I was looking through the stickey'd thread about making 15 round AK magazines out of 30 rounders using steel magazines. I decided to go out of state and give it a try with a 30 round polymer Pro mag magazine. I used a polymer magazine because it seemed like it would be easier to do, and you can sometimes find new ones on sale for as little as $10.00 each.
It's pretty easy to do, because I've done all the figuring for you guys. The Pro mag magazine has ribs on the side. There are 13 of them if you count the one that the floor plate slides on to. That's all you need to do is cut the magazine ON the 8th rib. Picture time.
Once you've made your cut, you need to make some grooves on the sides for the floor plate to slide on to. I used the file on my leatherman to make a nice groove on each side so that I was able to slide the floor plate on. Here's the groove.
Once that's done, you just verify that the floor plate slides on nice and snug. Go easy with the filing.
I chose to cut down the spring 6 loops. The magazine seemed to function fine with the full length spring, but since you've cut off almost 3 inches from the magazine the spring doesn't need to be so long.
This is what's gonna be left over.
And this is the end result.
It's just too bad that you've got to travel out of state to do this modification. The entire process took me about half an hour and that included filing the grooves by hand as well as using a dull hack saw to cut the magazine. Using a Dremel sort of tool will likely take 10 minutes off the job.