[Charlie98]

I now have 2 AR's, my original 1:7 twist 20" H-Bar barrel and my new RRA 1:9 twist 16" barrel...

What is the rule of thumb on twist and bullet weight? As I understand it, the 1:7 is more suited for the heavier (62- and 69grn bullets) and the 1:9 twist is for the lighter (55grn) bullets. I never really loaded anything other than the 55grn FMJ's prior to this for the H-Bar, not researching anything beyond the basic M193 duplicate load, so any suggestions are welcome.

 

[Pegasus]

Twist is used to induce spin on a bullet so as to confer static stability for the duration of the flight. The minimum amount of spin required to achieve this stability is influenced almost completely by the length of the bullet; the longer the bullet is for a specific caliber, the faster the twist it requires to stabilize it. Manufacturers do not market or sell their bullets by length, but rather by weight. In fact it's difficult to get the exact length of various bullets without buying them, but there is a database that has a lot of that info.

The .mil went to a 1:17 twist (from the original 1:12 of the early M-16) so that this rifle could stabilize the very long M856 tracer round, the companion to the M855 62gr w/ steel penetrator round. It is actually too fast a spin and it keeps the bullet too stable when it hits something, but for target purposes it's fine.

Now the 1:7 twist will stabilize virtually any .224 caliber bullet you put in it, except perhaps for the very long 90gr SMK or JLK, as these require a 1:6.5 twist. For your M193 equivalent, the 1:7 will do just fine. There have been reports of 40gr thin jacketed bullets flying apart as they exit the barrel because the spin is too fast; I have never seen that out of a .223 Remington. (I have seen plenty of that in 6mm Rem, and other such calibers.)

The 1:9 twist is actually a better twist for the .223 Remington, if you do not want to handload an 80gr SMK, Berger or JLK for it. The 1:9 twist will stabilize any loaded factory ammo you can buy with the possible exception of the Black Hills 77gr offering, especially since you have a 16 inch barrel. Yes, you can make up a bit of twist with a longer barrel because you get a higher MV which creates more spin. For your stated purposes, you will have no problems with either rifle, and you will be able to experiment with other loads, especially with the 1:7 twist.

 

[miatakix]

http://ammo.ar15.com/ammo/project/perf_whattwist.html

Your new middy will probably handle 69gr loads fine, if they're driven hard enough. 62gr shouldn't be a problem at all.

 

[Time Bomb]

I have a bushmaster varminter with 1in9 twist that shoots everything well except hornady 75 grain tap. the hornady box recommends 1in9 twist,do you think it's not stabilizing or my gun just doesn't like that load(2.5 moa vs .5 to 1 moa with everything else)?

 

[Pegasus]

The Hornady site shows the 75gr TAP but does not say what the bullet is. It looks like a V-Max but they do not show a 75gr V-Max. It's not an A-max because that bullet has an ogive so long it actually would be in the neck of the case if loaded to magazine length. It's also not a BHTP, because it has a polymer tip. So, I'm thinking it's really a V-max and they simply do not sell 75gr V-max bullets as components.

Either was it looks like it may be a longuish bullet, but I'm thinking it should still stabilize in a 1:9 twist. You should measure the twist in your rifle just to make sure it really is a 1:9 and not a 1:9.5 or some such. Do the holes appear oblong in any way? Have you fired it at 200 yards and what were the results?

 

[Time Bomb]

I have not shot the hornady's over 100 yard but they are hollowpoints of some kind.I wouldn't know where to start to measure it but my paperwork says 1in9 and I remember reading they tightened twist to 1in8 on the predator.With the exception of the hornady I only shoot 55 grain so I can practice with fmj and hunt with softpoints.

 

[Pegasus]

My eyes must have crossed. Of the 6 TAP offerings, the 75gr .223 Remington is the only one with a hollow point the others all have a polymer tip.

This means it's probably loaded with the 75gr BHTP bullet, which according to my notes is about .98 inch long. This is fairly long for a 1:9 twist so you are probably on the edge of stability with that bullet. THe Bushmaster site does show the Predator has having a 1:8 twist.

To measure the twist rate of a rifle, you need a cleaning rod with a patch, a marker and a ruler. Stuff the cleaning rod with the patch in the breech and once it's in there a few inches, use your marker to put a dot at top dead center on the cleaning rod near the handle. Make another mark on the rod at a spot right where it enters the breech. Now push the rod slowly and watch as the dot near the handle goes around. When it comes back to top dead center, stop. Make another mark on the rod at the same place where the rod is entering the barrel (as you did above.) Now push the rod through, remove the patch and pull the rod back out. Measure the distance between your two marks (not the dot near the handle,) and that will give you the distance for one full revolution of your rifling; so 1:x where x is the distance you just measured.