FSB Question

AR/M4 AR Technical Discussion
1

I have a barrel from a Colt 6520 that I want to install on a flat top upper. I found a source for “F” marked fsb but dont know which one to get. I dont know whether the barrel is .650 or .750 under the fsb.

Thanks
John Prescott

2

Your FSB bore is .625" (5/8") and is a F height FSB. If you use a flat top receiver with that barrel you’ll need the .040" Bushmaster front sight post.

3

I have a quick FSB question (and I appologize for the slight hijack, but I’d rather not start a new thread for such a trivial question).

I’m going to build an iron sighted 20" rifle. Should I use an “F” marked FSB or a nonmarked one? Does it matter?

4

If you don’t use a F marked FSB 20" barrel and zeroing makes the front sight post stick up far in the FSB, all you’ll need to correct it is the .040" taller Bushmaster front sight post which sales for $5.50