rate of twist on .223 barrels

jimmie

jimmie

New member
i see alot of people shooting a 1x8 twist on rockriver ar15's for varmint hunting. most people are using 55gr and smaller. i am surprised that noone is keyholing the target with the overspin of that lighter bullet? my 1x8 twist shoots 75 grainers and will keyhole the lighter bullets some are using. im just curious if anyone is shooting 75grainers out there?
 
Length has more to do with it than weight. Push 'em too fast or spin 'em too much and they go "pooof". I watched a guy blow up bunches of 115gr D-Tacs during a match. Him weren't too happy........grins
 
2much you made a good point. in the benchrest world twist rate is a main factor. maybe the shorter length of the ar barrels has alot to do with it. my ar is a sledgehammer within 200yrds using 75grainers, i dont shoot beyond that. another gun comes into play for beyond...
 
What rifling twist will stabilize what bullet is actually based more upon RPM than twist rate. Twist rate is just the easiest way for the mfg. to portray it to consumers. This is why some will argue that certain bullets will stabilize in certain barrels, even though they are twisted slower than the mfg. suggested twist rate. The mfg. recommended twist rate is "general" in that regard. Meaning, in most instances, that twist rate is required. The right handloader with the right components, with the right rifle, can change the equation. Hope that makes sense to somebody........
grin.gif
 
As long as the bullets are tough enough to take the rotation rate, most 40-75 grain bullets will not keyhole. Extremely long bullets need the fast twist, particularly at slower velocities. So far, I have had no problems with stabilizing 40-65 grain bullets in my RRA 1/8 twist. I suggest you try the 60 grain V-Max in your AR. I see no need to go heavier for a .224 dia. varmint bullet. With a COAL of 2.260 you can get them in most magazines. I'm using 24.6 grains of Benchmark, and CCI BR-4 primers @ 3100 fps.

I don't know what lighter bullets your using, but they should not keyhole due to over-spin. Very thin or fragile jackets can be shredded due to over-spin, and/or a rough bore. Make sure your bore is not fouled with copper or lead.
 
I went with a 1-12 twist for my dedicated varmint rifle, but used a teke-off 1-8 Colt match barrel for my coyote gun. I knew I wouldn't be shooting anything heavier than 55 grains out of the p-dog rifle, so there was no need to over spin the bullet and risk instability. Yes, over spin can cause instability, but usually not keyholing like under spin. The 1-8 barrel was just what I could get, but I know I can go to anything that will mag feed and still get it stabilized.

VarmintAR.jpg


CoyoteAR.jpg


Bullet velocity and barrel twist rate are the two things you need to calculate bullet RPM and therefore, stabilization. You can use a slower twist barrel if you push the bullet fast enough, and get the same bullet rpm as a faster twist barrel and a slower velocity.

In a 1-8 16" barrel pushing a 75 grain bullet at 2600 fps, for instance, the bullet rpm is 234,000. A 1-12 24" barrel pushing a 55 grain bullet at 3200 fps results in a bullet rpm of 192,000. A difference of only 18%. Substantially less rpm loss than the 50% reduction in twist rate alone would indicate. This is why some 1-9 twist 16" barrels won't stabilize a 75 grain bullet, while some 20" with the same twist will. The extra velocity from the longer barrel is just enough to spin the bullet fast enough to stabilize.

As far as single loading bullets, the only ones I know of that are too long to load in a mag are the 80 grain and up match stuff. Everything else, including the 75's and 77's should be loaded to mag length.

Fast Ed
 
Velocity X .720 divided by twist rate = RPM

This is the formula ballisticians use to base their recommendations upon.

As I stated earlier, you cannot load a 75gr A-max to mag length. The bullet will fall in the case. They have to be single fed.........
 
You don't keyhole with too fast of a twist rate. You spin the bullets faster than necessary and this brings out the variances between bullets more, causing your groups to open up.
 
i didnt realize it was called amax.here is what the box says:

hornady match
.223 rem
75gr BTHP
#8026

ordered them thru cabelas....are these considered amax?
 
If you can find out the actual twist rate that can help as well. I've seen a 1 in 9 that was closer to a 1 in 8 that did well with 75 gr Tap.

Just posted this to say I think that's one of the reasons everyone always says you have to test each barrel to see what it likes.
 
Those are not A-maxes, Jimmie. Those are match bullets, though. The A-maxes are longer, have a red tip, and a higher BC....
 
2muchgun is correct, and I was not. The 75 Amax shows a COL of 2.390, where the COL of a mag length .223 round is only 2.250. So it is the 80 grain and up Sierras, Berger heavy VLD's and the 75 AMax that must be loaded singly in an AR.

These really are the match bullets for 600 yard prone. I see no other use for them in an AR although in a .22-250 or a Swift they might be dynamite for long range stuff from the longer magazines in those rifles.

Fast Ed
 
They are a great bullet. I plan on trying them for 600yd prone at some point, I believe. I know several guys who have taken deer with them.......
 


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