.... I have some 50.4 gr. FN BBTs (NOE 250-51), and also some 58.3 gr. FN (NOE 250-60), plus the HP versions as well.... I tried them as cast (0.250" nose / 0.252" drive band), and they were hard to chamber in my TJ's .25 ACP barrel.... so I sized some of each down to 0.251", 0.250" and 0.249".... I tethered the gun at 2950 psi (it is modified for 3K plus) and both bullets picked up a lot of velocity between as cast and 0.251", gained more at 0.250", and then lost about 10 fps at 0.249", presumably from a small amount of blowby….
The 58.3 gr. FN were well over 900 fps, so I thought I would tune the gun for the 55.0 gr. HP version of that bullet, sized to 0.250".... Mean looking little devil....
(http://i378.photobucket.com/albums/oo221/rsterne/NOE%20Bobs%20Boattails/55%20gr%20BBT%20HP_zpshvh2zs9i.jpg) (http://s378.photobucket.com/user/rsterne/media/NOE%20Bobs%20Boattails/55%20gr%20BBT%20HP_zpshvh2zs9i.jpg.html)
I maxed out the preload and hit 993 fps (120 FPE), and then backed it off a turn and got 965 fps.... At that setting, off the tether, I got 2 equal shots and then a 3rd at about 940 fps, so I backed the preload out a bit more and ended up with 3 shots off tether of 956, 963 and 945 fps (111 FPE average) within a 2% ES.... Tethered, of course, it shoots 956, within a few fps.... which will give me a good platform to test these HP bullets.... The gun is loud, and not very efficient.... but at 110 FPE it should really lay a smackdown on a Marmot.... 8)
(http://i378.photobucket.com/albums/oo221/rsterne/NOE%20Bobs%20Boattails/Wound%20Channel_zpsb2onv1hz.jpg) (http://s378.photobucket.com/user/rsterne/media/NOE%20Bobs%20Boattails/Wound%20Channel_zpsb2onv1hz.jpg.html)
This is a penetration test of the 55 gr. BBT HP in soap at 110 FPE.... The nose came apart.... penetration of the base was 95 mm (3.75")….
Bob
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Nice looking bullet Bob. What’s your barrel length?
Peter
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Well, we know now that it is fast, dumps energy well for a varmint bullet. Now is it accurate? This is the most important thing well before the other criteria.
Knife
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OK guys
I am on the fence about a 25 cal AG,
And you guys are driving me nuts, with all the 25 info posted,
so out to the shop thru the cold and got to digging around in the bbl pile,
and guess what I find..................
A 25 caliber bbl, .257 bore (I think, haven't slugged it yet),
24.5" long long, the only possible problem is it's a Marlin Micro lands and groves, and FIRR the twist is 1x14".
Also found a 25/06 bbl, I had forgot about........... :o
Would this be a better for pellets or slugs, in a repeater action?
Thinking real hard on changing Leakin Lena to a 25 caliber HPA,
or my need to buy another donor AG, to change over, to fit this bbl.
Your thoughts or suggestions...........
I gotta to quit scratchin this itch............I am leakin blood..................
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Hmmm have never seen a .257 Roberts Marlin microgroove.... But do remember a friend having a Marlin that like your pistol used a necked .357 case... 22 lands 14 twist... so maybe that barrel is a .256 Win, Mag..?
I have a old model 60 22LR barrel (more modern are shorter at 19") very seasond but bet it will be fine... 16 land and twist...
New unused have a model 883n .22 WMR 16 twist also... 20 lands...
Been thinking they belong on a Marauder...
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I do think with proper sizing to ~.0005 over under groove... should be great subsonic barrels...
http://castboolits.gunloads.com/showthread.php?319002-My-Experience-With-Shooting-Cast-in-a-Microgroove-Barrel (http://castboolits.gunloads.com/showthread.php?319002-My-Experience-With-Shooting-Cast-in-a-Microgroove-Barrel)
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The one thing your going to run into with the 1:14 twist is the need to use long heavier bullets, starting in the 70+ grain area such as the Lyman or Arsenal 257400. shorter lighter bullets will start to loose stability at fairly short ranges. 50+ yards.
