Problems with front sight post installation

[HoneyBadger]

I started with two functional uppers. One of them did not have an "F" marked front sight, so it wasn't usable with any of the rear iron sights I have. I decided to fix this by taking it off, and then putting on the "F" marked FSB from the other upper. All went well until I tried to actually put the new FSB on. The holes for "pinning" the FSB didn't match up. See pics:

Upper that had the non-"F"-marked FSB:


Upper that had the "F" marked FSB:



Is this a common thing or do I have an anomalous barrel/FSB combination? The "F" marked upper was a new upper from PSA. The other upper I bought about 8 years ago from someone on another forum. I don't really know any of the details about the upper's origins. Any solution here? I suppose I could put the old FSB back on and then chop it down to just a gas block and hide it under a longer rail, but i really wanted to have a FSB on it (one that works).

Thanks in advance.

[thvigil11]

FSBs are considered part of the barrel unit and basically one time use sorts of things. Its rare to find two that match up exactly.

[HoneyBadger]

FSBs are considered part of the barrel unit and basically one time use sorts of things. Its rare to find two that match up exactly.

I had no idea... I've done a swap like this before with no issues. Maybe I just got lucky before?

[thvigil11]

I had no idea... I've done a swap like this before with no issues. Maybe I just got lucky before?

Basically, yeah you got lucky. 16 barrels are considered one piece replacement with the fsb, nut, cap all being one unit. With patience and time I have place a used fsb on a new barrel, I've also welded the holes and redrilled one, and I've also placed a new fsb on a predrilled barrel. All worked out pretty well, but took some time to get it right. Also helps to have all the equipment. (welder, mill, proper measuring devices)

[HoneyBadger]

Hmmm. I guess I'll just put a low-pro GB and a longer rail on it. Not what I wanted (and I was hoping not to spend any money on this project).

[SuperiorDG]

Hmmm. I guess I'll just put a low-pro GB and a longer rail on it. Not what I wanted (and I was hoping not to spend any money on this project).

That's how all my "projects" work out.

[BPTactical]

FWIW, I have done many FSB swaps like you would like done. No, the odds of the two sets of holes lining up are virtually nil. The barrel and FSB are match drilled as a unit and as mentioned generally are not interchangeable.
But it can be done without welding or any other black magic. A fixture for holding the barrel/FSB in alignment is critical.
PM me if interested.

[HoneyBadger]

Thanks for chiming in with your wisdom gents. I guess I learned something about FSBs today. I think I'll put the old FSB back on and Dremel it down, then hide it under a longer handguard. That way I won't have to buy a new gas block.

[Great-Kazoo]

Thanks for chiming in with your wisdom gents. I guess I learned something about FSBs today. I think I'll put the old FSB back on and Dremel it down, then hide it under a longer handguard. That way I won't have to buy a new gas block.

Take new FSB, tap the holes and use set screws. Or cut yours down, all is well in whoville.