Advice on barrel twist

[Turbulent Turtle]

Who am I to argue with Sierra? So at their website, for stock code 9290, the 90gr .224 SMK, it is stated:

Please Note!! This bullet requires at least a 1x6.5" twist barrel.

https://www.sierrabullets.com/store/product.cfm/sn/9290/224-dia-90-gr-HPBT-MatchKing

[br]
Denys, [br]
It's certainly true they recommend that, but Sierra is notoriously conservative regarding barrel twist. I've shot Sierra 240 Matchkings in a 10" twist barrel at ~2600 fps and they shot accurately at 1000. Sierra recommends at least a 9" twist. Berger's 230 Hybrid, .10" longer, has a 10" twist minimum recommendation. They do work well in 10" barrels. So, while Sierra recommends it, likely also is that a 7" twist will work.

That may be, especially in hot & humid conditions. However if it gets cold or coolish, funny things may happen. I believe in using enough twist but no more than needed.

 

[wrangler5]

FWIW, I shot some 3 shot groups of Sierra 77g SMKs recently out of a 26 inch barrel with 9 inch twist. The box says 7 or 8 inch twist, but I had a few of the bullets and just decided to see what they would do seated to magazine length (2.260) with BLC2 powder. At 200 yards, 9 groups ranged from 0.41 to 0.72 MOA (On Target software.) Two others came in at 0.14 and 0.27 MOA, but I think they were just flukes.

Temp was in the mid 80s, and humidity was normal for Missouri in July, but not oppressive. So I gather things may not work this well when it gets cooler. But if I could get more of these bullets, and they weren't so expensive, I'd be seriously tempted to see what they'd do with Varget, and seated closer to the lands.

 

[FroggyOne2]

It is just like the Berger 90's are supposed to be shot in a 7 twist, Jerry T. got them to shoot in a 7.7 twist. I some times wonder if I should have tried a 7.5 with 22BR.

 

[Turbulent Turtle]

FWIW, I shot some 3 shot groups of Sierra 77g SMKs recently out of a 26 inch barrel with 9 inch twist. The box says 7 or 8 inch twist, but I had a few of the bullets and just decided to see what they would do seated to magazine length (2.260) with BLC2 powder. At 200 yards, 9 groups ranged from 0.41 to 0.72 MOA (On Target software.) Two others came in at 0.14 and 0.27 MOA, but I think they were just flukes.

Temp was in the mid 80s, and humidity was normal for Missouri in July, but not oppressive. So I gather things may not work this well when it gets cooler. But if I could get more of these bullets, and they weren't so expensive, I'd be seriously tempted to see what they'd do with Varget, and seated closer to the lands.

The 77SMKs are a bit of an odd duck. I know of people who shoot them out of 1:9 twisted ARs with no issues and I know of people who just can't get them stabilized in their 1:9 twisted ARs.

As I said earlier, additional velocity can make up for a slower than needed twist. A 26 inch barrel will give you that additional velocity, especially compared to a 20 inch or shorter AR barrel.

Also not a 1:9 twists are 1:9. It's always a good idea to measure the twist of your rifle if you are on the edge. It's a quick and easy thing to do.

 

[wrangler5]

I have checked the twist (tape flag on cleaning rod) and it's right on 9".

The best groups did come at the top end of the velocities. I shot 7 initial groups covering the range of powder charges in Hodgdon's chart for this bullet and BLC-2 - 0.3g steps from 23.0 to 24.8 - and got velocities from 2497 to 2738FPS. Groups were 0.49 to 0.72MOA, except for that one three leaf clover at 0.137 which averaged 2681FPS with 24.5g of powder.

To see if that was just a fluke I made up 4 loads which straddled the tiny group, using 24.4, 24.5, 24.6 and 24.7g of powder. I shot these a couple of days later and got velocities from 2669 to 2714FPS. The 24.4g load went 2669FPS into 0.265MOA, and the other three went 2688 to 2714FPS into 0.413, 0.412 and 0.442MOA.

Of the powders Hodgdon lists for this bullet, BLC-2 and CFE223 are the only ones predicted to get (just barely) over 2800FPS at their top loads. I did go into the tests thinking velocity would matter, and I tried the BLC-2 because I have a bit more of it.

If I could get more of those bullets I'd be seriously tempted to load some more at 24.5g of BLC-2 and fiddle with seating depth. Even if I couldn't do better than 0.41MOA I'd be happy to take that load to the 600 yard F class matches at our club and see what happens.