It takes a fairly stout pcp to sling the heavier, longer bullets. ;)
Mike
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I think you will find that this boattail will be great in a 14" twist, as boattails need higher RPM than the same length flat base bullets do.... This bullet is available in three different diameters, 0.250/.252".... 0.253/.255".... and 0.257/.259" (which is actually 0.014" longer).... I did that because I consider this weight range (~ 60 gr.) to be the crossover point between .25 cal and .257 cal.... If you want to go heavier, go .257 cal, IMO.... Plus, you can get TJ's liners in both that are a 14" twist, which should be ideal for this bullet.... although it should also work in the 15.5" twist GM barrel on the MRod….
Bob
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KO
It is a 256 Win Mag bbl, 60 HP @ 2300 fps+, a Marlin 62 bbl. A 257 Roberts case is a lot bigger....... ;)
Have to find some 257 dia slug to check the bore diameter now.
Thanks Guys,
Have some thinking to do now............
Don
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Sorry, Peter, I missed your question.... The barrel is 24", the gun looks like a stock Disco, right down to the front sight mount with the scooped out bottom to clear the air tube, except it has a TJ's .25 ACP liner that is 7/16" OD and slides right into the Disco receiver.... Inside, though, it has a custom valve with bore sized porting and a rear velocity adjuster on the back of the tube....
Bob
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Sorry, Peter, I missed your question.... The barrel is 24", the gun looks like a stock Disco, right down to the front sight mount with the scooped out bottom to clear the air tube, except it has a TJ's .25 ACP liner that is 7/16" OD and slides right into the Disco receiver.... Inside, though, it has a custom valve with bore sized porting and a rear velocity adjuster on the back of the tube....
Bob
Awesome. Looking stock but has a nasty bite when needed. Just the way I like my rifles. I also have the 25ACP barrel in .500” on my Synrod. Very tight barrel but extremely accurate with rebated boattails.
Peter
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Odd, I had several members send me different sizes of BBT's in .25 and .257 and none would group in my 1:14 twist .25 factory RS or TJ 1:14 .257 barrels. It was simply too fast for the light bullets.
They worked on paper, but not actual shooting. Terrible spirling at anything near 50 yards and beyond. This, trying velocities from 790 all the way to above 1000 fps. Nothing worked. the faster the fps, the worse. Telling The tail that the Twist was simply too fast for the bullets.
A shame, as I had high hopes for them.
Knife
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Michael, could you please explain this statement?
the faster the fps, the worse. Telling The tail that the Twist was simply too fast for the bullets.
When Subsonic, you need a faster twist as you go faster, the opposite of what happens Supersonic.... Typical graph of optimum twist:
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/51%20gr%20BBT_zpspqzyyoel.png) (http://s378.photobucket.com/user/rsterne/media/Ballistics/51%20gr%20BBT_zpspqzyyoel.png.html)
Or, are you saying that you think that the bullet is Dynamically Unstable, like a pellet in too fast a twist?.... Static Stability, as in the graph, is what we need to avoid tumbling.... Dynamic Instability leads to spiraling (corkscrewing) as the bullet goes downrange and slows, and is impossible to predict or calculate.... It sounds like you were actually able to observe such spiraling?.... There is usually a very large range of twists that will work between these two conditions, as one is at one end of the stability limits, and the other occurs at the other end.... :-\
Do I remember reading that you had good success with some of the BBTs when you used a slower twist barrel?.... Which lengths did not work for you in the 14"?.... Did you find any difference between the FN and HP versions?....
