So yeah T6 is spot on, but if your pistol isn't able to shoot far enough(which is kinda silly) then it's an issue with the gun itself. You really only have to cover your MED distance with the backup, IE sub 100 foot. Most anything serviceable should be able to be accurate at 100 foot distances.
My own opinion, my favorite backup is a CYMA G18c AEP. It's simple, it works at any temp without issues, and it's got full auto like an SMG but fits in a typical pistol holster. In warm weather I generally will swap over to one of my KSC/KWA G18c guns since it uses the same holster etc. But I only need it for close up work, so it's sort of a non issue. Also one of the reasons I like the AEP is the low velocity because I can shoot someone nearly point blank and not hurt them. When I'm running my sniper rifle say in my ghillie there are enough times where someone comes upon me and doesn't know I'm there at all but ends up with a very close shot. So in those cases I'll either let them go or take them out with a handgun, and that AEP is a champ for that type of work.
Luke