Please ref general discussion thread: https://forums.bohemia.net/forums/topic/239401-hold-reload-key-mousewheel-to-select-magazine-underbarell-gp-type-to-load/

Since the scroll wheel menu is gone, there is no way to quickly load the desired magazine / unrebarell GP without going into inventory.

Simple fix suggestion hold reload Key + Mousewheel to scroll through available ammo types, change mag / unload and reload desired ammo type on release of the reload key

Please excuse the poor paint editing job, but should get the concept across

Thank you for reading!