Bob
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seems a bit weird to me that they are spiraling at short range... the 14 twist should be just about right for 60g rounds..?
the .256 Win. Mag was design as a 60g Varmint pistol cartridge for the Ruger Hawkeye single shot pistol (holy grail for Ruger collectors (Blackhawk frame swinging breech block instead of cylinder)).
and the T.C. Contender... was about 2200-2300 fps out of a 8.5" pistol barrel...was a big brother to the .22 jet and a flame thrower as a pistol also...
The Short throw Marlin Levermatic .256 came a few years later... I think the model 62 came out in 62... I only remember cause that was when I came out...
point is it was 14 twist barrels and designed for a 60g .257 round...
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Odd, I had several members send me different sizes of BBT's in .25 and .257 and none would group in my 1:14 twist .25 factory RS or TJ 1:14 .257 barrels. It was simply too fast for the light bullets.
They worked on paper, but not actual shooting. Terrible spirling at anything near 50 yards and beyond. This, trying velocities from 790 all the way to above 1000 fps. Nothing worked. the faster the fps, the worse. Telling The tail that the Twist was simply too fast for the bullets.
A shame, as I had high hopes for them.
Knife
Mike I thought you were having great success with your 1:14 twist on your RSII shooting lighter slugs? I’m shooting both 52 & 55gr slugs out of my .25 cal sized to .248 & .249. I get one hole at 60yds but it’s not consistent. That’s why I’m sleeving my barrel because my suspicion is the harmonics is changing during the shot cycle.
Peter
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The 40g .25 BBT well yeah can see that spiraling about 50 75 yards out... heck thought it would at about 75 yards out of a 15.5 twist barrel... from the begining I have thought of the 40g as a sib to a bevel base SWC... and yep bevel base are just as accurate as plain base IMO... Just do not try to +p them... The Buddy I had reloading for my .357 liked them for ease of loading...
so yeah I can see it with the 40g which I think wants about 1:19 twist...have seen it out of overspun short SWC...
50g .250-257 BBT would be over spun I think at 1:14... but not bad enough to spiral at 50 yards..?
here is the thing tho... over the years have seen stuff that is not s'posed to work actually work very well... also have seen things not work that should work ...
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My go to bullets are mostly 50.6 gr and lighter. They do fine at 70 yards. At 100, I am seeding spirling. Moreso with the 1-14 than the 1:17.7 barrel.
At night, with a spotlight on the gun and a dark target, I can see a beginning of a tail wag at 76 yards. Yet it is still very accurate. However, I would not expect it to be so at 100 yards +. No room for testing the 100 yards at the moment. Working on it again though. ;)
The heavier bullets I have here, 60 and 70 grain do well in the 1-14 twist.
The length of my favourite bullet is .447inch at 50.6 grains, and the other is .403" At 44.5 inch. I don't remember the length of the BBT's I tested, an they were in the one in 14 twist. But they were short!
I do have one other bullet, a spire point that is .526 at 57 grains and is single loaded. All the others fit a modded RS Evanix Mag.The really suprising bullet is the offering from the MP mold t;hat is a BT/round nose. It is 47 grains at .427 in length, and shoots very well in the 1:17.7 Twist. I have not tried it in the 1 in 14 twist so far.
Mike
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It must be remembered that what I may consider and what another my consider accurate may very well be two completely different things.
In good air, I am not satisfied if it cannot print under 1/2" at 80 yards. And I consider that marginal. The Talondor .257 with i:14 twist will shot several bullets under 1/4" at the same range. Consistently. I have not achieved this level of accuracy with the Rainstorm Yet. Trigger is a bit issue at the moment, as are the scope mounts.
I did learn today that the quick detach scope mounts are not as secure as I thought they were. Time to change them! And as the old saying goes, "Tonight's the Night! ;D
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This is the accuracy obtained with the new MP boat tail bullet at 47 grains/80 yards. Also, the second target at 35 yds, is two in the top hole, and 8 in the bottom hole. Excellent accuracy for a sporter I think.
These bullets are not showing the tendency to wag their tails at either range at all. 8)
The top two bullets were at 35 yards, adn the bottom two were at 80 yards.
