i kinda like the ideas :P

Assault Rifle - Need a more detailed explanation, mostly rate of fire to decide if it would be balanced or not.

Bayonet - Been suggested often, but it's a good idea and the game needs melee

Machine gun - I can't see it being used often with the 2 people requirement, your variant of the idea seems too underpowered.

Mortar - Just no. Mortars would render fortifications useless and make for massive frustration by being killed by something you can't see. The 40 block range one you described would be less game destroying than some described but I can't see any mortar making the game more enjoyable.

Explosives: Reduce planting time to 5 seconds, reduce number of pick-strikes to defuse to 3 and it sounds like you have a good addition. +1 to it making a ticking sound also

The explosives should have a ticking or hissing sound.

Assault Rifle: 75% less damage than rifle, 20 bullets in a clip. Player starts with 60 bullets (20 in assault rifle already)

Bayonet:25% more damage than rifle, instant death by stabbing in head, takes 2 hits to kill an enemy.

Machine gun: 50% less damage than rifle, 50 bullets in a clip, can be only operated by 2 people (one gunner and one who reloads). The gun must be deployed to be fired, can overheat by shooting for 5 seconds without end.

Mortar: Damage like grenade. Must be deployed to shoot, uses special grenades. Instant death within 3x3 radius of explosion and half damage at 4x4 to 6x6. It's max range: 40 blocks.

Explosives: 100% more damage than grenade to blocks and 50% less damage to players. Explodes within radiux of 5x5. Player can carry only one explosive, and it takes 8 seconds to plant. CAN be defused by hitting it with pickaxe 5 times, it's time before explosion: 10 seconds. Noise: Quiet.