I have a CMMG (9mm) flattop AR with a rail in place of the FSB. It looks like this pic (this is not my rifle)
I’d like to replace the rail in front of the hand guards with a regular “classic” (F marked) FSB. Is this feasible? If yes, is it relatively simple - like the guy at my LGS can do it - or does this need to be done by a true gunsmith?
I really appreciate any guidence!
I think the majority of the people on here would refer you to ADCO. But they “do NOT install FSBs on any surface coated/hardened barrel.” Scroll down to the middle of the page.
You have to have a jig and the machinery to drill the holes for the FSB pins properly. Most run of the mill gunsmiths at your local gun shop are not going to be set up to do this.
Have you considered a front sight that will attach to your current base? How about a FSB that has allen screws for attachment instead of pins? Unless you have your heart set on having a bayonet lug, these are viable options.
The last time I ran into this question locally, I told the rifle owner it would be more cost effective to buy a new barrel with the FSB already installed and sell his current barrel assembly.
If the barrel diameter under your ‘gas block’ is a standard size it should be pretty easy. There are three methods for attaching front sight posts: Pins, set screws, and clamps.
On a gas gun, the front sight/gas block is critical to the functioning of the weapon, and because of this, pinning is the best option. Pinning is the most positive, failure resistant method for attaching the front sight assembly. On a blow back weapon, if the ‘gas block’ moves, the gun will still work, but your hand guard may become unsecured.
I personally rate the ‘clamping’ style as the next strongest way to attach a FSP. I hate set screws, if I had a gas block with set screws I’d pin it anyway.
On my 9mm upper, I have no gas block/front sight; I just floated a rail over the barrel, and put a sight on the rail. I think that is a cleaner/easier set up, but if it’s not what you want, don’t do it.
If it were mine, and I wanted that setup, I’d buy the drill bit and tapered reamer and give it a go on my drill press with a GI taper pinned FSP. If that sounds like a horrible idea to you, send it to Adco.
guys, thanks for the advice and references. I only wanted this change for cosmetic reasons, I guess I like the ‘old school’ look of the FSB, but not enough to go through what sounds like significant effort or expense. The easy answer for me after reading your feedback is to go with BUIS on that front rail (Troy, MAGPUL or whatever) and then install an FSB if I decide to convert to an NFA SBR (so I can give my $200 to the govt). Obviously I can do the barrel & FSB and keep a 16" barrel too. Thanks for the feedback, this was a good learning opportunity for me and I appreciate it.