I cannot seem to find any information about this anywhere...
About eight years ago, I picked up a used 20 rd Pro Mag out of a parts bin at a LGS for a few bucks. I didn't notice at the time, but one of the feed lips was missing about a third of itself, as a result...it was rather picky about feeding. I didn't really think much of it...but I did notice the body of the magazine had lips for both 10 and 5 round options...fast forward to today, I decided to perhaps reduce the spring tension on the mag, and cut it down to the 10 round position. I cut the spring down a few coils less than 50%, ultimately ending up with a one coil less than 50%, yet it will not allow for anything but exactly 10 rounds, with no give to put it into a drop free modded gun.
Is this the way old Pro Mags were? Was it a safety thing where you HAD to install and chamber, or leave the bolt open?
I suppose it doesn't matter much to me, I have a detachable I can use from a bench rest, and load with stripper clips. I cut the thing pretty much exactly as a ten round mag should have been from the factory, maybe I should still remove more coils? If anyone has a PM 10 rounder, I would be curious how many coils are on the spring.