Keeping track of ammunition in a multi-caliber environment
Multi-caliber firearms are here. Some, like Faxon ARAK21 (above) have quick-change barrels in .223, 300Blk and 7.62×39. Others, like nearly every AR out there, have swappable uppers in an even wider variety of calibers. STANAG magazines, compatible or not, and with any caliber, fit all lowers.
Loading 458SOCOM into a 223 rifle isn’t a problem, the round just won’t chamber. 223 in a 300 Blackout gun would chamber, but the brass would fireform to the chamber and the sub-caliber bullet would rattle down the bore. 300 Blackout in 223 or 5.45mm upper might be forced to chamber with either a swaged bullet stick down the bore or a blown-up upper. Imagine other permutations at leisure. Lesser annoyances, like coming to the range with a 6.8mm magazine and a 223 upper hardly count.
Faced with this potential issue, Faxon came up with handy rubber bands for the three calibers their rifles chamber. These work very well for range use, but can move around the magazine body when placed into pouches.
ETS Group came up with an alternate solution: color-coded followers and floorplate markings. These take only a few seconds to swap: the magazine spring snaps into pre-set recesses on them.
You can get followers and floor buttons in several colors. While a streamlined solution, it differentiates between calibers without telling which magazines hold which calibers.
Another option is to write calibers on paper, then attach the notes with transparent office tape. The solution is relatively durable and water-resistant. None of these approaches work in the dark. It’s possible to use different shape floorplates for each caliber, so that they may be felt even in the dark, with magazines inserted into pouches, and with gloves on. Looking at the loaded ammunition is also an option, just slower in situations when time may be of essence.