Author Topic: .25 cal 51 gr. NOE Bob's Boattails (Read 43060 times))

rsterne · « **Reply #100 on:** August 05, 2016, 01:49:39 AM »

Quote

seems 1:16 is enough twist with a L.W. barrel

Sorry, where did I miss that the LW barrel Owen has is a 16" twist?.... LW barrels are usually 17.7" twist.... Not saying it isn't 16", just I've never heard of one....

Bob

K.O. · « **Reply #101 on:** August 05, 2016, 02:09:21 AM »

Quote from: owen on July 27, 2016, 09:48:43 PM

I just recently upgraded the power of my .25 cal. Airforce airgun to where the 51 grain BBT will work. It has a 1-16" twist LW bbl. with .254" groove dia. Both bullet's driving band sizes fall outside of my needs of .253". If I can help it I do not want the additional step of resizing the .255's. Ideally the bullets would drop out at .2535", according to what I've read. Would it even be possible for me to resize the driving band of the .252" mould? And even if I could, would that work well, or not, in a .254" bbl.?

and at pyramyd

https://www.pyramydair.com/product/airforce-condor-ss-pcp-air-rifle-spin-loc-tank?m=3063

I wish that all descriptions included twist rates... I think my trail XL 725 barrel is around 1:18.5 but that was a very rough measurement...

rsterne · « **Reply #102 on:** August 05, 2016, 12:54:38 PM »

Interesting, AirForce must be getting a custom barrel made for them by LW.... Thanks for that information.... With the success of the 51 gr. BBT in the 16" twist, and the marginal performance that Nick found in the 17.7" at just under 800 fps, I guess we pretty much have an answer for "how fast do you need to spin 'em?".... It will depend on the muzzle velocity, and the 17.7" may well work if you keep that lower (in the mid 700s or below?).... but for high powered PCPs the 16" twist appears to be the minimum.... at least based on current data....

Bob

nielsenammo · « **Reply #103 on:** August 05, 2016, 01:02:54 PM »

Bob if I get a chance to shoot with someone with a different gun/barrel I will try to see how they do. I wish they worked in the 17.7 but they are not far from working in that barrel, I did get one good group but could not repeat.

rsterne · « **Reply #104 on:** August 05, 2016, 03:19:05 PM »

By any chance, Nick, was the one group that was good, shot at a slightly lower velocity?.... Stability in the subsonic region decreases as velocity increases, that could explain your results with the 17.7" twist barrel.... Even if the velocity was the same, even just a change in air density might tip it one way or the other, if it was right on the edge.... I can remember stories of benchrest guys having problems at high altitude, or different atmospheric conditions, in the days where they were pushing the stability factors down to 1.1 or 1.2.... Current thinking is to stay as close as possible to SF = 1.5.... Just a thought....

If they are THAT close to working in a 17.7" twist, just a few thou off the base of the mould blocks to shorten the bullet and enlarge the base diameter (both of which increase stability) would do the trick.... Here is a drawing of the NOE 51 gr. that has been shortened 0.027" at the back....

The boattail and base diameter are the same as the 41 gr. BBT.... The change, at least in theory, make the 48 gr. bullet shown above as stable in an 18" twist as the 51 gr. is in a 16" twist.... It should be a simple thing to mill of the base of an existing 51 gr. mould to make this change so that it would work in an 18" twist.... You may not even need to go this far, just 0.010-0.015" might do the trick.... but only testing would tell you for sure....

Bob

nielsenammo · « **Reply #105 on:** August 05, 2016, 04:09:49 PM »

Well elevation could be a factor I shot a group in Colorado that was great at 50 yards, elevation 5k feet I think.

The second attempt that was not good was around 2500 feet but out of same gun which was a regulated cricket. Cricket shoots jsb 25 grain around 975 or so.

K.O. · « **Reply #106 on:** August 05, 2016, 04:45:01 PM »

well Bob he also cut off the choke which probably made it a bit easier...

Also I know it goes against the math but I feel each different round design there is a minimum rpm it likes... so getting the fps up a bit to 805+ fps MIGHT help in the 17.7 twist...

Deadguy is not reporting any keyholing at about 725 fps in a 1:16 L.W. (choked?) that gives about 32,600 rpm...

1:17.7 at 780 fps is about 1000 rpm slower (31,700)... but add about 30 fps (810) and that gives about 32,900 rpm...

this is just the sort of situation I want to experiment with moving the CoG forwards by hollowing the base as much as the psi will allow... dished ,cupped and maybe drilled about .1 about .3 deep after cupping... would also allow maybe just enough weight loss to hit closer to 800 fps...

But yep just chopping the tail a bit would be much easier..

Chokes well depends on harmonics because if it allready is having that sort of trouble 50g slamming into the choke adds a whole magnatude to the unpredictability... that a larger/stiffer/shorter... less active choked barrel will not face...

Then also probably have to watch for leading at the choke...

?

rsterne · « **Reply #107 on:** August 05, 2016, 05:35:27 PM »

Running the dimensions of the 51 gr. BBT through the Kolbe Twist Calculator, using a 17.7" twist, at 800 fps at 5000 ft. the Stability Factor is about 1.7.... while at 2500 ft. it drops to about 1.55.... and at sea level it drops further to about 1.45 (all at 59*F, normal barometric pressure).... These are extrapolated (guessed) numbers, because I have to estimate where 800 fps is, between the lines on the chart at 500 and 1000.... Regardless of the actual numbers, the bullet is about 10% more stable at 5000' than at 2500'.... That could explain your results.... FYI, in a 20" twist, at sea level, at 800 fps the SF is only about 1.1....

