The only way I could see this MP7/Leg rig set up work (with a belt), would be to add a sling from your MP7 to your shoulder (across your chest). This way the leg rig keeps your MP7 at your side but the sling supports the weight. Like someone said before, keep the holster very high, near the hand pocket on your pants. If you put the holster lower, it will flap around when you walk, run, and will hit the dirt when you crawl. Same with the sling, if the holster is to low the sling wont be able to keep the MP7 in its place and will get in your way.
If it was me, with something heavier than a pistol, it would be on my back near the shoulder in a holster (like a shotgun) or in the small of my back in a holster/sling combo. My suggestion, try different places at home, if it's not confortable at home, it wont work in the field. Once you found something confortable, try crawling, jumping, rollling on the ground. Next, try drawing it, if its not fast and easy, it wont work in the field.
Have fun