Left target is 20 shots tethered, on a one inch target dot. As you can see, 17 of the 20 came in under 1/2 inch. Woo-Hoo!!!
Knife
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Hmm all this .25 talk got me wondering... 4500 psi helium 160g BBT .256 Holland & Holland.. ;) possible with a reasonable barrel length..?
would certainly be the King of the 25 Cal... Falling block would be nice.. ;)
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There is no such thing as a 160 gr. BBT, for a start.... The heaviest one in .257 cal. is 113 gr. and 4 calibers long, designed for a 7" twist.... It takes about 4000 psi with air to get to 940 fps / 220 FPE.... There are VERY few bullets much longer than 4 calibers, some are up to 4.5 cal.... a few might be 5 calibers long (although I have never seen one), I don't know of any longer than that.... and a 160 gr. in .257 cal would be about 6 or more.... Play around with the Kolbe Twist Calculator, and see what it tells you for a bullet that long.... A boattail would need about a 5" twist @ 1000 fps.... :o
The 200 gr. "Whiteout" I designed in .308 cal. had an SD of 0.30, and was a good match for 4500 psi Helium.... That bullet scaled down to .257 cal is my 113 gr., but the SD drops to 0.244, which is why you can use air at that pressure, instead of Helium.... The problem with trying to get to an SD of 0.30 in .257 cal is getting a fast enough twist.... let alone a 160 gr. which would have an SD of almost 0.35....
Bob
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I realize everything would be custom was just wondering if it could actually be done... It is a somewhat famous one off rifle wondered about the twist rate myself... but yep it sent a 160g .257... and yep maybe just a flat base 160g cruise missile would be better for twist purposes .. ;)
in powder it was about 2100 fps I think I remember...
oops was wrong 2300 fps found a vid...
https://www.youtube.com/watch?v=-Uo5Ziaxhow (https://www.youtube.com/watch?v=-Uo5Ziaxhow)
so yep just got me thinkin just how far can the .257 be pushed with air/helium...
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If you used a simple round nose design (like the .256 H&H), with a cylinder 4 calibers long and a hemispherical nose, you get 160 gr, and an optimum twist rate of 9".... However, a 10" twist will provide an SF = 1.2, so it might even work in that barrel.... The LOA is 4.5 calibers, which is about the maximum L/D ratio I have found for any bullet....
The projected BC at Mach 0.5 is 0.28, and at Mach 1.0 is 0.10.... Therefore, my 113 gr. BBT would be significantly better ballistically…. You should be able to drive the 160 gr. on Helium at about the same velocity as the 113 gr. on air, at the same pressure with the same barrel length.... The trajectories would therefore be similar, but the Helium will deliver about 40% more FPE with the heavy bullet.... I guess the question is, what would you use a 300 FPE .257 cal gun for?....
Bob
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Now that would be something else!
changed the scope mounts and accuracy increased a bit. I am hoping that it wil help in the day to day poi.
Odd thing happened. I have been sizing to .2495 and it has been accurate at that size. This morning, not so much. I went back to the shop and sized some to .2500, (a size that did not do as well in the past), and she started shooting nice groups again. Weird,, really weird! I have no idea why the bbl. suddenly change it's preference in dia.
Knife
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Now that would be something else!
changed the scope mounts and accuracy increased a bit. I am hoping that it wil help in the day to day poi.
Odd thing happened. I have been sizing to .2495 and it has been accurate at that size. This morning, not so much. I went back to the shop and sized some to .2500, (a size that did not do as well in the past), and she started shooting nice groups again. Weird,, really weird! I have no idea why the bbl. suddenly change it's preference in dia.
Knife
Hey Mike....it could easily be leading in lands and grooves. With that small of a size change, I bet if you were to clean the barrel it would go back to liking the .2495.
