18" 7mm SAUM twist rate

[Radman]

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.

 

[Deleted member 115360]

I'll tell you who knows the answer to this question absolutely, Gunwerks. They are making and testing short barreled 7 saums like nobody on earth. You might give them a call, they seem like nice folks. Plus, they probably only charge like $249 for a phone call, or would they call it a "fon kaal".

 

[Buttermilk]

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.

 

[Elk Hunter 338]

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.

or simpler yet .... just do an 8 and be done!

 

[jebel]

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]

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]

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.

 

[Eman1982]

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.

What powder are you using? Thanks

 

[Buttermilk]

What powder are you using? Thanks

I've primarily used H1000, but have used some H4831sc. H1000 gives faster velocity and similar accuracy.

 

[Longtine88]

For those that have used them: is the 180 hybrid as reliable as the 215 hybrid for elk? And how does the 180 compare to the 175 elite hunter? Thanks

180 hybrid is king of the 7mm's to me for elk out of the Bergers. I've shot 7 elk with them. 4 bulls and 3 cows. Perform like a 215 from what I have seen. Last 2 have been behind the shoulders quartering away and one just dropped (650 yds) the other stood for like 3 seconds before staggering sideways and falling (340 yds). Grenade like destruction.

 

[HydroCarbon]

How do the 180 VLD Hunters compare to the 180 Hybrid. Finding the Hybrid is difficult, I've seen the VLD hunters in stock. I'm shooting the 168 VLD hunters and they're probably good enough.

I'm not sure I want to go any slower with the 18" bbl. I'm getting 2805 fps with the 168s and RL-23

 

[Buttermilk]

I personally think that the 162-168 weight range is a happy medium for the 7 RSAUM with the shorter barrels, from a ballistics point of view.

I'm getting 2850 FPS with H1000 and a Berger 168 VLDH. RL26 would likely net more velocity but my load shoots very well.

 

[Longshotjames]

I have a 7rm that is twisted 1:9.25 and it will NOT accurately shoot anything heavier than 162. Any 7mm I build will be at least a 1:8 twist in the future.
James

 

[grouse]

My 1-9 shoots 160 accubonds into tiny holes. Love that bullet