i was wondering if there was a unique anti material rifle, and if so, where is it located?

no

No unique version and no weapon mods.

There is ".50 MG Match Hand Load" ammo you can press for it and that's the only "unique" thing you can do to the AMR. At current you need the "Hand Loader" perk and 100 repair skill to press those rounds. If you have both it's just a matter of buying normal .50 ammo, breaking it down and re-pressing it with abit more lead+powder.

Their freakin should be.