A while back I was bemoaning the difference between loading a M12 compared to a '97.
Loading the '97, the shells stay flush with the end of the mag tube. With the M12, they pop back under the front of the lifter. That makes it a little awkward to push in the next round.
I'm going into a three gun match soon and I really wanted to use one of my M12's as I don't want to pound my old '97 with slug loads. So, I started playing with dummy shells and discovered the secret .....
Do not insert a shell all the way into the mag tube, making sure that the lifter holds the shell in place. This is accomplished by allowing the lifter to centre on the base of the shell, where it will happily stay, held in place by pressure from the mag tube spring. This allows you to insert another shell, doing the same thing, until your mag is loaded.
You have to do this dextrously or the last shell will pop under the lifter as per the design and now you have to be careful when you push down the lifter that the last shell inserted does not pop out. Takes practice.
Now I can use my M12's 6+1 rd capacity to the fullest.