Everyone that pre-orders a copy of Mass Effect 3 in either the UK or US will recieve a bonus in-game M55 Argus Assault Rifle – described as a quick-burst ranged weapon.
However, in an increasingly more common grab more relevant attention from consumers, if you pre-order from GAME you’ll get a bit more. Namely, the ‘N7 Warfare Gear’ which includes the N7 Defender Armour (increased shield, health, ammo capacity and weapon weapon) and the N7 Valkyrie Assault Rifle (fires two rounds per shot).
The offer is the same in the US too, but with the retailers in question being GameStop and EB Games rather than GAME.
Alternatively, if you’re a PC player in either region buying from EA’s Origin store you’ll get the AT-12 Raider Shotgun. Joy!