The recoilless rifle is actually a rocket launcher.
M40 106mm recoilless rifle.
106mm
106mm
106mm
If you mean a weapon fired from the shoulder, and exclude military anti-tank recoiless rifles, it is probably the 950 J.D. Jones, which weighs about 100 lbs. If you simply mean a rifled weapon, then the 18.1 inch naval cannon used by the Japanese Navy.
The quick answer is a .410 bore - but there are relatively low recoil 12 ga. guns as well. Browning made a "recoiless" trap gun which almost is recoiless. In general, a gas-operated semi-auto will have less recoil, and in some cases this can be combined with a recoil-absorbing stock.
222mm × 106mm × 73 mm
106 mm = 4.17322 " Direct Conversion Formula 106 mm* 1 in 25.4 mm = 4.173228346 in
A female is 108mm long and a male is 106mm long
9.4 kilometers = 9,400,000 millimeters:9,400,000 mm in Scientific Notation = 9.4 x 106mm
Depends on how you define gun. In WW II, a Japanese battleship was planned that fired shells 18 inches in diameter. The US Navy used battleships with 16 inch guns. The US Army had an 11 inch cannon (search the term Atomic Annie) that was a portable cannon with a 20 mile range (and could fire a nuclear weapon). If you mean fired from the shoulder, the 90mm recoiless rifle (an anti-tank weapon) would be in the running. If you mean a conventional rifle, either the Mauser 20mm anti-tank rifle, or one of the J D Jones specials, nearly an inch diameter bullet. The .700 Nitro Express was a commercially produced rifle- so there are a lot to choose from. Don't forget the 4 gauge shotguns, and the 4 bore Paradox guns. Bullet about the size of a golf ball.