.308 Win barrel twist rate vs. bullet length/weight

[Jasper1573]

I have searched THR and found no definitive and coherent information in one good post about barrel twist rate and bullet size.

I have a Rem 700 .308 Win with 11.25 twist rate and 24 inch barrel. The rifle shoots very nicely (out to 500 yards or so is as far as I have been able to shoot) with bullets in the 165-178 grain range. But I am contemplating heavier bullets up to 200 grains. I don't know that there is any real gain in doing this, I am just curious; and my Lyman reloading manual includes these heavier bullets in the .308 Win section.

So here's my question...

What twist rate(s) will stabilize those heavier rounds? Will it take a 1:10 twist to stabilize the 190-200 grain bullet, or can I get away with the 1:11.25? I would rather not buy a $30 box of 200 grain SMKs and find that they won't shoot accurately in my rifle because of slower twist.

For instance, the Hornady Amax 178 grain in .308 says it requires a 1:10 twist rate, but I fired some of these through my rifle at around 2600 fps and even with my movement got 12 round groups of less than 2 moa, and if I had done my part, the groups could have been sub-moa...possibly. I realize that Hornady puts that required twist rate on the bullet box for max accuracy and performance, but I found that the 178 Amax shoots about like the SMK 175.

Comments, opinions, experience?

 

[blackDdefense]

http://kwk.us/twist.html
Check this out. It should shoot alright but we use 11.25 and shoot M60 ball.

 

[MtnCreek]

IME, it will stabilize to a point, but as the projectile loses velocity at longer range, it becomes less stabile. I’ve found that a SMK tolerates a slower rifling than an A-Max of similar weights. For my taste, 190 gr bullets are too much for a .308; they just don’t provide enough velocity to perform well at extended ranges. Take that same 190 gr SMK and set it on a 300 wm, and it’s a different story.
Why are you wanting a heavier projectile?

 

[Jim Watson]

Berger's chart shows a 12 twist as adequate for their 190 gr VLD.
A friend was talking to the Berger rep at a LR match recently and was told that the 185 was what HE shot in .308. Twist not stated.

 

[Jasper1573]

Why are you wanting a heavier projectile?

I am just curious about performance of heavier weight bullets in .308. I know that a 175-180 grain bullet is sufficient for any purpose I can think of, but since Lyman put the 190-200 grain projectiles in the .308 Win section, I thought I would ask the question.

I have read that the 190 grain bullets perform well at long range, say out to 1000 yards, but wondered if my twist rate were sufficient for that weight bullet. I have also considered the BC on the heavier bullets...the difference between the 175 grain with a BC of around .5 and the 200 grain with a BC of about .56 might make it worth the effort to load up some 200s and see how they perform...just curious.

 

[MtnCreek]

I've loaded 190's, 208's and 220's for .308, but I never found any benefit, even w/ a 1:10” barrel. I think the gains in BC were negated by the lower velocity.
Just because I couldn’t make it work, does not mean you can’t. Give it a shot.

 

[kludge]

as the projectile loses velocity at longer range, it becomes less stabile.

Actually it's the opposite, as the bullet slows it becomes more stable, but it's not completely intuitive because bullets that are not stable out of a particular rifle can sometimes become stable out of a longer barrel or pushing them faster.

But this not a result of the higher velocity, it is a result of the higher RPM that the bullet is spinning; the extra velocity is shed faster than the extra RPMs and the bullet will stabilize, hopefully before it begins to yaw and tumble.

The denser the fluid (air, water, whatever) the more spin it need to be stable. Water is much more dense than air and this is the reason rifle bullets yaw, tumble, and fragment when they hit water (watch the mythbusters episode, 9mm pistol bullets penetrate water much much futher than any of the rifle bullets - all of the rifle bullets fragmented). A bullet that is stable in Denver might not be stable in Florida.

...at least this is my understanding. The aerodynamic/hydrodynamic shape also comes into play, but I can't pretent to understand that.

This was also the problem with the SS109 and the M4. The low velocity from the M4 short barrel is an example... the 1:9 twist stabilizes the bullets in the air, but when it hit flesh the bullets were not tumbling and fragmenting, there was too much stability in flesh due to the low velocity. They were not tumbling because the velocity was not high enough for the amount of spin to make the bullet yaw fast enough to fragment. The 5.56 relys on fragmentation for wounding. This didn't matter for CQC there was still enough velocity for tumbling and fragmentation, but at the long ranges encountered in Afghanistan it mattered a lot.

Granted, I don't shoot my .308 at 1000 yards, but you can get into the trans-sonic region at long range which will have different effects. A 168gr SMK at 2650fps MV will become trans-sonic right at about 1,000 yards give or take depending on elevation, temperature and head/tailwind. I shoot my 168's at 2550fps which will become trans-sonic about 50 yards sooner. It could wreck a match.

The 150gr SMK doesn't have a chance at 1000 yards.

A 190gr SMK even as low as 2350fps will most likely stay supersonic to 1000 yards, and above 2400fps or 2450fps will most definitely stay supersonic; this is why long range .308 shooters use the heavier bullets.

Jasper, The 1:10" twist will stabilize 190gr SMKs. My rifle has 1:10" twist so I can't answer you specific question, but the 1:11.25" will probably max out with ~175-180gr bullets. Hunting bullets with lead tips will be more stable than than longer match bullets with hollow points or polymer tips.

At ranges less than 1000 yards I would use the lighter bullets for less recoil.

 

[Mike1234567]

At "typical" velocities and shooting ranges I'm fairly sure a 1:11.25 twist barrels will stabilize 190gr... "maybe" 200gr but the latter is probably pushing it a bit. I know my Saiga with 1:12.5 twist "should" stabilize up to 178gr but I'm staying with 147/150gr. If I want to shoot heavier bullets I'll switch to .30-06 with 1:10 twist which easily stabilizes up to 220gr and "probably" 240gr... but I'm sticking with 180gr-220gr.

 

[dprice3844444]

http://www.tobystactical.com/2007/11/remington-700-ss-5r-milspec.html read this and subsections

 

[USSR]

The 1:10" twist will stabilize 190gr SMKs. My rifle has 1:10" twist so I can't answer you specific question, but the 1:11.25" will probably max out with ~175-180gr bullets.

When looking at charts listing twist rates in relation to bullet weight, the one thing you have to understand is, the twist rate listed is the "optimal" twist rate, and does not imply that a slower twist will not stabilize the bullet. I shoot and have known many others who shoot 190SMK's out of their 1-12" twist .308's, and in all cases, the bullet is stabilized. Obtaining a good velocity in a twist rate that may be marginal for a particular bullet is most helpful, as this increases the rotational speed of the bullet. In the case of the .308 with a 1-12" twist rate, I have found VihtaVouri N550 to be the ideal powder to give maximum velocities, and in the case of my 26" barreled FN SPR, I am able to reach a velocity of 2700fps with the 190SMK bullet, while remaining within SAAMI pressure specs for the cartridge.

Don

 

[kludge]

Wow, I never would have thought to use a powder that slow in the .308, and 2700fps with a 190gr is smokin'!