I just purchased a Mossberg 100 ATR and i found a good deal on some 7.62x51 NATO rounds. I was wondering if my gun can fire these.

It should be no problem,however,, just to be on the safe side call mossberg.

NO.......that's in CAPITAL LETTERS !!! 

Yes you can.    check it out here.
http://en.wikipedia.org/wiki/7.62%C...751mm_NATO

Be my guest, but read a few of these first...................and there is a lot more where these came from !

http://www.thefirearmblog.com/blog/...2x51-nato/

http://www.303british.com/id36.html

SDOWLING.

Both of those indicate the problem you are bringing to light is the not shooting 7.62x51 in a 308 but shooting 308 in a 7.62x51. 
Both cartridges are same.  The 308 is loaded to a higher pressure and some nato chambers are longer.  Niether of these are a problem when shooting 7.62x51 in a 308 winchester chamber.

the only problem is in reloading (not with standing the pressure differences) and that comes down to correctly resizing and careful examination of the fired cases..nato rounds have a (normally) thicker web and walls, which pose li'l if any difficulties for the reloader. if you collect cases.. from say a range were once fired NATO rounds have been fired then full case resizing is required as with length sizing...this is due to the varibles in the chambers of NATO firearms...first run or factory (armory ammo) should feed and fire without any problems in your non-military firearms...(becareful of your selection of ammo brands (nato) as some may not be as quality as what you would hope for...

Then again with the specs of the Nato rounds one may have a comercial firearm that has a chamber that is to the lower end of it's tolerances..and some of the ammo may not chamber correctly...