203mm Howitzer M110A2 (8 inch - heard ALL over). Very loud especially when firing Charge 8 (humongous amount of powder). The M109A5/6 is almost as impressive firing Charge 8 Super and I've done that a time or two as well 8)

Other than that, M2HB .50 cal MG (my all time favorite MG)
and the usual assortment of other "small" arms.

60mm mortar, 84mm Carl G recoiless rifle, M203, 12 gauge shotgun, C7A1 and C7A2 5.56mm (M-16), C9 5.56mm (M-249 SAW), C6 7.62mm (M-240). Those would have to be the biggest boner makers I have fired.

Apart from the usual 84 Carl Gustov, 81 mm Mortar, Jimpys etc, the nastiest rifle I ever fired was a 55 Boys Anti Tank Rifle owned by a fellow rifle club member in UK. The recoil was horrendous!

Well, the M68 105mm rifled tank gun is the biggest I fired. Fired probably close to 100 rounds of that. Also 20mm training ammo(fired from the 105 with an adapter), which is kind of redicules after the 105. 60 mm mortar.

Actual small arms wise...I guess the 0.5 M2 MG, although i fired it with strings, not actually touching the MG.

So the biggest one i actually manualy fired was the FN-MAG 7.62... Probably about 30-40 thousand rounds all togather.

Mine would be the K201 (Korean production M203 for use with K2 rifles) and the K3 which is pretty much South Korean production FN Minimi.

Barrett Model 95 .50 Cal

Multiple Launch Rocket System....

Biggest I've ever played with was a privately owned WWI Ordnance QF 13 pounder.

In terms of firearms.... Finnish Lahti L-39 20mm anti-tank rifle... ammunition if about $80 a round. I fired off one round.