>>64106526
If there is no double stack tower, and instead the stack hits the feed lips as soon as 4 stacks zipper down into two, this would have the effect of doubling the capacity of the surefire’s tower, which is about 15 rounds.

Picrel. The Surefire is composed of 3 fundamental segments. The bottom (blue) segment is quad stack. The middle (yellow) segment is where the stacks zipper down into two, and the top (red) segment is a simple double stack tower. You can imagine cutting off the red section, doubling its width/capacity (so it’s quad stack) and adding it to the blue section. And just move the feed lips to the top of the yellow section. This keeps the mag the same length. So, +15 rounds.

With smaller case diameters, capacity would increase. For example if you moved to a 30 carbine case head (9mm vs 9.6mm for the 5.56), you’d gain ~5-6 rounds for the same mag length, and mag width would decrease by about 0.1”. Doesn’t sound like a lot but it is both a gain in capacity and reduction in magazine weight per capacity.