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- Wed Nov 16, 2016 12:39 pm
- Forum: Rifles & Shotguns
- Topic: Building my First AR-15
- Replies: 35
- Views: 7197
Re: Building my First AR-15
The reason the 16" and shorter barrels went to 1:7 twist has to do with velocity. At a lower velocity due to barrel length and 1:9 twist the bullet isn't spinning fast enough to stabilize the heavier bullets. Slower muzzle velocity equals a slower bullet RPM given the same twist of the barrel. A 1:9 out of a 20" barrel will most likely stabilize a 75gr bullet but it seems 68-69 is about perfect.