Optimus - Bullets Bumblebee - Plasma Grenades/Missiles Ratchet - Bullets Ironhide - Grenades/Bullets Jazz - Energy Blasts Sideswipe - Bullets Jolt - ? Skids - ? Mudflap - Bullets Arcee Sisters - Bullets Dino - ? Que - ? Sentinel Prime - Rust Grenades Roadbuster - Bullets/Missiles Topspin - Missiles/Bullets Leadfoot - Bullets Brains - Plasma Bullets? Wheelie - ? Jetfire - Missiles Megatron - Plasma Blast Starscream - Bullets/Missiles Soundwave - Plasma Blast Shockwave - Missiles The Fallen - ? Long Haul - Missiles Demolishor - ? Rampage - Plasma Grenades? Scrapper - ? Crankcase - Bullets? Crowbar - Bullets? Hatchet - ? Bonecrusher - Bullets Brawl - Missiles/Bullets Ravage - Bullets/Missiles Laserbeak - Bullets Sideways - ? Blackout - Plasma Burst/Bullets Grindor - ? Alice - Bullets/Plasma Bursts Reedman - ? Scorponok - Bullets/Grenades Frenzy - Bullets Apologies if I missed anything/one, but that seems to be the type of weaponry each character uses, and the majority seem to use bullets and missiles, so take that as you will.
I believe it was said in the Reign of Starscream comics that "They regenerate ammo like we regenerate hair, nails and skin." Seems like a legit explanation.
I've always wondered whether the Autobot's guns were lasers that had a muzzle flash similar to our guns or if they were just giant machine guns like ours.
They probs use the weapons from their Alt-Modes like Starscream uses a Vulcan machine gun and missiles. On Cybertron they probably had laser weapons, but on Earth they use materials around them and Use projectile weapons. Even though at the beginning of DOTM in the Ark you see empty cartridges flying around the turret, I find it hard to believe they would use the same Brass case cartridges with internal explosives we use.
Probably they regenerate for their energy weapons and built in guns / swords (which transforms from their body parts hence Megatron's cannon in first movies, Bumblebee's arm cannon, Starscream's weapon, Sideswipe's swords) and they also use handmade ammo for extra weapons like Optimus' arsenal of hand guns and miniguns, axe, shield Megatron's shotgun, Sentinel's rust cannon etc. Ironhide is the weapons specialist, he's probably responsible for designing weapons and making ammo for them.