18" 7mm SAUM twist rate

Radman

Well-Known Member
Joined
Nov 23, 2019
Messages
187
Location
TN
I’ve got a 17.5” bbl with a 1:9” twist in my 7mm RSAUM.
If I were to do another it would likely have a faster twist. I’d probably do something in the 8.5” range even if I only shot 162-168 gr bullets. If I were to shoot 175-195, I would go faster.

The Berger stability calculator will help you decide a good twist rate.

I get 2830 to 2850 fps with 162-168 gr bullets from my 17.5” barrel.
Good article on twist rates in the Mar/Apr 2021 Rufleshooter magazine.
 

Buttermilk

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Joined
Dec 19, 2005
Messages
647
Location
Good Ol' Oklahoma
The answer is really simple. Not like we are trying to launch a bullet to the moon here....

For bullets up to 160 class bullet weights, 1:9” minimum

For bullets in the 175-195 range, 1:8.5ish” minimum.
 

jebel

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Joined
Jun 15, 2018
Messages
326
Location
Oregon
The answer is really simple. Not like we are trying to launch a bullet to the moon here....

For bullets up to 160 class bullet weights, 1:9” minimum

For bullets in the 175-195 range, 1:8.5ish” minimum

It’s convenient for us to think twist correlates to weight, but that’s not really right. Holding all else constant, increasing weight actually improves stability. But things aren’t constant when we increase weight, length also increases and the increased length erodes stability (swamping any improvement from increased weight).

So using a rule-of-thumb for weight to twist is a crude measure. Easiest way to see that is look at some monolithics where, in this case, you likely need 1:8.5 or better to stabilize 155gr bullets.

If you need a rule-of-thumb, longer bullets require more twist and shorter ones require less.
 

Buttermilk

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Joined
Dec 19, 2005
Messages
647
Location
Good Ol' Oklahoma
You’re correct. It is convenient.

it’s a given that heavier bullets are longer. It’s also a given that monolithic are longer than non monolithic.

We typically don’t use bullet length to gage bullets, just weight.

I’ve never asked a fellow what length bullet are you shooting...
Hence my reference to bullet weight
 

jebel

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Joined
Jun 15, 2018
Messages
326
Location
Oregon
You’re correct. It is convenient.

it’s a given that heavier bullets are longer. It’s also a given that monolithic are longer than non monolithic.

We typically don’t use bullet length to gage bullets, just weight.

I’ve never asked a fellow what length bullet are you shooting...
Hence my reference to bullet weight
Fair enough. I also never ask those in camp, “what bullet length are you shooting?” We talk in terms of bullet weight.
But twist rate seems to be a confusing enough topic for some that I don’t like to see the simplifications. People are trying to figure out what barrel to buy, which they don’t want to regret soon after.
Rules of thumb are fine around the campfire, but I don’t want to give builders advice that will hamper their choices.
I suspect we agree.
 

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