That's the problem. The answer is both. For NV I'd probably go with a PVS-14 of some sort. Unfilmed white phosphor Gen 3. For therm, something like an AGM Rattler TS35-640. Many years ago I did some exploring out on the Bay at night with an older green PVS and had a blast. I also goofed...
No problem on the length since the 91/93 receivers are longer than MP5 receivers. Using a longer side folding UMP stock puts it right at 29" and a hair. An A2 and A3 stock puts it at 28" unless I wanted to weld on the suppressor mount. Or I could make it a pistol and only use my sear with a...