I know Nelson has recently made the Nightraider, Fusilaut, and Augewehr able to take Ranger or Military rounds, but what I suggest is specially-made magazines for each caliber, where an Eaglefire can now use .22lr, 5.56, or (With a Barrel Attachment (So that it has drawbacks for using it), it could use a 7.62 or .50 Cal bullet instead ONLY as long as that barrel attachment is in use). This could help if you have a gun that only uses a certain ammunition (In 3.0 terms), and loads of Civvy Ammo or Ranger, maybe even Hi Cal ammo, you could use those instead at the cost of a suppressor or muzzle. I’m not sure if it could work in 3.0 but I’m fairly sure Nelson could make the Receiver and Barrel change from a gray to black or tan or something, and change the size, but I’m confident he could work it out by the time 4.0 rolls around. This would also offer more complex ballistics, as Nelson’s Trello page says he wishes to implement, but it’d also make gunplay and looting more interesting, adding even more ways for you to personalize your Freshspawn killing machine Favorite gun.
TL,DR: Guns now take magazines that look the same, but the name tells what caliber ammunition it takes. Caliber changes on a single gun require you to sacrifice using a barrel attachment to change the gun’s used ammunition for as long as you keep it using that caliber.