Why does a XPR 308 have a 1:12 twist

[norcal300]

For the Winchester XPR rifles, only the 308 has a 1:12 twist. Any rational for that?

The others have the following: 243, 270, 300, 30-06 =1:10, 7mm-08 = 1:9.5

Our Colt AR-15 SP1 (wife got it in 1985) has a 1:12 twist and guidelines for a .233 with a 1:12 twist basically say to stay 55 grains or below.

Would a 308 with a 1:12 twist have similar limits on how heavy the round could be?

PS. I fully admit my lack of knowledge so any flaming is acceptable and no offense will be taken

[highpower790]

Probably done that way to load for a greater range of bullets.my 1:12 is good for a range of 125-175/178gr.

[Kwikvette]

Faster twist = stabilizes lighter rounds

This generally has to do with the role the rifle will partake in for example; my light weight hunting rifle has a 1:12 twist (pencil barrel, etc) whereas another one has a 1:10 twist (heavy barrel). The 1:10 stabilizes 168/175gr well out to 800 yards.

[Jimmy's]

It's a hunting rifle and there is not much need for heavy bullets for hunting deer size game.

[norcal300]

I'm confused. I thought later versions of M-16s and ARs went to a lower twist. If they can stabilize a 556 with less, why is it a different case for a 308? Would it be for a case of shooting a lower grain 308 farther then you would try an 556. (I think I just need to learn basic ballistics better)

[norcal300]

Jimmy answered my question. (Read his after I post my last rely)

Thanks

[sfvshooter]

My Scar heavy is 1:12 twist as well. Guess it's meant to shoot M80 ball (145-150 gr?). Seems to be most accurate with 175 gr match though.

All my AR 15's (14.5 to 18" barrels) are 1:7 twist. They do fine with 55 gr, but I get close to 1 MOA with the 18" DMR using 77 gr OTM.

I've only been shooting for about 1.5 years so I'm always learning something new.

[Whiterabbit]

It's a hunting rifle and there is not much need for heavy bullets for hunting deer size game.

This. For hunting CXP2, light is good, and running light lets you shoot faster for better opening and flatter trajectory ie longer MPBR.

One could ask the same questions about why varminters prefer light for cal bullets, similar idea.

[FLIGHT762]

I'm confused. I thought later versions of M-16s and ARs went to a lower twist. If they can stabilize a 556 with less, why is it a different case for a 308? Would it be for a case of shooting a lower grain 308 farther then you would try an 556. (I think I just need to learn basic ballistics better)

Yes, you're confused. The later version of the M-16 A1 (1-12" twist) went to a FASTER twist, 1-7". The M16 A1 used M-193 ammo with a 55 grain FMJ bullet.

The M-16 A2+ was given a 1-7" twist to fully stabilize the M-856 Tracer round and the M-855 62 grain round.

The .308 Winchester works very well with a 1-12" twist. The 1-11.25" and 1-10" twist was used to better stabilize the 175+ bullet weights.

Yes, you need to learn basic ballistics better. To help you, look here : http://www.bergerbullets.com/twist-rate-calculator/

http://www.accurateshooter.com/technical-articles/calculating-bullet-rpm-spin-rates-stability/

M-16 twist rates explained here: http://www.gunsandammo.com/ammo/pair...st-rates-ammo/

[Whiterabbit]

If they can stabilize a 556 with less, why is it a different case for a 308? (I think I just need to learn basic ballistics better)

FYI stability is a factor of bullet length, velocity, twist, and caliber. (caliber not to be confused with cartridge)

since bullet length is dominated by weight, the common vernacular is to be primarily concerned with bullet weight and twist. Particularly when the discussion revolves around common cartridges, where caliber is (of course) fixed, and provides a fairly predictable guideline for velocity (given weight).

Thus the focus on weight and twist alone.

However, since caliber is in the mix for 308 vs 223, that explains why twist rates can be radically different between a 22 cal, a 30 cal, and a 45 cal or 50, etc etc. Before even considering bullet speed or weight(length).

IOW, can't think about a 1:12 308 in the same way as a 1:12 223.

In fact, can't even think about a 1:14 35 cal rifle in the same way as a 1:10 9mm barrel. Even if the 9mm is a rifle.

[norcal300]

Thanks FLIGHT762. I actually knew about the FASTER twist ... BUT HAD a senior moment and just reverse it. Now that you mention it, I had read the thing about changing the twist on the M-16 a couple of years ago but simply forgot it. I definitely will read the nice references you provided. My apologies to the group for being brain dead.

[MongooseV8]

Faster twist = stabilizes lighter rrounds

Got that backwards. Faster twist stabilizes heavier bullets.

[Kwikvette]

Got that backwards. Faster twist stabilizes heavier bullets.

I forgot I replied on here this morning

Thanks for fixing that; was getting ready to go to the range.

[LynnJr]

The twist rate in the barrel determines which bullets you can use.
With a 308 Winchester most shooters will never shoot a bullet heavier than 190 grains so you don't need anymore twist.
If your plan is to shoot 210-240 grain bullets you would want more twist.
The 30-06 and 300 Winchester Magnum both have more case capacity and people shooting those cartridges commonly use heavier bullets thus the faster twist rate of the barrel.
If your shooting a 52 grain 22 caliber rifle go with a 14 twist barrel.
If you want to shoot heavier bullets in the 80-105 grain weight range it makes more sense to go with a 6mm/243 bore than it does to use a 224 bore but alot of shooters are required by the rules to stick with certain cartridges.
That is why you see 223 barrels in 7 twist rate.
The average shooter wanting to shoot a 80-90 grain bullet would be much better off switching to a 243 or 244 for those weight bullets.
A 10 or 8 twist 308 barrel is meant for heavy for caliber bullets or subsonic rounds.

[norcal300]

Thanks to everyone for the answers - clearly I have a lot to learn.