This can be found in a chest in Cid's house. When compared to the W Machine Gun, this weapons earns Barret an extra Materia slot and a little more attack power in exchange for the loss of long range attacks.
Heavy Vulcan Long range weapon
Attack: 39 Attack%: 100
Materia growth: Normal
Chainsaw
Attack: 52 Attack%: 100
Materia growth: Normal
Solid Bazooka Long range weapon
Attack: 61 Attack%: 100
Materia growth: Normal
Rocket Punch
Attack: 62 Attack%: 110
Barret's "novelty weapon" can be found in a chest in the bottom left-hand corner of the Temple of the Ancients labyrinth area. However, the slight increase in strength over the Chainsaw isn't quite enough to counter the loss of Materia slots.
Microlaser Long range weapon
Attack: 63 Attack%: 101
Materia growth: Normal
A·M Cannon Long range weapon
Attack: 77 Attack%: 103
Materia growth: Normal
The A·M Cannon, available from Mideel, is one of Barret's toughest gun-arms able to level up Materia. It loses out to the Max Ray in terms of strength, however.