Just found a 20 round mag from my old ex service SLR (FAL L1A1) which I had totally forgotten about.
And guess what ?! I think with a bit of filing it could readily be made to fit a 308 Winchester 100.
Top round lifts to a similar position and angle, the magazine cross section is the same width and length. Different mounting and locking system though, but no welding required, only filing.
I'm not inclined to do the modification as plenty of buddies will jump through hoops to get their hands on an original un-shortened mag.
Also the FAL weighed 10lb and with another few pounds rattling around underneath it just brings back bad memories.