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Rifle barrel twist on an AR-15

2K views 8 replies 7 participants last post by  drymag 
#1 ·
I keep seeing 1:7 to 1:9 twist. I'm looking at a 1:9 twist weapon. What effect would that have? I've read people say it would cause heavier bullets (>70gr) to not be as accurate at range. I'd probably be shooting mostly 55 and 62 grain rounds.
 
#2 ·
The longer a bullet is the faster it needs to spin, since heavier bullets also tend to be longer, they need a faster rate of spin to stabilize.

Shooting shorter (lighter) bullets in a barrel that has too high a twist rate can cause overstabilization, meaning that the bullet will retain the same angle it had when it left the barrel rather than maintaining the angle of it's trajectory, this will cause tumbling as the bullet travels down range and the nose of the bullet is no longer pointed in the direction of it's path, or they can even be ripped apart by centrifugal force.

You can compensate for rate of twist by using different loading data, a lower velocity also lowers the rate of spin, a higher velocity will raise it but you can only adjust the load so far before you are loading too light or too hot.
 
#3 ·
#5 ·
Stabilization by spinning a bullet is a minimal thing (to a point). You can not over stabilize a bullet but you can spin it so fast that centrifugal force pulls it apart (really, I have had it happen!!). In my case it was a bad choice of bullets and the too fast spin for that bullet construction. It was a hunting bullet never meant for a fast spin. They never made it to the target. When I used bullets of newer design and better construction of the same weight, the bullets made it to the target but the accuracy was not as good as much heavier bullets in the same gun.

In the case of 223 if you stick with varmint weight bullet around 55 grains then slower twist rates are adequate. When you start using bullets in the 60+ grain weight class then you need the faster twist barrels. Since the military moved to 69 grain bullets they changed the required twist rates closer to 1 in 8. Even heavier bullets may require 1 in 7. But exactly what a bullet needs is estimated buy the Greenhill Formula that uses the bullet length not is weight for a given caliber. Long bullets take a faster spin.

LDBennett
 
#6 ·
Your choice of OPTIMUM barrel twist depends on the exclusive use of the rifle. What do you need the rifle to do? There is no magic twist rate that will deliver optimum results with every bullet weight in any caliber. If you use your stated 55 to 62 grain bullets, you are fine with the 1:9 twist. It all depends on just what you need the rifle to do.
 
#9 ·
Check out the Hornady 55 grain TAP FPD. It is a defensive round, that according to Hornady tech supt, is designed for varmit work and will shatter when hitting things like sheetrock. It's design tends to keep it in the house and not visit your neighbors. Their 75 TAP, according to their ts, will some times visit outside your walls.
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If you buy budget bullets and read their velocity recommendations, they seem out of whack w/ the reloading manual powder/velocity. You call their tech supt (ie xtreme), and they will say they work fine w/ reloading manuals but stray no further than mid range loads. Then the obvious questions is according to who's manual. I had a good recommendation of punting w/ the middle manual and watch for stress as you build up from start. Besides the usual watching of casings, watch for the barrels residual copper amounts and pay attention to the feel/sound of the barrel as you test.
 
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