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I am in the process of building a 20" AR also....It's going to be kinda of an odd ball.....something like a 300 whisper XL for super-sonic 110 to 125's.....Pug had listed a post a while back concerning a longer cartridge similar to the 300 whisper.....most of my 300 whisper brass comes out to around 34.4mm....HK developed a 7.62 subsonic load a while back based on a .223 case that works out to 7.62x37mm.....the load that I'm working on for my AR is based on a .223 case and works out to 7.62x39mm....I have formed up dummy loads and am having a reamer cut and getting all of the parts together to cut the barrel and assemble the upper....hope to have it running in two to three weeks...am trying to decide on a twist for the barrel, your last post on the 1:10 has me leaning in that direction....I have also seen the additional length adding several fps....the difference between my 16" and 24" bolt guns is about 200 fps average on super-sonic loads for a given charge....you wanna hear something funny, the 16" bolt gun shoots sub-sonic loads about 100 fps faster than the 24" gun with a given charge...
kurtz
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