I have a Bushmaster 7.62x39 and have had it for about 1 1/2 years now and absolutely love it. It has performed flawlessly since the day i bought it and don't let people scare ya by saying they always jam and don't function. I have about every brand of magazine thats available for them and id say that the ASC is probably the best built and most reliable magazine besides the factory Bushmaster magazine, which is probably the best.
You also have the option of using a standard .223 magazine which will work fine with 7 rounds or less. Also can use the 6.5 Grendal magazine, but im sure how well they work as i haven't used or tried one. The only magazine i have had trouble with once in a great while is the C-Products.
Out of the box, mine wont shoot the cheap steel cased ammo cause the primer is to hard ignite it, but if thats what you want to shoot there are enhanced firing pins you can get to shoot them.
The lower is interchangable with any other ar15 caliber small than a 308. I just bought a .223 upper and just swaped them back and forth for awhile till i got a complete .223 ar.
I have tons of experience with this rifle and reload for it and can answer any questions or concerns you might have regarding this caliber ar15. Hope this helps you out some