eh, not soo much of an ammo truck thing. Although, the rx79 has that giant backpack and you can guess what it stores in there..
Probably should go into more detail...
Some fluff info(please ignore all sillyness as magic):
Beam rifles use a latice of particles known as minovsky particles to fire out plasma shots contained in this latice. Which is also how the gundam "beam saber" works. However, the beam rifle required massive generators(look at the zeke generator) so it was only able to be equiped on a battle ship. Now, when it reaches a distance it will dissipate and be lost in all strength(represented by the weapon range as spring as the weapons fade away)
However, the Federation develops a beam rifle that can fire at the power of a battleship gun using a limited charge e-cap clip placed in the rx78's beamrifle. This had about 7-12 shots of massively damaging beam blasts and then was out, forcing the rx78 to engage into close combat. However, I have not seen a working close combat weapon yet
so I cannot do the whole fire off a volley then charge in and destroy stuff bit
The other units(gelgoog and rx79beam) use what looks like a clip. It can be reloaded in battle just fine. However, there is a reload time(hence the slowdown of fire rate). I could do a scripted reload that happens every so often but I wasn't sure if people would hate it.
Now, the gm sniper and sniper II use a "long beam rifle" however, there is a difference:
The gmsniper is using a prototype long beam rifle which is effectively a steady stream weapon that requires a cool down time or it will burn out. I need to do some smoke coming off of the thing and stuff but I have not gotten to that yet
The sniperII uses a finished version of the LBR(can I use this for discussion purposes) it can use the long range and has a slower fire rate however, it fires a shot powerful enough to keep integrity at great distances.
Now bear in mind that i am not saying that any of this makes sense but it has it's "logic" if you could call the crazy gundam fluff that(the gaw uses thermo nuclear jet engines.. don't ask just lol at it please). There are no reload trucks for the MS of gundam. When they have used up their ammo they usually have reached the range to kung-fu fight with light sabers in mechs that supposedly have just recently been developed and are first generation mass production suits.
Now that being said, there is a way to setup a gmsniper as a constant fire weapon. You'll find this neat because it is possible with the new lua stuff. The gmsniper can be paired with a coolant truck and it can(while stationary) fire CONSTANT! That might me neat. Hell if you want I can give gm snipers an on/off option that would represent them saying fuck it and the beam rifle could start over heating and damage the suit :).