You wanna build a carbine pistol? For outdoors? Then you have options. For indoors all your gonna do is build a good indoor gun then make it bigger with no to the gains. Here is what I am doing.
I have a glock, it's an 18c (not important, I have AEG's that shoot 30+ rps and I don't ever use full auto)
I have a stock, it's made by TacArms Irael. It fits into the opening that a lanyard plug would go in normally. This its super light, and super compact, the stock is the length of the gun, when collapsed it easily can be fit into a pouch or something when not in use.
Now on the business end, where it really matters to this build, is going to be an SAS type strike face, basically your standard potatoe masher but with some -14 mm threads on the front for a silencer and lower rail for a vert grip. Couple that with a ~230mm inner and yer pushing 400fps, some hop up mods and some .30's or .36's and a nice red dot now we are in business. We will have a light weight easy to maneuver pistol carbine. With similar shooting ability to a decently tuned gbbr. Grab some 50 round mags and have at er. Is it practical? Perhaps depending on application but you could do just as well with an m4 in CQB length or some other small aeg or an smg gbbr of some kind. My build will be pretty unique and I think that shoved in a was pack will be a good tertiary when deployed in milsims incase shtf then you would have a Decent weapon in a compact space easy to deploy.