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Posted: 6/29/2003 6:55:53 AM EDT
I have a quick question on barrel twist rates for a .223 varmint/target AR setup. I'm looking to build a 24" BBL kit, and am not sure which barrel twist to go with. I will be shooting mostly at 300 yards, but also want to be able to load for distances over 500 yards as well. I understand the heavier bullets are needed for the longer ranges, and want to make sure I get the proper twist rate to keep them stable.

Thanks!
Link Posted: 6/29/2003 8:40:29 AM EDT
Rifling twist is dependent on the length of the bullet you're trying to stabilize. Assuming constant construction a longer bullet will be heavier than a shorter one of course, but technically it's a length factor. Exact twist can only be determined by experimentation. Greenhill's forumla is a fine rule of thumb but you'll end up in the 1/7 to 1/9 twist (one turn in 7 or 9 inches) range most of the time. Your ammunition will determine the correct twist. Match barrels (real match barrels) are very often 1/8 twist and seem to do well out to 1000 meters. -- Chuck
Link Posted: 6/29/2003 10:09:02 AM EDT
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Link Posted: 6/29/2003 12:13:12 PM EDT
Tango27: Chuck's answer is right on the nose! Very good info. You will find that quite a few target/varmint barrels come with a 1:8 twist... largely because this is a very good, overall twist rate for the longer (heavier) bullets. I think that Kreiger pushes a 1:7.7 twist barrel? To be honest, the 1:7 twist rate is faster than needed for almost any ammo you will use... Granted, as Chuck pointed out, the OAL of the bullet is the determining factor and consistant density plays in as well, but given you are limited by the dimensions of the magazine, I would say that you will be more than pleased with the 1:8 twist rate. BTW: The Greenhill formula (with a constant of 150) is intended for velocities much lower than what the AR/M16 shoots -- remember that the stabilization is a result of the actually speed of the bullets spin (RPM) and that the velocity is a determining factor in that speed... Most would suggest you use a constant of 180 with the Greenhill formula for your purpose.
Link Posted: 6/30/2003 7:02:18 PM EDT
Thanks guys! Looks like the 1:8 will be the one. Now it's time I learn how to do my own loading...
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