As I have discovered AMC is "Armed Merchant Cruiser". It's a rather big and bulky, and heavy bugger and well armed. Look it up in the museum.
As Kylaina has pointed out, and I also concurr that they can be found mostly in the southern end of the patrol area just north of Scotland, and if one ventures in closer to Lock Ewe even more so. The spawn area is very near that base.
I concurr on the area because the only one I found and sank was just north of Scotland. I nearly didn't get him due to the fog and heavy seas at the time.
I was in this mission before patch 1.2, and the mission description didn't say AMC, but just said sink 5 ships, thats all.
After the patch it now reads "Sink 5 AMC". Heh, had it said this before I wouldn't have wasted torps earlier in the mission.
Im on my way back down to this area, and hope to wrap up this part of the mission soon.
It's this one. :
