AR middy 7.62x39 16"?

The larger bore diameter of the 30 cal AR-15 barrels mean that they lose pressure a lot faster. My experience has been in line with this - carbine length gas systems are best for 16" 30-cal (300 AAC/7.62x39) AR-15's.

I tried a 20" 7.62x39 build about 10-12 years ago with a rifle length gas system, and I could never get it reliable.

My 10.5, 7.62 39 ar puts out commie ammo at about 2100 fps. So I am losing about 300 fps over same setup with 16 inch barrel. I was surprised it did not lose more.

Bolt breakage in the AR15 configuration under sustained usage can be a problem as the bolt head has to be opened up to take the bigger cartridge head which is also true of the 6.5 Grendel etc. Alexander Arms had to redesign the AR bolt head for the Grendel (which is based on the 6.5 PPC round that was based on the .220 Russian which is a necked down 7.62x39 round). I also believe that several of the better manufacturers of AR uppers in this format also have had to deal with this issue--considerable thread on it at AR-15.com.

Magazines can also be an issue as the feeding geometry is different for the round--fatter, different COL, and different taper to 7.62x39 cartridge.

I tried a 20" 7.62x39 build about 10-12 years ago with a rifle length gas system, and I could never get it reliable.

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Me too. My 16" carbine-gas runs better than my 20" rifle-gas.

C-Products mags solve the mag issues. The weakness is the bolt, since the rim diameter is wider on the x39 than 223. I had one bolt lose the two lugs next to the extractor.

ditto

carbine length gas works better for 7.62x39.

M

I did build a 16" Dissipator style 7.62x39 once, to get the sight plane of a rifle-length gas system and the reliability of a carbine length gas system. It was too heavy, so I sold it.