"Stens have the magwell welded to the barrel trunnion IIRC."
No, the magwell of the sten is not welded to the receiver or barrel trunnion. It rotates around the reciever and acts as a dust cover for the ejection port.
To me, if you made an "AK" that took an AR magazine, then it wouldn't be an AK. The foundation of the gun, the receiver, would be pretty highly modified from that of an AK.
A guy I work with has an Arsenal AK chambered in .223. It seemed like a great gun. It took some kind of proprietary magazine which is the reason I lost interest in buying one. I personally don't see any good reason for buying an AK in .223. Why would you ?
I have heard for years about how M16 magazines are disposible. I have heard that the weakest part of the M16 weapons system is the magazine. I have heard many interent gun gurus talk about how weak the M16 magazine is. But, I have never known anyone that ever damaged one. I have in excess of 70 AR15 magazines: USGI, Thermold, SA80 mags, Orolites and every single one of them works fine and I never had any issues with them at all. I have served in the peace time US Army, stateside as well as 8 years in the National Guards of two different states and never encounted anyone that damaged a magazine. I am sure it has happened, no question that it has happend. I am sure that someone will feel obligated to tell me a story about how it happened to someone they know: but I would say it is extremly rare. And, telling a story about it doesn't really tell us much unless the person had several different magazines subjected to the same treatment and one of them was uneffected while one or more of the others was damaged.
I PERSONALLY put this whole thing down as a myth. You can choose to believe what you will.