Rifle Twist Ratio Question

[SteveUSA1]

Just picked up a used Howa 1500 Varmint 24" heavy barrel .308win cal.with a rate of twist 1:12. Seems like other manufactures of rifles use the 1:10 rate of twist for this cal. Am i to be limited to a certain bullet weight because of the 1:10 rate of twist?

 

[MCgunner]

It'll likely do best with the lighter bullets in the caliber. Seeing as it's called a "varminter", I reckon that's why they chose 1:12. Faster twists stabilize longer bullets.

 

[R.W.Dale]

It'll likely do best with the lighter bullets in the caliber. Seeing as it's called a "varminter", I reckon that's why they chose 1:12. Faster twists stabilize longer bullets.

One turn twelve would be a slower twist than the 1 in 10 barrel.

 

[powderbrass]

This is good info for me. I joined this forum to learn more about rifles so here might be a dumb question: twist ratio = the twist that the bullet gets coming out of the barrel? Please, no snickers. I have never heard this terminology before and would like to learn.

 

[SteveUSA1]

Rate of Twist

Krochus,your right on the rate of twist. I was going to use the Federal Gold Match 168gr. BTHP, but at $25.00 a box for twenty I understand that this weight may be a little to heavy. Your Thoughts?

 

[Bullet]

powderbrass
The twist rate is how far a bullet travels down the barrel by the time it makes one revolution. Hence 1 (revolution) in 12 (inches of barrel) in this instance.

SteveUSA1
168gr bullets should be fine. You can even go heavier. Here is a link to Berger bullets that gives twist rates for their bullets. –

http://www.bergerbullets.com/catalog.htm

Here is what Shilen recommends in their barrels -
.308
- 8" for bullets heavier than 220gr.
- 10" for bullets up to 220gr.
- 12" for bullets up to 170gr.
- 14"* for bullets up to 168gr.
- 15"* for bullets up to 150gr.

 

[R.W.Dale]

Krochus,your right on the rate of twist. I was going to use the Federal Gold Match 168gr. BTHP, but at $25.00 a box for twenty I understand that this weight may be a little to heavy. Your Thoughts?

I don't think it would be an issue untill you move up into 180gr+ bullets. Way back when 30-06 and 308 used diffrent twists 1-10 for the 06 and 1-12 for the 308. Now most rifles use the faster 1-10 but don't ask me why.

 

[L.Puckett]

.308 Twist rates

Steve,

The 1:12 twist will be just fine for 150 grn to 175 grn.

Exceeding 175 grn go to a faster 1:10 twist, below 150 grn use a slower 1:14.

If you are not demanding absolute accuracy, you can exceed these recommendations some with no problem.

The USMC spec's a 1:12 twist for the M40A3, using M118LR rounds. These are 175 grn SMK (similar to Fed GMM 175). The Army's using the new 1:11.25-R for the same round.

Fed. GMM 168 grn is quite similar to the 175's but typically give way to the wind too much when used beyond 500-600 yrds.

Take care,
L. Puckett

 

[Rem700SD]

SteveUSA1,
shop around for a case price on the federal, or use Black hills ammo. Both SHOULD run you around $15.00+shipping per box for Sierra 168grn BTHP match. Try Natchez or Midway, or look in Shotgun News for an ad from Hoplite(iirc). they seem to have good deals on Federal GM.

 

[MCgunner]

One turn twelve would be a slower twist than the 1 in 10 barrel.

And therefore better with lighter/shorter bullets in the caliber.

 

[MCgunner]

I don't think it would be an issue untill you move up into 180gr+ bullets. Way back when 30-06 and 308 used diffrent twists 1-10 for the 06 and 1-12 for the 308. Now most rifles use the faster 1-10 but don't ask me why.

It might have something to do with the logistics of ordering barrels or tooling for the slower twist running the cost up per barrel. I ain't a manufacturing guru, but what I'm trying to say is, the manufacturers probably save money by making the guns with the same twist.

There's very little need to worry about the .308 for the really heavy .30 cal bullets. It's not real efficient with bullets over 180 grains. That's where the .30-06 has an edge with it's greater case volume. The heavier bullets seated into the short little .308 case invade on case voluming and powder capacity unless the gun is free bored specifically for heavier bullets which would be sort of a dumb thing to do. If you want to shoot 220 grain bullets in .30 caliber, you need an 06 or a .300 Win Mag.

In my M7, I shoot a 140 grain Barnes for heavy-ish game such as hog where I need the penetration. I shoot 150 grain Nosler partitions on thin skinned game. The Barnes X bullet is a penetrator and it actually has a higher sectional density "is a longer bullet" than the Nosler because it's all copper, no lead. I would use this bullet on any game that I thought the 150 wasn't sufficient for. I don't need no stinkin' heavy bullets.

 

[Gun Geezer]

Rate of Twist is not everything

Turns in however many inches is not everything. The real key is "rps" or revolutions per second.

Rate of twist combined with velocity is the key. For a given caliber, in a given barrel (fix rate of twist), a range of bullet weights and therefore lengths can be stabilized more or less by varying velocity.

When "working up a load" that is essentially what you are doing: playing with various bullets, type of powders, primers, and amount of powder to get that pefect combo for a particular barrel/action.

I have always felt that rate of twist is the dominant factor. But velocity counts, too.