Im thinking about making an infantry character who uses full auto weapons and small explosives. But figure i would ask what do you guys think is the best fully automatic weapon big or small?

for infantry the various service rifles would be best or the laser rcw if energy weapons.

Take the grunt perk if you have the right dlc for it, between it and other perks. A grenade launcher, service rifle, combat knife, and hand grenades would do you well.

Main reasons for the service rifles - Ammo is easy and cheap to get, handloader gives you the best ammo for them as well. Plus weight if in hardcoe would be a factor, you don't want excessive amounts of ammo holding you down (in normal mode, the grenade machine guns are a good choice with the service rifles as backup to them)

If you want a full auto weapon your best choice would be the .45 Sub Machine Gun, fully modded. Sure, you have to go to Zion to get it, but it works pretty well. The ammo can be a tad bit expensive though. I would advise to stay away from Fully Automatic weapons though, you will burn through your ammo fast.

My favorite is the assault carbine with the mods. Pretty high dps, but you burn through ammo fast, so make sure you have lots of it.