twist and barrel

[toby]

Don't get hooked on the notion that heavy bullets are better within 600 yrds.

[milotrain]

In a .308 without a 30" barrel the 175 will outperform a 155 with significance at 600 yards.

[subscriber]

Rolly,

Before you buy a .308 rifle you need to decide what barrel length you want and what ammo you want to use. Longer barrels provide (marginally) more velocity and therefore the bullets have higher rotation speed for the same nominal twist rate. This means longer barrels can use lower twist rates (1:13 rather than 1:12) - Palma competitors don't use a 1:13 twist to lose matches; they know from experience that just enough twist to ensure stability provides maximum accuracy. You also need to know your desired maximum effective range, as bullets need higher spin speed to remain stable as they approach the speed of sound at long range. In other words, best twist for for peak accuracy at 600 yards and for 1000 yards may not be the same value...

The M4 uses a 14.5" barrel with a 1:7 twist. This fast twist rate is required to be able to shoot heavy for caliber 77 grain bullets out of the short barrel. Not only do heavy bullets require faster spin speed, they are almost always slower than lighter bullets out of any given barrel due to the same peak pressure limit (and possibly due to case volume limits).

Keybord jockeys insist on a 1:7 twist on their ARs, despite the fact that they have 16" or longer barrels and shoot mostly 55 grain bullets. They do this because it is "MILSPEC". Insisting on a 1:10 twist for the .308 may be due to similar thinking.

Now if you are planning on shooting 180 to 200 grain bullets exclusively, then a 1:10 twist may be exactly what you need. Or perhaps 220 grain subsonic loads; then you need a 1:8 twist...

As someone else has mentioned, it is not the weight of the bullet that determines twist requirements, but the length. The more exact requirement is the distance between the center of mass and the center of form, or center of drag. In other words, the material density (lead VS copper) and the shape of the bullet also come into play. A blunt 220 grain .308 bullet may be perfectly stable with a 1:12 twist, while a 220 grain spitzer almost certainly won't be stable. This is due to the length difference between bullets; and the difference in distance between the respective centers of gravity and their respective bullet tips. (Consider that hollow based shotgun slugs can be fired from a smooth bore because their center of gravity is forward biased).

If you are planning to shoot mainly 150 grain ammo then a 1:12 twist will not be the limiting factor for accuracy. Consider that even the SCAR-17 has a 1:12 twist; and it only has a 16" long barrel: http://www.fnhusa.com/l/products/car...ries/scar-17s/

Another data point is that the M16 started life with a 20" barrel and a 1:14 twist. Arctic testing showed that in colder denser air this twist rate was not stabilizing the bullets properly, so the twist was changed to 1:12. As service rifle barrels became shorter and bullet weights were increased (from 55 to 62 to 77 grains) the twist rate went up from 1:12 to 1:9, then 1:7.

[toby]

In a .308 without a 30" barrel the 175 will outperform a 155 with significance at 600 yards.

You think?

[subscriber]

In a .308 without a 30" barrel the 175 will outperform a 155 with significance at 600 yards.

The .308 doesn't have the case volume (powder capacity) to utilize a 30 inch barrel. Such long barrels will work better with the .300 Win Mag.

a 30" barrel will whip around like whip saw unless it is extremely heavy, so the accuracy gains with such long barrels are also dubious. Consider this barrel length VS rigidity study: http://www.riflebarrels.com/articles...est_rifles.htm

[subscriber]

The Ideal Barrel For A .308 Caliber Tactical Rifle

http://www.riflebarrels.com/articles/longrange_shooting/ideal_barrel_308_tactical_rifle.htm

[Ahhnother8]

The .308 doesn't have the case volume (powder capacity) to utilize a 30 inch barrel.

A 30" barrel will whip around like whip saw unless it is extremely heavy, so the accuracy gains with such long barrels are also dubious.

Neither statement is true. We shoot 30" barrels, with bullets that are fast and accurate.

[subscriber]

Neither statement is true. We shoot 30" barrels, with bullets that are fast and accurate.

More details please.

What is your barrel contour?

Are you shooting factory ammo or only custom reloads?

What bullet weight are you using at what velocity? If 155 grain, I would be surprised if they are going over 2900 FPS.

Is your barrel bedded or free floating?

[ExtremeX]

More details please.

I don't shoot it but i'm guessing 30" barrels and the 155gr bullet are common amongst the Palma crowd.

[milotrain]

How about, he's on the US international Palma team. Or how about, run a balistics calc.

You think?

Run the numbers. There is a reason why all of us who shoot 308 Garands and M1As don't shoot 155s at 600 yards.

[toby]

How about, he's on the US international Palma team. Or how about, run a balistics calc.


Run the numbers. There is a reason why all of us who shoot 308 Garands and M1As don't shoot 155s at 600 yards.

Nope, I shoot real life. I know what I and countless others can do with light bullets 150's 155's-165's out to 600 yrds it's fun easy shooting.

[milotrain]

It's not that they are not effective or fall out of the sky, it's that when you are fighting for xs the 175s buck wind better. Implying I don't shoot "in real life" is both childish and incorrect.

[Garandimal]

U.S. RIFLE CAL. .30 M1: 24"/1:10 Bbl.
(as God and JC Garand intended)






GR

[subscriber]

To each their own. When I was shopping for a .308, I chose a stiff barreled rifle with its balance point far back to facilitate offhand shooting - my favorite position . I doubt I will ever use this rifle for anything further than 500 meter metallic silhouette rams. http://www.riflesilhouette.com/about/targets.html

http://www.remington.com/products/firearms/centerfire/model-700/model-700-sps-tactical.aspx

Mine has a 1:12 twist, which is fine for the 150 grain ammo I have. There is a variant of this rifle with a 1:10 twist. As it is threaded for a suppressor I dropped it from consideration.

[subscriber]

The question was about twist: http://www.riflebarrels.com/articles...ance_twist.htm