I got a 500 several months ago and was going to convert it to a bullpup but it won't fit into the lower section of the kit. I have just found out that it's actually a 500A. Here's where I need an expert. Is there ANY difference between to 500 and 500A. I'm thinking maybe there's some difference in the receivers. Another question. Are the barrels interchangeable between the 500 and 500A?