Okay, I just finished putting a 21 together.

I have tried Alpha Wolf and KKM for barrels, my preference is the KKM, specifically the threaded barrel. It's 5.375" long. I haven't tried it yet, but Carver Custom makes a 4-port comp that ought to really tame slide velocity down.

I have 4 mags with SLR rifleworks +4 extensions, and several OEM capacity with +20% springs. The SLR extensions seem to be reliable and do allow the mag to hold 17.

Gun with the SLR extension, comp is a KKM 4-port, another efficient design. The only issue I have with that barrel setup is the threads are a proprietary pitch and size so you gotta run whatcha brung. It does work good.

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