If your mags won't go into your rifle with 5 rounds on a closed bolt you unfortunately have mags that were pinned to 5 instead of the usual 5.5.
You have 2 options, you can either always insert a loaded mag on an open bolt and then release the bolt without riding it home (which you should never do anyway), or you can remove the rivet and install a new rivet slightly lower on the opposite side. You will have to take the mag apart and do a bunch of measuring to the bottom of the follower with 5 rounds in the mag (no spring) and messing around to get it right but it really isn't that hard.
Don't worry about creating a prohibited device by removing the rivet. What you do in your home is your business and unless you go to the range and run 30 rounds through your rifle in front of a bunch of witnesses no one but you will even know you did it. You are repairing your magazine not altering it.
If you find that the new rivets are interfering with insertion into the magwell you can file them down till they don't rub but make sure you don't go too far. Don't use a dremel as it will heat up the rivet and soften the plastic. Also make sure your rivets go through far enough to block the follower but not far enough to interfere with the spring.
Good luck
