I'm always thinking about mechanical things, so yesterday I was thinking "why not build an AR upper that would totally convert the weapon into something else all together?"

OK, I know they make .50 BMG uppers, but I'm talking about something more high speed-low drag, like a STEN gun upper, that takse STEN mags from the side and is made from a tube like a STEN, instead of with AR HGs, or it could take AK mags from the top ala Bren, ala Stoner 63.

How about an upper that looks like a Bren, or 1919 or something, why does everyone just keep playing with the few variants we allready have? And then trying to make the mag well work?

Somebody has already converted a sten, IIRC.  I saw a scanned magazine article about it.  There are belt-fed .22lr (1919ish) uppers as well.  As for why people stick with the basic AR upper, it is modular, parts are readily available, and parts are cheap.

IIRC if you make the upper house the ammuntion feeding then IT becomes a firearm - thus losing the primary advantage of the AR-15 (where you only have to go to the FFL for the lower).  Case in point the 8mm belt fed uppers, and the soon to be relased 5.8x28 upper that takes P90 mags (empties drop through the magazine well of the lower).

I believe there is an upper for 5.7mm that uses the P-90 magazines, laid flat on top.

That sounds a little like the kind of approach you are thinking about.

I was thinkin about a weed eater leaf blower combo.

MGI
Modular Lower Receiver


www.mgimilitary.com/

Forest, 8mm belt fed uppers!?!? Where??

Quoted: I believe there is an upper for 5.7mm that uses the P-90 magazines, laid flat on top. That sounds a little like the kind of approach you are thinking about.

See here:
www.ar15.com/forums/topic.html?b=3&f=15&t=275658&page=1

Quoted: Forest, 8mm belt fed uppers!?!? Where??

Google 8mm belt fed upper.  It's the first hit.