20" Barrel Twist Rate Comparison...

parabellumdude

parabellumdude

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Hey guys,
I'm looking at building a .308 for long range target shooting based on a rem 700 sps tac. action & barrel @ 20" long. Doing a comparison today because of availability; I can easily grab a 1:12 twist ( tac.) but not a 1:10 ( AAC- SD ).

I fed some generic data ( picked some variables not too far off what i would be shooting ) into my ballistic calc. ( Isnipe ) and this is what I got:

Hornady 190gr bthp match, .308, 0.53 G1 BC, 4500fps & 1in projectile ( Fudged it largely here for the last two if somone can pull this back to a more common number please do)

Then i set the gun up as: 100yd zero, 1.5 turret height, .5 mil adj's, BARREL LENGTH:20" & TWIST: 1:10

I set daily variables as : 8mph wind @ 270 deg. 200m altitude @ 15*C and 29.53 in.hg press.w/ 10% humid.

The output was interesting as isnipe calculated; @2000 yards
A bullet drop of 41.92 and velocity @ target of 1,224fps with a DRIFT of 236" or -11.29 MOA

Once I changed the value for barrel twist to 1:12 (700 tactical not the AAC tactical) The only value that changed was Drift = 223" -10.88 MOA


Do any experienced long range shooters think its worth it to go 1:10 over 1:12 on a 20" barrel system when your looking to shoot out to 2 grand? I have to say there's comfort in the numbers but how well numbers translate to real world shooting would be up to my data and the opinion of a shooter who's had experience at these ranges.
With a .308 will the spin drift on the bullet play against me or with me and at what ranges? Is there a specific range of target distances where more spin is beneficial and vise versa? Is there a difference in the AAC-SD barrel rifling that is not the same as other barrels with the same twist rate?

Thanks in advance to those who contribute.
-P
 
Ummm you may want to try some where closer to 2400fps for that muzzle velocity cuz you'll be looking at more like 170moa drop.one the bullet goes subsonic (some where around 1100yds, as I didn't change any of my dailies ) it drops like a rock ...... Just sayin

Using a 200gn projectile I can't even get 4000fps out of my .338 lapua
 
You may need to check your numbers, you are way out to lunch, the 308 might make 2000 yards with a hail marry shot 1000 is more realistic. Most 20 inch barrels dont make 1000, more like 850-900 yards accuratly, you need to run 155 lapua or 175-178 gr bullets to make it, with loads tending to be hot. A 1-12 twist may or may not shoot a 175 grain bullet. The general rule is a 24 inch tube with 1-10 twist and 175 smk. Dont fall into the trap of using 168 gr smk. they are more for shooting 500 yards or less. I shoot 2650 fps with 175 smk and they make 1100 yard accuratly.
 
We were recently testing an electronic target system at 1000m. Velocity of a long barrelled .308 shooting a 155 with a muzzle velocity of just over 3 grand was about 1300fps at that distance. I really can't see a 20" .308 being a good choice for very long range shooting. Makes it that much more difficult to obtain maximum muzzle velocities.
 
I like fast twist barrels; my .308 is a 1:8 and under favorible conditions, I can stay sub MOA well beyond the magic 1000 yards, although shooting groups isn't what I'm about. The long range shooting I find enteraining is attempting to get first round hits on realistic sized targets at estimated ranges. Spin drift, like gravity, elevation, and vertical target angle above or below horizontal, is a constant, and constants can easily be allowed for in your firing solution; whereas wind, mirage, and light can be challenging variables.
 
You need to get your numbers a little more on target to figure out what you will need. I just put in a generic 190gr load into my calculator and came out with 882 FPS at 2000 yards and a 136 MOA drop.
 
juanvaldez said:
Lots out their to read on bbls. The 6mmbr faqs mentions velocity and twist rate

http://www.6mmbr.com/barrelfaq.html
Click to expand...

"What Barrel Twist Should I Use? The most versatile barrel twist rate is 1:8". This will shoot everything from 62gr FB bullets to 107 VLDs with great accuracy. If you shoot mostly at 100-200 yards, a 1:14 twist will deliver 60-70gr match bullets with unrivaled accuracy. Take-off 1:14 6PPC barrels rechambered to 6BR work great at low cost. For shooting out to 600 yards, in calm conditions, you may get best accuracy with custom 75-85gr FB bullets. These work best in a 1:12 twist tube. A 12-twist is perfect for varminting with the 75-grain V-max and the Berger 80gr MEF. Our initial testing shows the 1:10 twist to be very effective with the new low-drag 85-88gr FB bullets, and 90gr boat-tails. The 1:10 twist is a great choice for varminting and Egg Shoots because you can run the 87gr Hornady V-Max and shoot Lapua's very accurate 90gr factory-loaded ammo. If you need a barrel to shoot both light and heavy bullets, the best choice is still a 1:8. For the 115gr DTAC bullet, 1:8 will stabilize in most conditions, but 1:7 is a better bet.

Does Twist Rate Make a Difference in Velocity? Yes. Our tests show that, with the 80-90gr bullets, a 1:10 or 1:12 barrel will give you as much as 80fps more velocity than a 1:8 barrel, shooting the exact same loads. Fast twist (1:8) barrels have more drag and friction, which can slow the bullet down. Ideally you want to use the slowest twist rate possible that will stabilize the bullet you choose to shoot. For a dedicated "point-blank" 100/200 yard Benchrest gun, you want a twist rate from 1:13 to 1:15. But if you want to shoot both light (60-80gr) and heavy (100gr+) bullets, stick with a 1:8."

These are good points... Thanks to those who commented. I have plugged a few other numbers into the solver and the drop is larger. The article Juan posted makes some good points when it comes to the twist rate being effective ENOUGH on different bullet weights. I think thats a concept I originally overlooked. I'd say that even with the 1:12 if the shooters using correctly matched ammo with a powerful consistent load things will be a lot tighter down range and the effect of an unfavorable twist rate can be minimized. I think I'm gonna stick with the 20" 1:!2 after some final homework and I think eventually It will give me what I want out of the system.
 
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