[ValaskaAirsoft]

Bolt action and SMG's have been a thing done in the real world, heck sometimes soldiers would sling a rifle on their back and an assault rifle even. Secure both weapons firm and tight, get them real snug and you'll be perfectly fine.

Practice and train slinging and drawing your combo, accept the fact from time to time you will need to actually put the Mossin on the ground and you're going to be fine. An MP40 isn't going to weigh you down too much, but if you are willing to break period pieces the MP7 would be perfect for an SMG that would keep you safe when engagement ranges get too close.

But yeah, wear the gear around, train with it, keep in shape. I don't understand why people act like 3-4 spare KG is going to break your back or something haha, we're Canucks we're made of sturdier stuff than that I'd hope! All kidding aside, this loadout is perfectly feasible and shouldn't get in your way as long as you can manage to stow your second weapon and carry a back holster for your Mosin.

Aside from the MP7 other possibles could be;

MP7
MP9
Kriss Vector
MAC11
UZI
ARP9 CQB
UMP (Folding Stock)
PDR

Others that are a bit larger bit might work still!

P90
EVO (CQC variant)
L85 CQC
Cybergun Taurus PT92F (Auto Pistol)
M93r (Burst Pistol)
Slingshot with a ton of BB's.

Long as you can stow the weapon securely and tight enough, grab yourself a back holster for the Mosin... you'll be good.