The b-square base has a slight curve in the side of the base where you'd load the gun. It's got 3 or 4 slots on the tang area and 3 or 4 slots on the front ring. I don't have one on hand, but I'd guess that it gives you around an inch, maybe more, of movement back and forth for the scope, that you wouldn't get with regular bases, or extension rings.
It also allows you the space the rings to give the most support to the tube. Sometimes, it will even allow you to use an extra ring if you want to. I had one a Savage 16 in .308, and I had a Simmons Whitetail Expedition scope on it, a 3-9x42, which was a bit longer in the tube than average for a scope in that range. I was able to use three Weaver Quad rings on it with that base. Needless to say, that scope never moved at all. It was a slightly heavy scope on a light rifle, so I figured that I'd nothing to lose with using the extra ring.
I haven't had any problems with loading the magazine, although I will admit that I have slightly smaller hands than the average guy. I have put the bases on guns for other guys as well as mine, and they've never complained about loading problems.