Hmmm, well I own 3 of the mentioned rifles.
I have the closest to a sniper rifle in a 308 semi-auto made here:
I have a civvie version of the M14 that has been accurized and worked by a master M14 smith here:
And then a plain jane DSA FAL here:
The pros and cons of these 3 rifles are:
AR10 -
PROS: excellent optics platform and out of the box accuracy. Sub MOA using Match grade ammo is easy. With homeloads I am sure .75 MOA can be achieved. Simple maintanance since I am familiar with ARs/M16s.
CONS: mag problems, mag expenses, and available spare parts would make me leary of this one as choice for a MBR even in a different configuration. Lack of track record in real world. A bit ammo sensitive.
Verdict: This is a really cool range rifle and tack driver. If you try to make it something else let me know how it turns out.
M14/M1A -
PROS: Everyone loves to shoot these. With iron sights it will outshoot either of the others listed here. Assuming of course you have accurized the rifle. Excellent track record of reliability and real world use as MBR. Available and cheap parts (beginning to change now). Simple maintenance.
CONS: In my personal opinion (shared by many) this is a platform not suited to optics. I won't go into detail just suffice it that it is a pain in the ass to do. Recoil is heavy. And, believe it or not, of the three accuracy is the worst if it isn't properly accurized and maintained. All those trophies at Camp Perry were won with specially worked rifles. No big wonder ARs rule there now.
Verdict: And excellent platform for a MBR. Just don't expect it to be a tack driver or to give you MOA accuracy without special mods and maintanance (read annual bedding of the stock).
FAL -
PROS: Very reliable and proven MBR design. Very low maintanance and tons of spare parts and cheap magazines (like $5 each versus $40 each for the other rifles maybe). A decent enough platform for optics. Many options and choices.
CONS: Not a tack driver. Again, this is a MBR and asking a MBR to shoot MOA is not reasonable IMO. Weak iron sights. Trigger not as good as the other two (smooth).
Verdict: The best choice for a MBR.
YMMV.
Oh, the AR10 and M1A could both be bought it you are interested. I'll keep the FAL thanks.