If a barrel has been drilled for a FSB, will it only take the FSB that was drilled for it? I often see used barrels for sale where the original FSB has been removed  and is long gone, can you buy another surplus FSB that's been drilled with pins and install it, or do you need a un drilled one and have to send it off?

this is true for modern barrels where you have so many different folks doing their own thing but early colts used a jig that drilled both holes at the same time.  i've had good luck swapping early colt FSBs with each other.

I bought a barrel with an FSB attached but just had to have the Colt fsb on the barrel for my build. Well, I'll never attempt that again as it was a pain in the butt to get straight and get notches and pins to match up. Guess I was lucky and it worked out, but if you must switch one out for another, I'd suggest paying someone with jigs and equipment to do it right.

My two cents:  The FSB is designed to be specific to a particular barrel but they can sometimes be interchanged.  Most reputable gunsmiths will prefer to fix the barrel first, probably by filling in the old drill holes with some sort of weld (someone who knows better than I can tell me what they use), and then re-drill the barrel for the new (old) FSB.  That will give you the BEST fit.  John Thomas of Retro Arms Works (he's on this site) does this kind of work and it's not too expensive.  I'm sure there are probably others here who can do it.  Maybe JT or someone else who does this kind of work can chime in here an explain what he thinks is best.  I have heard a few sad tales of loose FSBs from people who simply tried to take an A1FSB from a parts kit and put it on a predrilled barrel.  It may work...or it may not.  

I got really lucky in one I swapped. I'll never attempt it again.

Same.

It's "hit or miss". Sometimes they line up,sometimes just a tad off, and sometimes no where near close.

I had 3 20" barrels I bought that had been drilled for the FSB but, they had been removed.
I just ordered a dozen of the FSB's on EE that were new take offs.
Two of them fit perfect, 1 had one of the holes off a very small amount, It was on the front hole on the barrel at the top of the hole.
I hit that one spot with a small jewelry file and it fit perfect.

I was able to use two of the FSB's on some 16" barrels.

I still have some FSB's left and some that were never drilled yet.

They are all tight and straight.

Go very slow, watch what you doing. A BFH is not your friend with this...........I use a very small hammer and punch to drive in the pins.

Ron

Although not retro - grade , I've had good luck swapping out FSB's on FN barrels.

FN must have a jig also.

The three I installed the FSB's on I think were FN barrels.
I would have to dig them out of the safe to be sure, but I cannot get to that safe now.

Ron

The guy I got them (FSB) from on EE is "HOCHIMIN"
You might check to see if he has any left.
Great to deal with.........