1) Is this firearm available to civilians with the short barrel?
2) I read that it can only hold 30 rounds - not 50. Is this true? Is the magazine smaller in size?
3) What types of accessories are compatible with the accessory mount? Is it a universal mount?
Yes, great job - the conversion to make the mags from 30 to 50 takes less than 30 seconds, and the new plate is like $5 or so from Magazine Parts. I think there are a few other sources too - but that is what I used.