Compact Carbine, Marsec Lasgun & Heavy shotgun AP shells aren't yet implemented as loot, are they? First two are fully functional bottlenecks atm. Was about ask for their IDs to put on my Brainers' desks for research when I got wind to search them for myself and what do you know found em I did:
STR_COMPACT_CARBINE (unimplemented as loot as of L6, rule or save file edit required)
STR_MARSEC_LASGUN (Viper Fighter - 50/50 chance of crash/destruction, determined at 1st of month or ship spawn)
STR_SHOTGUN_PA_SLUGS_AP (!Highway House! and !Mansion Invasions!)
Put them in Hideout storage for research if you don't want to wait for a patch (under desired Hideout "items:" section in save file). Don't forget to add ": 1" for quantity, and be careful with sensitive formatting as well (best to copy/paste existing entry above or bellow and just rename it).
UPDATE
Added possible spawn locations in brackets next to IDs. Even after appropriate mission spawns the loot soldiers carry is taken for a sizable pool of possibilities so there's only a 20% chance for each item, for that one rank of soldier that receives said equipment pool. This explain why I wasn't getting the latter two well into my 3rd in-game year! Still waiting on those AP shells, fml.