I have a mil spec Ar-15 A 2 upper. I cant find markings on the barrel. I purchased it during the AWB and it does not have a bayonet lug. What size A2 Front Gas Block should I buy?. I appreciate the information.

probably a .750 no f-marked like this one.  unless its a pencil barrel. probably be tricky putting a new one on thou


https://www.riflegear.com/p-22388-schmid-fsb-front-sight-base-non-f-marked-750-id.aspx

If it uses taper pins to mount, it is going to be very difficult to replace it with another one.

The FSB is paired with the barrel once those holes are drilled, since being even a fraction of a mm off will make the holes not line up.

I've heard some gunsmiths have had luck drilling the holes out to the next size up, but that will depend on their skill, willingness, and barrel diameter

I don't know how attached you are to your barrel but you might want to consider keeping what you have, or looking into getting a new barrel + FSB assembly (they usually aren't to hard to find for a decent price)

Its very common in the AK world to weld up old cross pin holes, dress down, and drill new holes for new barrel components. I've done the same for AR barrels, mostly during the plandemic when components were scarce. Its pretty cost prohibitive now, Get a new barrel and FSB. Installing a new one is like 80 in my shop, welding, lathing, and drilling for new sight would be like 250 in labor alone.

If you're really dead-set on keeping the barrel, there are several companies that make a clamp-on A2 FSB. I had one at one time from Dez arms and it worked fine and has the bayo lug.

Cordless drill, 29 gauge drill bit, 8-32 tap and holder, and 8-32 thread x 3/16" long cup point set screws.

Maybe 10-32 if the barrel has taper pin slots?

I'd check with ADCO about the work OP or just buy a new barrel assembly.

As previously mentioned the barrel with a Fized FSB is essentially a fingerprint to the barrel. It’s in your best interest to just buy a new barrel.