Got my MC1 used. Looks like it was never shot Came with the 2 regular mags and 3 Glock 43 mags: one with a Pearce finger ext (still a 6 shot) and 2 flat bottoms. Ordered 2 plus one ext. When they came I put the +1 ext on one of the flat bottoms. Filled it with seven cartridges and it would not seat. Thought maybe it was the seventh cartridge. I took that out and tried again. No luck. To make a long story short I shaved 15 thousands off the top of both ext.s and now they work fine.