Bob

rsterne · « **Reply #108 on:** August 05, 2016, 06:04:41 PM »

Kirby, rpm isn't everything, because a bullet is the least stable right at Mach 1, by a huge margin.... Look at the charts on page 3, in post #43 and you will see what I mean.... The rpm increases with velocity, but the stability still drops in the subsonic region....

Bob

K.O. · « **Reply #109 on:** August 05, 2016, 08:13:23 PM »

Bob of course it is not everything but in some cases the added rpm from the added velocity may let you run bit a slower twist especially if it is right on the edge like this situation...

yes that 30 fps may change the drag profile on the nose to much for it to matter... but I have a bit of faith in the round flat nose up to the mid trans sonic region not changing much with that 30 fps change... and the bit of boost to the gyroscopic stability just may be enough..

Probably in the higher trans sonic the less likely it is to help...

Bunch of maybes I know but this subsonic boolit stuff out of air rifles is breaking new ground...

I did look to see if I could find a link that refers to this and though it is at very high velocity it is alluded to in this link...

scroll down to

"Implications for Gun Builders and Reloaders"

http://bulletin.accurateshooter.com/2008/06/calculating-bullet-rpm-spin-rates-and-stability/

I know in years past I have talked with others that have the same thought but it was a good while ago...

owen · « **Reply #110 on:** August 05, 2016, 10:45:12 PM »

Kirby, I'm sure that I have that piece in the scrap bucket. When/if I find it then I'll PM you. I already believe that when sized these bullets will shoot 1 moa or better, but I'll have to wait and see.

rsterne · « **Reply #111 on:** August 05, 2016, 10:45:48 PM »

If you look at those graphs in the post I mentioned, you will note that when Supersonic, you can use a slower twist the faster you go, because increasing the velocity increases the rpm and stability.... This is the region where what you are talking about (increasing the rpm) works.... However, when Subsonic, the reverse occurs.... as you increase the velocity, the stability decreases, even though you are increasing the rpm.... The overturning forces increase faster than the rpm when you are below Mach 1.... hence trying to get more stabillty by increasing the velocity is the wrong way to go when Subsonic....

Bob

rsterne · « **Reply #112 on:** August 05, 2016, 10:52:28 PM »

Here are a pair of 51 gr. BBTs recovered from soap after impacting at 100 FPE.... The upper bullet is a FN, and the lower one is a HP....

It is quite obvious that the boattail portion never contacted the rifling, it looks like it did when it came out of the mould.... Those are from a TJ's 14" twist barrel....

Bob

aceflier · « **Reply #113 on:** August 05, 2016, 11:09:47 PM »

What rifle are you hiding thats pushing 100fpe?

rsterne · « **Reply #114 on:** August 05, 2016, 11:44:29 PM »

My Disco Double.... I have three 25" barrels for it, .250 cal 14" twist, .300 cal 26" twist, and .357 cal 26" twist....

Here are the best tunes with pellets and bullets in each caliber.... 4% ES strings from a 3000 psi fill....

If I crank it up in .25 cal, tethered at 2900 psi, here is what it is capable of....

The highest I have obtained with it, in .357 cal, is 192 FPE with 132 gr. bullets....

Bob

aceflier · « **Reply #115 on:** August 05, 2016, 11:55:28 PM »

That thing is sexy! Got to get me one of them CF shrouds on my Mrod.

rsterne · « **Reply #116 on:** August 06, 2016, 12:00:41 AM »

It's actually not a functional shroud.... It is in compression, and the barrel is in tension to stiffen it.... LDCs are illegal in Canada....

Bob

K.O. · « **Reply #117 on:** August 06, 2016, 02:07:20 AM »

Quote from: rsterne on August 05, 2016, 10:45:48 PM

If you look at those graphs in the post I mentioned, you will note that when Supersonic, you can use a slower twist the faster you go, because increasing the velocity increases the rpm and stability.... This is the region where what you are talking about (increasing the rpm) works.... However, when Subsonic, the reverse occurs.... as you increase the velocity, the stability decreases, even though you are increasing the rpm.... The overturning forces increase faster than the rpm when you are below Mach 1.... hence trying to get more stabillty by increasing the velocity is the wrong way to go when Subsonic....

Bob

Bob if it did not have a relatively short nose and a 70% meplat it would be very true... My thinking is that the shorter the lever(CoG/CoP) the less that is true... the longer and the pointier the more twitchy...

also I think on the pointy bullet the faster you push in subsonic into trans sonic the further forward the CoP moves... with a 70% meplat not so much until higher into the trans sonic...

Point is I do think it can gain a bit of stability because the CoP is more static as it punches thru the air rather than cutting thru with a Knife edge...

rsterne · « **Reply #118 on:** August 06, 2016, 12:18:37 PM »

Believe what you want.... I'll follow the advice of Robert McCoy.... Those graphs are specifically for the 51 gr. BBT, calculated using his program, developed at the Aberdeen Proving Grounds.... If he says the stability when Subsonic decreases with increasing velocity, I believe him as to the trend.... How the actual SF number relates to the twist required may be up for debate, but IMO not the general principles.... You will find that general shape of graph for every projectile, complete with the discontinuity at Mach 1....

Bob

owen · « **Reply #119 on:** August 06, 2016, 11:47:05 PM »

Bob, What shell holder will I need to get in order to use NOE's ram, for the sizing operation? This is for a Lee Hand Press.

Here's an image of the first three shots, with the NOE/Rsterne 51 FN bullets, at my 121 yard target:

I then decided to clean the barrel and shot another three shot group at the 50 yard target. It measured approx. .593"

Thank you to our advertisers!

.25 cal 51 gr. NOE Bob's Boattails

Author Topic: .25 cal 51 gr. NOE Bob's Boattails (Read 43060 times))