theres no reason one couldnt turn 2 10-round magazines into 1 ~17-round magazine.
you can chop the feedlips off one mag. and chop the bottom off the other.......and weld the 2 together.
as for the spring, you can either braze the 2 springs together, or you can try and find a replacement spring long enough.
reliability will be as good as your fabrication skills.
although personally, i would just tape 2 mags together and simply flip them when i run out.
If they pass a law banning manufacture of high cap mags and you build your own probably looking at ten years in the federal penitentiary. Prison don't sound worth it guys either get your hi caps now or learn to do fast mag swaps. If you need more than 10 rounds to get out of a situation your either a bad shot or in deeper than a couple more bullets in the magazine will fix. Get real, real life is seldom like the movies.
2 things:
1) you are assuming only good guys have a want for high cap mags.......they are favorable by criminals for the same reason 'good guys' like them.......and an additional 10 years for an illegal mag is nothing when you are facing life for murder.
2) your statement
"If you need more than 10 rounds to get out of a situation your either a bad shot or in deeper than a couple more bullets in the magazine will fix." is woefully ignorant and is a statement right out of the mouth of Sarah Brady.
there are many instances where more than 10 rounds is ideal....unfortunatelly not everyone is a master marksman, so should they be denied the right to defend themselves if they happen to miss?
what about if there are multiple attackers?.....say you have 3-5 attackers, thats 2-3 shots each, better hope you dont miss, and you better hope 1 round does the job...or else you are screwed.
and how about the Korean merchants who defended themselves durring the LA riots?.....should they have been limited to 10 rounds when they were facing hundreds of violent looters?...hell, even if they were expert marksmen, 10 rounds wouldnt have been enough.
you are right, life is not like the movies....in the fact that the good guy doesnt always come out on top w/out a scratch.....perhaps its time for you to "get real"