Twist Rate vs. Bullet Weight


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Jayhawker
May 23, 2006, 04:55 PM
Brand new RRA Varmiter AR 15 with a 24", 1-8 twist bull barrel. What is your advice as to the maximum weight bullet this barrel will stabilize? Has anyone heard of an ideal weight bullet for this configuration?

Thanks

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Jim Watson
May 23, 2006, 05:08 PM
80 grain SMK or similar is both maximum and optimum for 300-600 yards from what I have seen the Highpower shooters doing.
It might well be more accurate with a lighter bullet at closer range or in a flat calm where ballistic coefficient is less important than sheer grouping ability. It just depends on how well any particular combination fits that individual barrel.
Sorry, you will have to shoot the gun.
I'd try some 75 gr A-max in there someplace.

Bear in mind that such bullets will do best if loaded long, longer than the magazine will accept. If you want to shoot it as a repeater, the 77 grain bullets are meant to be loaded to 2.26" and shot from the magazine.

Frohickey
May 23, 2006, 05:13 PM
http://www.shortmags.org/shortmags/ref_data/TwistRateCalc.asp

Bullet length and bullet weight are inter-related, as you get to smaller calibers, more of the volume of a bullet is used by the jacket material, which is less dense than lead.

Art Eatman
May 23, 2006, 06:53 PM
From a thread at The Firing Line, referenced to Shilen:

".224 CF:

8" For bullets heavier than 70 gr.

9" For bullets up to 70 gr.

12" For bullets up to 63 gr.

14" For bullets up to 55 gr.

15"* For bullets up to 55 gr. driven 4,100 fps or more"

My guess is that an 8" twist wouldn't give good groups with bullets much under 70 grains. Certainly I'd not expect tight groups with <63-grain bullets.

Art

Jim Watson
May 23, 2006, 07:12 PM
I dunno, Art.

Ed Harris once did some shooting back when the 9 twist and 69 grain bullet was the hottest thing going. He found that a 52 grain match bullet was STILL more accurate at 100-200 yards even though the heavy bullet bucked the wind better at longer ranges.

45crittergitter
May 23, 2006, 11:03 PM
My 8" bolt gun shoots 75 gr. Sciroccos pretty darn well.

Chris Rhines
May 23, 2006, 11:18 PM
My guess is that an 8" twist wouldn't give good groups with bullets much under 70 grains. Certainly I'd not expect tight groups with <63-grain bullets. Not in my experience. My 1:8" JP Supermatch barrel prints sub-MOA five-shot groups at 100 yards with 55 grain bullets. It's pretty well impossible to spin a bullet too fast, unless the jacket seperates in flight.

With 77-grain Mk262 ammo, it does sub-0.5" at the same distance.

- Chris

rangerruck
May 24, 2006, 05:06 AM
a one in eight twist should let you shoot up to about 78 or 79 grns very well, lighter grns will be more accurate allways, but for rounds from 40 to 55 grns may spin apart once they leave your bbl, it is not that they wont be acc. I would say that you should find a 69 to 75 grn bullet will be perfect for you, just find the right one. Start with black hills in the blue box.

Swampy
May 24, 2006, 07:42 AM
Jayhawker,

Standard setup for Across the Course Highpower at 600 yards in .223 AR-15 Service Rifle is an 8" twist barrel and just about any 80 grain match bullet available, including the long VLD's.

Most common powders seem to be Re-15 and Var-Get.

Note: The SMK 80 is usually seated to jump a short distance to the lands. The 80 grain VLD's, such as the JLK and Berger, generally like to be into the lands. YMMV. Load testing is the key with seating depth.

Walk the 600 yd firing line of Camp Perry or just about any CMP-NRA Service Rifle event in the last 5 years and you'll find that the vast majority of the AR's are a combo of 8" twist and 80 grain match bullet...... though in the last couple the 6.5" twist and 90 grain bullets are coming on strong.

Best regards,
Swampy

owner Swampworks Inc. / JLK Bullets
http://www.swampworks.com

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