I was actually thinking about this for HK rifles, I tried something with an aluminum mag, but it was way too flimsy. I'm trying to acquire some steel tubes to play with now, hopefully they'll be a bit sturdier than the aluminum mags.
To answer the 10 round question, the mag would be blocked to hold 10 rounds total, so realistically, you could prolly load 8 or 9 from the hole, and maybe the 10th through the ejection port.
"When they kick at your front door, how you gonna come? With your hands on your head or on the trigger of your gun?" - Paul Simonon