The one big thing you’re gonna run into trouble with is if you want to mount a light. I have an M3S90 and still haven’t found a method of mounting a light. (Surefire doesn’t make a forend, etc.)
The other issue (for me anyways) is that the stock LOP was rather long. This I was able to fix, thanks to reading a member post about chopping the stock down here.
If you want a nice semi, it’s a good pick, especially for that price. After a little break in, mine has runs decently well, although it prefers stouter loads and not light target stuff.
It all kind of depends on what you want to use it for in the end.
I picked up an M3S90 a couple of years ago…by far, my favorite long gun. Never a problem with the pump or semi-auto actions…pretty easy to switch between both.
Looks good, I wonder if it’d work with the M3 though. Every once in a while I look up barrel clamps like this one, and then get dismayed because the description says M1, M2, and M4, deliberately omitting the M3. Makes me hesitant to order one just to try it out, since if it doesn’t work, I’m hosed.
If I could find someone who has a working one on an M3, I’d order it and get a light on it in a heartbeat.
On the M4 you remove the sling plate retainer, discard the attachment plate and install the BLAM4.
Went to Brownells and checked out the schematic for the M3 and it certainly looks like it will work. M3 has a forend cap then the sling ring magazine, reference below link. I would order the BLAM4 from Brownells and see if it will work on the M3, if not you can return without any questions.