Re: Question for special forces(and anyone else who ca
Subguns are still being used for hostage rescue, urban missions outside of designated war zones, covert missions, maritime operations in very close quarters where ricochets are a large threat (oil rigs, ships, etc), and other situations.
It's on a per-unit basis. There is likely no S.O.P. that dictates when a subgun must be used. Each unit will use them according to their own needs, or their mission demands. Additionally, certain units use them more than others. SEALs are more likely to use subguns than Para Rescue or SF (aka Green Berets), since they work more in open environments like jungles, fields, mountains, etc. But again, it's up to the unit.
Subguns of the future are basically here. Compact weapons that have the size, recoil and magazine capacity of a pistol round, but the power of a rifle round. The FN P90 was a big asset to this for European SpecOps. The HK MP7A1 is another option that has emerged. These are weapons that can penetrate body armor at up to 200m, but have the handling and feel benefits of a pistol round.
There is a unit out there that does extensive undercover/covert recon and surveillance in urban areas. The operators of said unit regularly carry hidden pistols and HK MP7A1s or MP5K-PDWs.
As has been pointed out, rifles like the Mk18 and other SBR variants are effectively usurping the subgun roles. Still, there is a place for the subgun. Such is why SWAT teams still employ subguns. This includes elite teams like the FBI HRT and LAPD SWAT.