Edit: this post is about being able to use the weapon customization system to change what types of clips, magazines, and belts a weapon can feed from and to extend how large of an en-bloc clip or how many shells can fit into it’s internal magazine. It has nothing to do with bump stocks, “full auto receivers” or imaginary types of mosins and very little to do with fundamentally changing what the weapon customization system is.
Perhaps there should be a type of attachment for magazine adapters and extensions of integral magazines. To be more clear, I’ll use some real world examples below:
The Beretta Cx4 storm and Mx4 storm can be fitted with magazine adapters (both production and third-party) to feed from a variety of different magazines.
5.56 variants of the Galil can be fitted with an adaptor to feed from STANAG magazines.
Berthiers can be converted from only being able to feed out of 3 shot en-bloc clips to being able to feed out of either 3 or 5 shot clips.
M1 Garands have been converted from feeding from 8 shot en-bloc clips to feeding from detachable box magazine. (No I’m not talking about the m14 or m15, which have about as much commonality with the m1 as an AKM does with an AK-74.)
DP machine guns can be adapted from magazine feeding to belt feeding.
MG-34 machine guns can be adapted from belt feeding to magazine feeding.
Various lengths of magazine tubes and magazine tube extensions can be installed onto many firearms (mostly pump-action or lever-action shotguns and rifles)
Although technically some of these examples are changes to the magazine itself, or even conversions of what are technically magazines into magazine wells, these magazines -in terms of game design, atleast, are more equivalent to magazine wells with the en-bloc clips that they hold being more equivalent to the detachable box magazines held in magazine wells.
Once again, I’m not sure what a good name for this type of attachment would be.