Caliber appeared in the first game and would in my opinion make a good addition to the roster, boasting higher damage than 5.56, but less accuracy and more recoil.
To compliment the new caliber I suggest 2 new rifles:
AKM - An iconic rifle, no zombie game is really complete without one, would be full auto with a 30rd mag, heavier than the AR15 and with more recoil, but dealing more damage, would be a stepping stone between the AR and the XM7, allowing the XM7 to be a more rare and powerful rifle worthy of it's experimental and futuristic design. Would also only have access to the suppressor and maybe a flashlight for attachments.
SKS - A classic choice from the first game, the SKS could be a common low tier rifle due to it's immense popularity in the US, would make up for it's fixed 10rd mag and lack of attachments by being lighter than other choices and having a unique bayonet attachment for extra melee damage.
I think these would be a great addition to the game, increasing weapon diversity, adding back a classic from NMRiH1 for OG players, and filling a more niche gap in the roster of rifles.

