Hello,
Although 9mm AR15s are blowback, including CMMG's radial delayed blowback, does anyone know the diameter of the barrel? I am thinking about attaching a FSB to the barrel and was wondering if the barrel is 0.750" or smaller? Likewise, I was thinking of using a delta ring assembly for 2 piece handguards as a stylistic choice. For that I would need an end cap help in place by the FSB.

Thank you

my CMMG RDB barrel is .730"... purchased as barrel and bolt not complete gun

Ah, so then a 0.750" gas block/FSB won't fit? Is it possible to have a setup so that I can use the delta ring, end cap, and FSB on the CMMG barrel? I'm looking at the 16" 9mm barrel and BCG combo.

Thank you

I've added a FSB that was not originally on a barrel where the tapered pin holes did not line up.. you could make up the diameter difference with a piece of .010" brass shim stock with enough opening to make it fit, and a little black rtv silicon to hide it....I tapped the hole on the bottom of the FSB that is used to drill the gas tap hole in the FSB... I used a set screw to hold the FSB in place to cross drill for a pair of roll pins... align the barrel in the mill with an index point on the alignment stud and run the mill table out to the front sight and adjust to make sure it is vertical... tighten the set screw... I used a milling machine to plunge through the FSB and start the barrel, then drilled...

since you don't use the gas port anyway, the single set screw with a dimpled barrel and a tight fit with a shim may not need the cross pins to hole the fron end

there are clamp on FSB available..

Hi lasnyder,
Thank you for the info! I like the idea of using a 0.01" shim. I probably won't go so far as to use a mill on the FSB, but I like your description. I will probably use a clamp on FSB with a 0.01" shim. I was hoping to use an end cap, like I said in my original post, and maybe a (non functioning) gas block with a rail. I know that the "correct" way is to use a free floated handguard, but I kind of like the look of the 2 piece handguard with delta ring.

Thank you again