Peter
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" I guess the question is, what would you use a 300 FPE .257 cal gun for?.... "
well If I had the $$$ to build it... would also have the $$ to hunt Roos in Australia.. ;) if legal with an air rifle..? always wondered what they taste like...
here in Wa. State my choices are limited to non game birds/animals... so it would be for the invasive giant rodents.. ;)
Nutria...
but seriously just to do the experiment.. ;)
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In good air, I am not satisfied if it cannot print under 1/2" at 80 yards. And I consider that marginal. The Talondor .257 with i:14 twist will shot several bullets under 1/4" at the same range. Consistently. I have not achieved this level of accuracy with the Rainstorm Yet. Trigger is a bit issue at the moment, as are the scope mounts.
Did you change the sear geometry in your RS2? It helps a lot when the sear does not push back the hammer when you pull the trigger. Adding a abalanced valve should also help quite a lot as it allows half the spring force for the hammer.
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Now that would be something else!
changed the scope mounts and accuracy increased a bit. I am hoping that it wil help in the day to day poi.
Odd thing happened. I have been sizing to .2495 and it has been accurate at that size. This morning, not so much. I went back to the shop and sized some to .2500, (a size that did not do as well in the past), and she started shooting nice groups again. Weird,, really weird! I have no idea why the bbl. suddenly change it's preference in dia.
Knife
Hey Mike....it could easily be leading in lands and grooves. With that small of a size change, I bet if you were to clean the barrel it would go back to liking the .2495.
Peter
That is exactly what I did before the change Peter. lOL. I used the lead remover that Tom at AAA suggested. Man it works fast!
Wet one patch with it. Wait 10 minutes, use a very stiff nylon brush, followed by a dry patch and it comes out dark grey. Next patch, nearly clean. One more and clean. Run one thru with FP-10, and one more dry.
Back to shooting fantastic! 8)
Mike
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" I guess the question is, what would you use a 300 FPE .257 cal gun for?.... "
well If I had the $$$ to build it... would also have the $$ to hunt Roos in Australia.. ;) if legal with an air rifle..? always wondered what they taste like...
here in Wa. State my choices are limited to non game birds/animals... so it would be for the invasive giant rodents.. ;)
Nutria...
but seriously just to do the experiment.. ;)
Here in Central Texas, Wild Pig of course. 8)
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In good air, I am not satisfied if it cannot print under 1/2" at 80 yards. And I consider that marginal. The Talondor .257 with i:14 twist will shot several bullets under 1/4" at the same range. Consistently. I have not achieved this level of accuracy with the Rainstorm Yet. Trigger is a bit issue at the moment, as are the scope mounts.
Did you change the sear geometry in your RS2? It helps a lot when the sear does not push back the hammer when you pull the trigger. Adding a abalanced valve should also help quite a lot as it allows half the spring force for the hammer.
Yes I did Ric. However, Travis sent one for me to try when they were developing them for the Krals. It does a fine job!
I wish I had the talent to make a balanced valve, Well, that and a lathe. LOL! I would love to have one!
Mike
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In good air, I am not satisfied if it cannot print under 1/2" at 80 yards. And I consider that marginal. The Talondor .257 with i:14 twist will shot several bullets under 1/4" at the same range. Consistently. I have not achieved this level of accuracy with the Rainstorm Yet. Trigger is a bit issue at the moment, as are the scope mounts.
Did you change the sear geometry in your RS2? It helps a lot when the sear does not push back the hammer when you pull the trigger. Adding a abalanced valve should also help quite a lot as it allows half the spring force for the hammer.
Yes I did Ric. However, Travis sent one for me to try when they were developing them for the Krals. It does a fine job!
I wish I had the talent to make a balanced valve, Well, that and a lathe. LOL! I would love to have one!
Mike
The problem with RS2 is how to attach the thimble to the valve block. There's scary little material in between the tube o-ring groove bottom and the opening inside so machining is not a good idea IMO. I have some ideas for hanging it in the valve block inlet but no smart solution yet. I'll let you know if I figure it out.
