Frankly it's no surprise it ended up undergassed with that combination. That's an important thing to consider before shortening an AR barrel, and if you do, you need to be prepared to open the gas port.
You'll need to buy an assortment of wire gauge drills and open up the port in small increments until it runs right. Don't rely on someone else's guess on port size, best results will come from verifying what works on your rifle.
Changing springs, weight, etc to non-standard parts (especially for someone who doesn't know ARs well) is a band-aid fix around the gas port issue and sure to cause confusion and/or problems later.
