Author Topic: 55 gr. BBT HP (Read 4218 times))

rsterne · « **Reply #20 on:** March 10, 2019, 08:52:01 PM »

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....

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

K.O. · « **Reply #21 on:** March 10, 2019, 09:39:29 PM »

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...

so yep just got me thinkin just how far can the .257 be pushed with air/helium...

rsterne · « **Reply #22 on:** March 10, 2019, 10:09:13 PM »

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

KnifeMaker · « **Reply #23 on:** March 10, 2019, 10:50:36 PM »

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

PeterL · « **Reply #24 on:** March 10, 2019, 10:58:54 PM »

Quote from: KnifeMaker on March 10, 2019, 10:50:36 PM

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

K.O. · « **Reply #25 on:** March 11, 2019, 05:32:55 AM »

" 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..

rkr · « **Reply #26 on:** March 11, 2019, 06:29:40 AM »

Quote from: KnifeMaker on March 10, 2019, 03:06:31 AM

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.

KnifeMaker · « **Reply #27 on:** March 12, 2019, 09:41:05 AM »

Quote from: PeterL on March 10, 2019, 10:58:54 PM

Quote from: KnifeMaker on March 10, 2019, 10:50:36 PM
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!

Mike

KnifeMaker · « **Reply #28 on:** March 12, 2019, 09:41:55 AM »

Quote from: K.O. on March 11, 2019, 05:32:55 AM

" 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.

KnifeMaker · « **Reply #29 on:** March 12, 2019, 09:43:30 AM »

Quote from: rkr on March 11, 2019, 06:29:40 AM

Quote from: KnifeMaker on March 10, 2019, 03:06:31 AM
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

rkr · « **Reply #30 on:** March 12, 2019, 10:19:55 AM »

Quote from: KnifeMaker on March 12, 2019, 09:43:30 AM

Quote from: rkr on March 11, 2019, 06:29:40 AM
Quote from: KnifeMaker on March 10, 2019, 03:06:31 AM
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.

KnifeMaker · « **Reply #31 on:** March 12, 2019, 12:48:13 PM »

Thanks, It will be interesting!

Travis is working on a solution now.

K.O. · « **Reply #32 on:** March 12, 2019, 07:26:31 PM »

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

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

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=

K.O. · « **Reply #33 on:** March 12, 2019, 08:57:28 PM »

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...

Gut2Fish · « **Reply #34 on:** March 12, 2019, 10:10:44 PM »

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

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.

K.O. · « **Reply #35 on:** March 12, 2019, 10:12:53 PM »

Quote from: KnifeMaker on March 10, 2019, 03:06:31 AM

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!

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...

K.O. · « **Reply #36 on:** March 12, 2019, 10:55:17 PM »

Quote from: Gut2Fish on March 12, 2019, 10:10:44 PM

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

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

rsterne · « **Reply #37 on:** March 13, 2019, 12:59:57 AM »

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

KnifeMaker · « **Reply #38 on:** March 13, 2019, 03:30:45 AM »

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.

rkr · « **Reply #39 on:** March 13, 2019, 07:09:47 AM »

Quote from: KnifeMaker on March 12, 2019, 12:48:13 PM

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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55 gr. BBT HP

Author Topic: 55 gr. BBT HP (Read 4218 times))