Twist for subsonic 5.7x28.

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ranb
Silent But Deadly
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Twist for subsonic 5.7x28.

Post by ranb » Wed Nov 25, 2020 8:37 am

I'm looking into getting a Contender barrel (MGM) for my G2 SBR in 5.7x28 for shooting heavy subsonic .224 bullets suppressed. A 70 grain bullet at 1050 fps has 171 ft-lbs at the muzzle; far better than any subsonic 22lr. I'm thinking that a 3-4 grain powder charge in the 5.7x48 will be nearly as quiet as a 2 grain charge in the 22lr when suppressed.

For fast twists I have a choice of 1-6, 1-7 and 1-8. While playing around with the required twist calculator at JBM, it says that even the short Speer 70 semi-spitzer is not stable at subsonic speeds with a 1-6 twist. I've shot the 70 grain .224 bullet in my 1-7 twist AR-15 at 1050 fps and it did not keyhole or cause baffle strikes.

I don't plan on shooting anything but subsonic in the SBR, so should I just go with the fastest twist I can get? Anyone with experience with subsonic .224 bullets? Thanks.
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Capt. Link.
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Re: Twist for subsonic 5.7x28.

Post by Capt. Link. » Thu Dec 10, 2020 4:33 pm

I would call Manson to make the throating reamer you will need for heavy bullets. Subsonic 60gr Aguila needs a 1-9 twist so you will need at least that. A 1-7 should cover up to 90gr .224 bullets. The terminal effect would be best if the projectile is barely stabilized...a good reason to think 1-9 twist.
-CL
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Historian
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Re: Twist for subsonic 5.7x28.

Post by Historian » Wed Jan 13, 2021 12:18 pm

I second the Good Captain's sage advice.

In the literature the twist for the FN 5.7×28mm is 1:9 as stated.

Like most guys who never read carefully I did not see the barrel length
that you are considering. One calibration point is derived from
the stellar Ray Brandes, no longer made, Bully Barrel for S&W 41,
5.5" long with a 1:8" twist. It stabilizes the Aquila 'Fatty'
consistently.

Best.

Historian
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Re: Twist for subsonic 5.7x28.

Post by Historian » Wed Jan 13, 2021 12:36 pm

A thought about the use of 5.7x28mm v. 9mm v .45 for home defense.

What are the Db values for the 3 above rounds? If
one does not have a can to attenuate the sound
in an actionable situation and if sound level about the same then
'old dinosaurs' prefer the 1911A1 if no can available.

The having a pair of 'clip-on' ear plugs* next to John Browning's
gem is prudent.


Please upload data if possible.

* E.g., << https://www.grainger.com/product/3EAU4? ... P:20501231 >>

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