Bullet stability??

jtoews80

jtoews80

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I built a rifle with a slow twist and am wondering how decreasing velocity affects stability. If the bullet only stabilizes with a MV of 3165fps+, will it lose stability once its velocity drops below that point as it travels or will it stay running true anyway??

Rifle is a 6.5 RUM with a 1-12 30" barrel. It won't group with a MV lower than 3165 if I use a 140 SMK. Load testing at 100yds. If all else fail, I'll find out when I play out further.
 
Your barrel's twist is to slow to stabilise the 140's at longer ranges imo.
You would be better off with a lighter bullet ,I think you should have at least 1-8.5 to stabilize 140's evn a 1-8 .
 
With your 1-12" twist and the bullet doing 3165FPS and 189 900 RPM I wouldn't worry to much. Unlike the speed the bullet spin doesn't change much if at all while traveling down the range. Roy Weatherby's 7mm magnum was way faster than 7mm Rem b/c he was useing 1-12" twist to Remington's 1-9,25" so there was no spike of presure at the start of rifling and longer throating was also helping to reach more speed. Twist of the barrel doesn't stabilize bullets, the proper bullet spin does J T try to boost your bullet speed a little or use slightly shorter bullet not nessesarily lighter one.
 
When i played around with subsonic ammo, stability was much worse(critical) at 1100 fps, then at 1700fps. .300 Whisper...210 gr. cast bullet,1-13" twist, accuracy was far better at top end velocity, with all bullets.
 
Here is some data about Sierra 6.5 142gr MatchKing stability from Brian Litz.
All other 140gr match bullets are almost identical to this bullet.
http://www.appliedballisticsllc.com/264Cal142grSMK.html
Only in the best case will a 1-9 stabilize the bullet.
The 1-11 case is really really bad and 1-12 will only be worst.

If you're truly interested in bullet stability go to page 8 of
http://www.appliedballisticsllc.com/index_files/Understanding_part1.pdf
Basically, large caliber bullets are very easy to stabilize while smaller caliber bullets require a tight twist.

Alex
 
This is the weirdest wildcat I've ever played with, 15 rnds into a new barrel, I hit 3316 fps and didn't even see any flattening of the primer; however, I was only grouping 6"@100 yds until I worked up over 3165fps. And while I cleaned with gun juice to treat the barrel I was getting some deformation at 3-3100fps. It seemed to almost make more pressure.
As soon as I started vleaning with normal cleaner and a Boresnake almost all indication of pressure disappeared. Thats when I took it up to 3316 and still mothing for pressure.

JT
 
gunrunner8 said:
With your 1-12" twist and the bullet doing 3165FPS and 189 900 RPM I wouldn't worry to much. Unlike the speed the bullet spin doesn't change much if at all while traveling down the range. Roy Weatherby's 7mm magnum was way faster than 7mm Rem b/c he was useing 1-12" twist to Remington's 1-9,25" so there was no spike of presure at the start of rifling and longer throating was also helping to reach more speed. Twist of the barrel doesn't stabilize bullets, the proper bullet spin does J T try to boost your bullet speed a little or use slightly shorter bullet not nessesarily lighter one.
Click to expand...

This answered my question, if the rpm changes along with velocity. And then if it would take off as a result.
 
Nice wildcat but some have a 300 round barrel life. A 1 in 12 IMHO would work better with a Lapua 108gr bullet. But if your trying to use up some 140gr SMKs there are some guys on long range hunting that use this cal. www.longrangehunting.com/forums/f17/6-5-rum-16330/index2.html
Barrel makers love this cal.

I love the 6.5-284. I shoot the 6.5-284 out to a mile with 108s and 139s. Barrel makers like my cal too.
 
N570 powder might be a good one for this beast. I had a 6.5/270 WSM. 500 rounds is a good estimate for accurate (competition) barrel life.
 
What kind of grouping are you getting at 100 or what is the best group you have gotten.

I had a .243 win a few years ago with a 1-10 twist and I wanted to try and stabilize 105's when I got the speed way up there they did stabilize a bit but it was a short run and inconsistent results.
Gunrunner is correct about spike pressures staying down with a slow twist but to be honest I really feel your twist rate is to slow for the 140's if you find a 140 that is shorter it will stabilize much better.

Bullet stability is as a shorter bullet will stabilize with less twist while a longer vld bullet will require more.

Like others have said trying a shorter bullet may work great ,best of luck.
 
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I have a couple hundred 130's and 140's, to be honest I am a little surprised that it shot the 140 at all, groups were 6"@100 until I passed 3165 fps. I was just doing the gun juice treatment with the 140 and it started stabilizing them. The first two at 3165 & 3170 cut the same hole .357" ete.
 
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If you really put the speed to them they will stabilize but you will only know for sure if you try them at longer ranges .
I've tried 75 grainers in a 220 swift with a 14 twist and they would stabilize out to 250 yds or so but then would keyhole or miss the paper altogether at 300.

With the kind of speed you are pushing these bullets at maybe you should try them out further and decide for yourself if they will work or not ,but dont try to many until trying a shorter bullet your barrel is in the oven lol.
 
Supersonic projectiles should gain flight stability with distance because the spin rate (rpm) does not decrease nearly as fast as forward velocity.

This is not true for subsonic projectiles which can experience degrading flight stability with distance. I'm not sure why this occurs but I have seen it enough times to know it is true.

Flight stability is mostly related to spin rate (rpm). That rpm is related to how fast the bullet travels down the barrel. Higher forward velocity inside the barrel increases the spin rate. As soon as the bullet leaves the barrel, forward velocity and rotational rate are no longer connected.
 
If you can get the 140's to stabilize at 100 yards they should then remain stable until the velocity drops into the transonic range. With your 6" groups were the bullet holes oblong?

I also remember reading about possible pressure spikes in large cased rounds with lower powder charges. I know it was in one of my reloading manuals, just can't remember which at the moment.
 
LongBomber said:
If you can get the 140's to stabilize at 100 yards they should then remain stable until the velocity drops into the transonic range. With your 6" groups were the bullet holes oblong?

I also remember reading about possible pressure spikes in large cased rounds with lower powder charges. I know it was in one of my reloading manuals, just can't remember which at the moment.
Click to expand...

It is possible that the starting point was too low and causing a spike. The holes were cut straight, no keyholing. Just diagonal strings limbing left to right. First to about 2.5" apart and the third 4" further right and up. Almost in a line. I went up In large jumps and on the third set of 3 it stabilized. Then I went up a bit more one at a time and the first two good ones and the two singled were still inside 2.5-3". They walked right an inch or so with the extra velocity that was it.

I'll test it further, but I am really optimistic about the results to date. It still not a target rig, way to hot and waaay to much powder, but if I can not the heck outta a coyote or two across a quarter, I'll have done what I set out too.
 
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