Im pretty sure the highest size in terms of cartridges for Call of Duty is the .50 BMG. The XM-25 from MW3 doesn't count since it is a singleplayer only weapon. The Halo Sniper Rifle fires a 14.5mm x 114mm APFSDS. I'm usually against video games putting in cartridges that are .50 BMG or higher since they are made weak for gameplay balance, which defeats the purpose of putting in a weapon that fires a round that large in the first place. Highest cartridge I think for a video game weapon should be .338.
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I will make exceptions for mini/gatling guns provided they have serious limitations (Ex, placed at certain points, NO move speed, must be found in the map, etc.) just because of how fun they are to use.
Auto Turret of some kind. (Taco you can pick the model )
Could be a game type as well. I think people would like it and always thought it was fun in halo as well.
maby a 50-cal. would be cool too