The 10 rounders are solid mags. I recommend buying as many as you can get your hands on. I carry with 12 rounders but if the SHTF... having 30 10 rounders laying around ain't going to be a bad thing. They are reliable, tough, and steel.
I'm sure you could convert one... but not in any way that I would trust it to carry so what would be the point. You couldn't get it to hold 12 rounds unless you added some body to it. They have a gay little plug in them that you have to have in order to attach the base plate... so you can't just slide on a 12 rounder base plate. I think Big Bore made some 20 rounders by welding to 10 rounders together... again... not a carry item... just fun. I've got like 20 12 rounders but I have a box full of 10 rounders. They're solid and cheap.
What I would suggest is figure how many mags you intend to carry on a regular basis and triple it. Buy those in 12 rounders for carry, rotation, and to have a few extras.... and then buy a boat load of 10 rounders. If you need more mags than 10 in a given scenario... you aren't going to care if they hold 12 or 10.
The ones I welded together only held 17, but you are correct, even though they have always worked I would not trust my life to one; not on a dare.
People ask this question it seems once a week around here and there are no really good ways to convert the 10 round mags. Some people have taken out the inner plug and glued the base plate on, but you cannot take them apart to clean any more and what if the glue fails and dumps your load of ammo on the ground in front of you. How embarrassing to prematurely dump your load right there in front of God and everyone!
One should either live with the ten round mags which are dirt cheap as mentioned, or quit being a penny pincher and buy the full caps. Also as said, with ever full cap HK mag CDNN throws in a free 10 round mag. That should soften the blow.
I use the 12 round mags for carry and train with the 10s...doesn't bother me if I break a baseplate doing a reload or malfunction drill and we don't have any courses of fire that require more than 10 rounds in the mag anyway.
As soon as the boss signs my HK45 authorization I'll be carrying 10 round mags at work anyway.
I agree with everyone else about CDNN...freebie 10 rounders is a good thing.
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