I am considering purchasing a RRA 16" 9mm upper after the AWB ends to get cut down to use as a pistol upper. Ideally, I would like to put an Oly Arms shorty FIRSH (3.75" long) and have the barrel cut down to as short as I can get (maybe 4.5-5").

I have heard that the 9mm system does not need to use the gas system since it is a simple blowback design, so does this mean that I do not need a gas block or gas tube at all, even for short barrels?

Are there any problems with reliability that I would have to worry about with a barrel that short?

What is the diameter of a 9mm barrel? Are they tapered at all? Would a FIRSH barrel nut (which is designed for the .750" HBAR) fit without modification?

No gas system, no problems with a short barrel. Olympic runs a Firsh on some of their 6 inch barreled carbines. I think you ought to stick to 6 inches or so, so you dont blow your finger off.

I have a 5 inch 3-lug on mine that allows me to use my Gemtech Raptor Suppressor on it. It works and looks great!
Keep in mind that the FIRSH won't work with the Tango Down grip. I ended up using a standard KAC grip on it that works really well.

Good luck!

M_J

I put the 3.75 FIRSH on a 9mm upper and had KKF make a faux block that matched the top rail of the FIRSH....barrel is cut to 7.5".....entire upper is RRA 9mm...9mms are blowbacks.....no gas tubes involved...Any reliability issue will not arise from the length of the barrel but the block / mag set up you go with....just my 2cents worth...

Thanks for the info.

I was also wondering about the diameter of a standard 16" 9mm RRA barrel. Does it get thicker near the chamber? Or is it the same all the way out to the muzzle?

If I get the barrel cut down to around 6-7" can I move the front sight up and have it still fit around the barrel? I will probably use a 1913 railed gas block vs a standard front sight since they are easier to remove for gunsmithing work and I would prefer a flip down front sight anyway. Just wondering if this is possible to do and what diameter gas block I need to look for... (.750"?)