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Thanks, It will be interesting!
Travis is working on a solution now. ;)
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was thinking more of a Jack of all trades design than needle nose long tailed... or just round nose...
Declared data
Bullet diameter (inches)
.257
Bullet length (inches)
1.2
Nose length (inches)
.4
Meplat diameter (inches)
.130
Drive-band diameter (inches)
.257
Base diameter (inches)
.22
Boat-tail angle (degrees)
10.4
Boat-tail length (inches)
.1
Secant radius (calibres)
(Estimated) Bullet weight (grains)
156.5
Bullet density (gm/cc.)
11.4
http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=Custom+bullet&re_calculate=yes&boundary_layer=L%2FT&diameter=.257&length=1.2&nose=.4&meplat=.130&drive_band=.257&base_diameter=.22&angle=10.4&boat_tail=.1&secant_radius=&weight=159.2&density=11.4 (http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=Custom+bullet&re_calculate=yes&boundary_layer=L%2FT&diameter=.257&length=1.2&nose=.4&meplat=.130&drive_band=.257&base_diameter=.22&angle=10.4&boat_tail=.1&secant_radius=&weight=159.2&density=11.4)
well actually this is a hunter so a 60% meplat would be nice. so lets change things should not mess with the B.C. much...
http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=Custom+bullet&re_calculate=yes&boundary_layer=L%2FT&diameter=.257&length=1.2&nose=.4&meplat=.154&drive_band=.257&base_diameter=.22&angle=10.4&boat_tail=.1&secant_radius=&weight=156.5&density=11.4 (http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=Custom+bullet&re_calculate=yes&boundary_layer=L%2FT&diameter=.257&length=1.2&nose=.4&meplat=.154&drive_band=.257&base_diameter=.22&angle=10.4&boat_tail=.1&secant_radius=&weight=156.5&density=11.4)
same 1:7 twist needed by the 112g
http://www.geoffrey-kolbe.com/cgi-bin/barrel_twist.cgi?diameter=.257&length=1.2&nose=.4&meplat=.154&base_diameter=.22&angle=10.4&boat_tail=.1&b_twist=7&secant_radius=&unit_length=inches&temperature=59&humidity=0&local_pressure=29.92&pressure=29.92&altitude=0&density=11.4&custom_density= (http://www.geoffrey-kolbe.com/cgi-bin/barrel_twist.cgi?diameter=.257&length=1.2&nose=.4&meplat=.154&base_diameter=.22&angle=10.4&boat_tail=.1&b_twist=7&secant_radius=&unit_length=inches&temperature=59&humidity=0&local_pressure=29.92&pressure=29.92&altitude=0&density=11.4&custom_density=)
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I feel funny talking about looking for the top when all I have actually done so far is at the bottom...BBT 40g at ~50 fpe... & 30g BBT at ~19 fpe (16 twist)and then at ~25 fpe-and 30 fpe with 19 twist...
reading reported results on the 30g .22 BBT up to ~60 fpe it seems the bore riding round flat nose is a twist/velocity versatile round with a meplat useful for hunting...
I do not have the 50g and 60g .25 BBT nor the rifles with fpe to push them... but seems they would be similar in behavior...
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How's this KO?
Changed your boat tail, made the meplait bigger and shortened the OAL to 3 caliber. Much better BC (.63) and still a whopping 130 grains. Crazy heavy in .257.
http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=Custom+bullet&re_calculate=yes&boundary_layer=L%2FT&diameter=.257&length=1.03&nose=.48&meplat=.18&drive_band=.257&base_diameter=.18&angle=8.7&boat_tail=.25&secant_radius=&weight=129.7&density=11.4 (http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=Custom+bullet&re_calculate=yes&boundary_layer=L%2FT&diameter=.257&length=1.03&nose=.48&meplat=.18&drive_band=.257&base_diameter=.18&angle=8.7&boat_tail=.25&secant_radius=&weight=129.7&density=11.4)
Shorten the shank to a more manageable .93 OAL and it comes in at 115 grains with respectable BC of .58 at mach .5. That's a nice hunting boolit. Almost use it for shooting rams on the mountain across from you.
