As someone who has owned an M14, FAL, G3 and Scar-h at one time or another I can definitely say this:
M14's are good for a DMR base. Midcaps are easy to find and there are quite a few internal parts available at any reputable dealer. External parts aren't super common but most places have a t the very least a passable selection.
G3: Don't do it. It's really not worth it in comparison to an M14 or the FAL A) The gun itself is very creaky and there's very little to be done about it (The metal Kit on Evike might help here a bit - however the long front end is very toyish feeling) If I remember right the outer barrel is metal. B) Hop up system, discussed below C) The midcaps I tried fed poorly and were a bit overpriced.
The range was decent but overall, in my opinion, the hop up stopped it from being the tack driver I wanted it to be. It slipped very easily and even after replacing it it struggled to find consistency. I ended up breaking the gearbox trying to upgrade it. (It was my second gun and I got it for a pretty good price - no real loss there)
FAL: In my personal opinion, these are by far the best choice of the 3. I've owned 2 versions: The short RIS one, which despite being very heavy in the front and requiring some type of battery holder/PEQ was by far my favorite rifle I've ever owned.
I've also owned the longer version which I used at OP: Blacksheep with huge success. I added another 4 inches in barrel length to get it out to the 509mm length I wanted. It was by far my best long gun and it was a CONSTANT attention grabber by guys on the field and after a solid tune up from T6 it was easily able to compete with most any rifle out there.
The Lancer Tactical/JG ones are solidly built and use V3 parts - meaning internals are pretty much in endless supply.
