Quoted:
Where can you get these magazines, I looked on the bushmaster site.
Moreover, let me get this straight, this upper will plug-n-play on my .223 lower? No special bufferspring etc? Also, includes bolt/carrier/charging handle?
Magazines: Use the CProducts ones. They're the best in the 7.62x39 caliber. I tried and failed for years to fine magazines that were remotely reliable in my 7.62x39 AR15, then they come out with these last year or thereabouts. They're awesome and WORK! CDNN has them or you can just order them direct from C Products.
Link!
http://www.cproductsllc.com/shop/index.php?cPath=22_36
do I have to modify it to use steel case ammo?
You shoudn't. I've run steel and brass through mine without a hiccup. Granted mine is the 20" from a different manufacturer, but if a DPMS will eat it, a Bushy shuldn't have an issue.
Yes the upper will plug and play on your standard .223 lower receiver, no hassle or screw around. Push pins in, insert magazine, pull charging handle, rock and roll. There could be slight differences between receiver manufacturers though. Example: My DPMS 7.62x39 upper receiver will not fit on my RR lower, but vice versa works fine. I don't know if there could be any conflicts regarding Bushmaster vs. other manufacturers.
I always laugh at people that are here on AR15.com who forget that the AR15 magazine well is quite a bit longer than the rounds you're feeding it, unless you're throwing those 88 grainer VLD bullets that you've got to single load anyway. 7.62x39 will happily fit into not only the magazine, but your magwell too.
My only beef with this upper is that I absolutely hate the M4 profile barrel. What's the step for? I don't see too many people outside the military running a grenade launcher on an M4. What's wrong with either just going pencil barrel or CAR barrel?