personally would not buy the MSAR - the reputation of the company online is just bad. Their main customer service rep left the company a few months ago. And, it's just gone downhill since. Some people have sent in defective E4 mags and such a year ago and are still awaiting replacement. Go read the posts at the AUG forum and on AR15.com. Admittedly, there are some guys who ONLY bash the company. But if you read everything as a whole, it's still very bad...
My STG-AUG gun ran 100% with 30 round mags. But, the 42 rounders didn't work so well. If the gun does jam - it's a real pain to get it to work again. Ya gotta pull the barrel, and maybe even try to get the receiver out the stock. A jam can take 5 min to clear - which is very BAD in a self defense situation. I was always had the fear of a jam after my 2 experiences of trying to get the gun running again when those 42 rounders jammed.
I had MSAR replace the 42 rounders that gave me issues, and then I decided to sell the gun. I have real doubts as to the longevity of the company, and getting spare parts is getting to be very tough in the past 6 months or so.
After all the stuff I keep reading about them, I decided that I prefered my PS90 and bought a 2nd one.