I have a stag gun with an A2 FSB. I want to convert it to a low-pro gas block and use a longer free float rail. I got a Spikes A2 FSB that I cut down to use as the low pro gas block. My question is, will I have any problems taking off my old FSB and replacing it with the same kind? FSBs aren't fit to the gun are they? I wanted to save my old one just in case I ever want to convert back.

Edit: feel free to move this, I wasn't sure whether it was build it or general.

Yes, they are generally fit to the gun with regards to pin locations...

So my new one most likely won't fit correctly? That's a bummer. On the upside, I got some good practice cutting one down.

That is correct.  Most likely the pin holes in your new cut-down FSB will not match your existing holes.

Correct. but- there is a chance that it will. I got lucky with my first AR. Long story short the ears broke off my F type FS/GB, I bought another and was able to swap it out.and regain zero with proper gas flow as well. I did some research on it before I did it tho and 9 times out of 10 it won't work out. But you could get lucky like I did

Also, you can cut the one on it down to free float, and buy a clamping or set screw gas block as a backup to "switch it back" in a sense
, as well as just install the clamp/set screw GB and free float, while keeping the F type you have on it now for switching back.