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It must be remembered that what I may consider and what another my consider accurate may very well be two completely different things.
In good air, I am not satisfied if it cannot print under 1/2" at 80 yards. And I consider that marginal. The Talondor .257 with i:14 twist will shot several bullets under 1/4" at the same range. Consistently. I have not achieved this level of accuracy with the Rainstorm Yet. Trigger is a bit issue at the moment, as are the scope mounts.
I did learn today that the quick detach scope mounts are not as secure as I thought they were. Time to change them! And as the old saying goes, "Tonight's the Night! ;D
right now I am playing at 30g and 30 fpe kneeling was at .8 ish so a touch over 2 moa... well that would get a rabbit dinner... sub moa at 50 is always a goal... I am pretty sure once it warms up rested at 35 yards will be very close to moa...
sub Moa at 80ish fpe, 80 yards... so yeah looking for sub moa 35 yards with 30 fpe seems sort of equivalent.. ;)
would set longer goals but fpe handled right can equal accuracy at more distance...
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How's this KO?
Changed your boat tail, made the meplait bigger and shortened the OAL to 3 caliber. Much better BC (.63) and still a whopping 130 grains. Crazy heavy in .257.
http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=Custom+bullet&re_calculate=yes&boundary_layer=L%2FT&diameter=.257&length=1.03&nose=.48&meplat=.18&drive_band=.257&base_diameter=.18&angle=8.7&boat_tail=.25&secant_radius=&weight=129.7&density=11.4 (http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=Custom+bullet&re_calculate=yes&boundary_layer=L%2FT&diameter=.257&length=1.03&nose=.48&meplat=.18&drive_band=.257&base_diameter=.18&angle=8.7&boat_tail=.25&secant_radius=&weight=129.7&density=11.4)
Shorten the shank to a more manageable .93 OAL and it comes in at 115 grains with respectable BC of .58 at mach .5. That's a nice hunting boolit. Almost use it for shooting rams on the mountain across from you.
you are using g1 B.C. for a g7 round.. ;) I think a boattail that long wants to wag out of the barrel more than the short boattail... I thing once past about a 65% meplat it wants to catch an egde...and with less drag at the rear slower recovery...my gut says it will be picky about spin...
It is really hard to explain my gut but WFN noses are said to be finicky...and a long boatail will add to that...
https://www.shootersforum.com/handguns/30129-wfn-threashold-velocity.html (https://www.shootersforum.com/handguns/30129-wfn-threashold-velocity.html)
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In addition, that bullet is 4 calibers LOA, not 3.... You won't be shooting across to the next mountain at 560 fps, regardless of what the calculated BC is....
Bob
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What I want to know is what are you going to shoot these heavy bullets out of?
Dyotat100 makes the most powerful .257's out there and that is too heavy for them!
This is starting to sound more like a what if session. LOL!!
Unless you are set on using a steady diet of Helium, 156 gr, 130 gr. far too heavy! Unless you have developed a new unobtainium valve. ;D
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Thanks, It will be interesting!
Travis is working on a solution now. ;)
Answer is wingnuts. A thread bar through the valve block inlet and modified wingnuts on both sides to clamp it in place. Then screw on extension nut modified to a thimble on the poppet side of the threadbar. The poppet is metal so make a groove for o-ring and add ventilation.
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Michael, it's a given that a bullet that heavy in .257 would require Helium, that was stated in the first post about a 160 gr. by Kirby.... I was trying to point out that a bullet that heavy would either have to be so long that the twist rate would be in the 5" range.... OR would have to be 4-4.5 calibers long and basically cylindrical (with a bit of tapering on the ends, maybe)…. and so would not have as low a Form Factor as, for example, my 113 gr. BBT.... Since you would be driving either at the same velocity (113 gr. on air, 160 gr. on Helium), if they slowed down even at the same rate (same BC) then ballistically they would have the same trajectory, the only advantage to the 160 gr. would be FPE....
