flat top 20" needs what type of FSB?

[gt5966b]

Do i need a F-marked FSB for a 20" barrel'd flat top upper? I think i've read in the past that i do. I guess that brings me to my next issue...

I bought a 20" colt HBAR barrel off of the EE on ARFCOM and it had a regular FSB. I knocked it out and tried to install an F-marked FSB that i had on hand. It didn't work out to me. The F-marked FSB didn't want to line up with the existing drill holes in the barrel and the pins didn't fit in that looked like it was going to work out - so i was reluctant to try and hammer them in for fear of damaging the barrel.

Should i expect FSBs to be interchangable? Any tips? Thx.

I'll post up some pics a little later.

[Robb Jensen]

Unless the FSBs were made by the same manufacturer around the same time 99% of the time they won't work.

I've run 20" flat top upper with and without F marked FSBs and haven't had a zeroing problem either way. Honestly I wouldn't worry about using a F marked FSB (which I don't view as necessary), if you have trouble zeroing just get the taller front sight post.

[gt5966b]

once again - gotm4 to the rescue. thx.

[gt5966b]

Honestly I wouldn't worry about using a F marked FSB (which I don't view as necessary), if you have trouble zeroing just get the taller front sight post.

one more Q - i just noticed that the existing non-F FSB is a pre-ban (no lug) and in fact the seller stated that the barrel was originally a pre-ban that he had threaded.

I was hoping to build it into a fairly high-fidelity M16A4 clone and wanted a lug on it. If that's the case, what are my options for getting a new one installed. Can i get an un-drilled FSB installed on an already drilled barrel?

I'd be willing to pay somebody. Thx.