I have a Century (yeah,yeah stop laughing!)R1A1 with an Inch pattern receiver.Everything I've read indicates that Inch rifles will work "reasonably well" with metric mags. This is true as long as I apply rearward pressure on the front of the mag as the rifle cycles. My only metric mags are somewhat worn blued steel of unknown origin.Are some metric mags more "Inch Compatible" than others? With Inch mags,the rifle is 100% reliable and around 2 MOA with Port surplus.

I have a Century L1A1 (with no shame) and it took some "cleaning-up" of the milling.  The barrel had some problems too, but it was all fixable.  It is really a great rifle, and you can't beat the price.  I picked up the L1A1 and a CETME for the price of a higher priced DSA (which are really nice don't get me wrong) and with a little tweaking they are reliable and fun to shoot at the range.

I hope somebody responds with a "these mags work and these don't because..." BUT I never have had a good answer.  I really like the metric mags because they are cheaper.  Most work really well.

My (novice and not a gunsmith) advise would be to try a newer mag and see if it works.  The rear lug-tab on your worn magazine may be not holding it up high enough.  If you put the mag in and the bolt head is not sliding into the mag low enough to strip a round off on a new mag, the mag catch may need work to hold the back up higher in the receiver.  So, new mag lever/catch?...talk to a gunsmith?

I wish I could help more.  Good luck!