There is always the challenge of building the biggest, baddest .257 around, I get that.... My .308 Whiteout 200 gr. bullet (using Helium) was the same idea, and all it did was ruffle a bunch of feathers at the EBR.... Too bad the fact that we duplicated the ballistics of the .300 AAC Blackout was overlooked.... Different calibers have different advantages and disadvantages.... Here is a bullet that is 6 calibers long....
(http://i378.photobucket.com/albums/oo221/rsterne/Bullet%20Casting/172%20cal%2050%20gr%20BBT_zpsjrzsp2t2.png) (http://s378.photobucket.com/user/rsterne/media/Bullet%20Casting/172%20cal%2050%20gr%20BBT_zpsjrzsp2t2.png.html)
The SD is fairly high, at 0.241, but using 4500 psi of air you could push that into the high 900s.... The problem is that the twist rate required would be less than 4".... :o …. The get a similar shape to work in .257 (it would weigh 166 gr.) you would need Helium....
Bob
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I completely missed the mention of Helium to push it. My bad!
Considering the dwindling supplies of Helium, I don't think I would want to use it.
Mike
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They just discovered a huge source of Helium in Africa.... When they stop selling it for use in party balloons, I would think about not using it in airguns…. JMO....
Bob
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I had a chance to shoot these at 100 yards today from my .25 cal Disco with the 14" twist TJ's barrel.... Here is a 3-shot group I got while sighting in the gun.... It is actually the smallest group I have ever shot with an airgun.... and rivals the best group I got decades ago with my .222 Rem handloads.... It was shot from bags, front and rear....
(https://i378.photobucket.com/albums/oo221/rsterne/NOE%20Bobs%20Boattails/55%20gr%20BBT%20HP%2025%20cal%20at100%20yds_zps4vwte6au.jpg) (http://s378.photobucket.com/user/rsterne/media/NOE%20Bobs%20Boattails/55%20gr%20BBT%20HP%2025%20cal%20at100%20yds_zps4vwte6au.jpg.html)
It gets lost under the dime.... 8) …. These were sized to 0.250", and shot at 950 fps, like the ones in the first post above.... I shot a Ground Squirrel with one and the results were NASTY.... I'll be posting a pic in the Hunting section....
Bob
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That has to be rewarding after all the effort you have gone to since what about 2013 developing the BBT concept... Thanks for the work
I like down at stock ish fpe and love the 30g .22... this 55g .25 looks to be a very good round also if you have the fpe for it... has to fly very well for that kind of accuracy...
I do think the design is about the best that can be done for an exceptional general purpose/hunting round...
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Well I can tell you, it hits like a .22LR HP.... WOW !!!!
Bob
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They just discovered a huge source of Helium in Africa.... When they stop selling it for use in party balloons, I would think about not using it in airguns…. JMO....
Bob
Yeah, No kidding GRRRR!
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Congrats on the Group Mr. Bob! 8)
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Awesome group you got there at 100 yards Bob !!!
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Great shooting Bob . Good to see you’re getting some time behind the trigger shooting at range . Looking at the bullet design , I’m wondering why I can’t get the 51grain to group well . I’m going to have to revisit them once I get the .22 bullets sorted out .
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The 55 gr. in .25 cal is 2 calibers long.... I am beginning to think that and longer is where boattails really begin to shine.... Just a gut feel, no hard data yet.... other than that stellar 3-shot tease above....
Bob
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What is amazing to me is your best group in decades was out of a 7/16 barrel.. :o and a modded Disco. ;) 8)
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I guess the bullet and barrel are a good match !
Bob
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Well I can tell you, it hits like a .22LR HP.... WOW !!!!
Bob
well with its , B.C., and M.E. it should match well enough you could call it a Pnu 25lr.. ;)