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9mm AR Bolt weight, and buffer choice


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So I was reading about the all the different combinations in thread below, but could not really determine the consensus on what was working, and nothing really sounded like what I have, so I thought I would post this.

 

I have a 8.5" upper, and carbine stock (with spacer in back for less bolt travel).  I was reading about removing the weight from the bolt, and using a .308 buffer spring with the 9mm Buffer.  Does this sound right?  Obviously I want to try and reduce dot jump as much as possible, but I don't want to beat up the parts.  What combination seems to be best for a short barrel set-up or does it really matter in 9mm?

 

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I run a 15.8 oz. bolt with a 5.5 oz. buffer and a .308 flat wire buffer spring with my 132 pf loads through a 16" barrel. The bolt just barely clears the bolt hold back when fired, but doesn't "slam" into the buffer tube spacer. It feels very soft to me. I haven't tried this setup with my 8" upper yet, but when I do, I'll let you know if it feels any different.

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The shorter the barrel the less buffer weight you "need". The bolt/buffer mass in a blowback gun is designed to keep the bolt closed long enough for pressure to drop to safe levels that the case can withstand. The shorter the barrel the less mass you need for safe operation because the pressure in the barrel drops sooner. If you look at an SMG design book you should see the formula demonstrating that this is the case.

I've tried removing the mass from the bolt and running a 9mm buffer in a 16 inch barrel and I liked the combo a lot. I would like to go lighter, eventually, but with a 16" barrel I don't want to get too close to the edge.

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Hello: If you go too light in either spring pressure or buffer weight you will get a velocity change since the round will not seal well or will fire slightly out of battery. I found this out by using my chrono with my reloads and different buffer and spring setups. It has been driving me crazy over the past couple of matches since I was getting jams and also causing trigger problems. I think 18-20oz buffer bolt setup is the perfect setup if you are shooting 125-140PF loads. Try the 5.6oz buffer with the 223 spring. You may also try removing the bolt weight and a 4.3oz buffer with a 308 spring. Playing around with the different setups and loads is half the fun. Thanks, Eric

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