STI mag capacity differences

[kcobean]

Why is it that I have one .40 mag with a Gram's spring and follower that will take 20 rounds and still seat easily, and 5 mags that have stock follower/spring that will only take 17 and still seat without pounding them into the gun? The Gram's kit is only a +1 kit, right? So where are the other 2 rounds coming from?

I've also noticed that on the 5 mags that only take 17 currently, they are VERY hard to load (I can't get the last couple of rounds in without a loader), whereas the 20 rounder I can load by hand no problem.

The answer, I suspect, is that the 5 mags are out too narrow and are not allowing the ammo stack to sit properly in the mag, so as the the spring tension goes up, so does the sidewall pressure of the stack trying to settle properly into to single columns of rounds side by side. But because the mags are too narrow, the stack cannot settle out. Instead in binds on the mag walls more and more.

So....when I get around to replacing these mags, is there a way to ensure that I get properly sized mags that will go to 20 reloadable with a Gram's kit or is it the luck of the draw?

I'm going to put a micrometer on them tonight to confirm my suspicion, but this isn't the first time I've seen this (hello, Caspian).

Kelly

[Pat Miles]

The answer, I suspect, is that the 5 mags are out too narrow and are not allowing the ammo stack to sit properly in the mag, so as the the spring tension goes up, so does the sidewall pressure of the stack trying to settle properly into to single columns of rounds side by side. But because the mags are too narrow, the stack cannot settle out. Instead in binds on the mag walls more and more.

Kelly

Actually it is just the opposite, the magazines are too wide. They are allowing the ammo to get close to being side by side and jamming against the sides of the tube instead of laying in a staggered position that loads and rises smoothly.

[Sarge]

I think part of the capacity puzzle is the springs as well. They collapse within themselves as opposed to just stacking coils on top of each other?

[yoshidaex]

Have you confirmed the capacity of those questionable tubes with the Grams follower/spring kit from your 20 rounder?

[kcobean]

The answer, I suspect, is that the 5 mags are out too narrow and are not allowing the ammo stack to sit properly in the mag, so as the the spring tension goes up, so does the sidewall pressure of the stack trying to settle properly into to single columns of rounds side by side. But because the mags are too narrow, the stack cannot settle out. Instead in binds on the mag walls more and more.

Kelly

Actually it is just the opposite, the magazines are too wide. They are allowing the ammo to get close to being side by side and jamming against the sides of the tube instead of laying in a staggered position that loads and rises smoothly.

The micrometer will tell. It is standard practice with the Caspian .40 mags to put them in a vice and squeeze them front-to-rear to "bulge" the sides out a few thousandths to solve this exact problem. I'll post my findings tonight.

[andy27al]

Springs and spring tension play the largest role in capacity

[Dr Mitch]

STI will fix any defective mags. Just FYI. Don't assume you have to go through the expense of buying a bunch of mags and tossing out the old ones.

[EkuJustice]

All mine are 20+1 with grams followers and springs and Dawson +1 base pads(non SNL) fit the gauge with no filing which is why i didn't want the SNL. With the factory base pad and the grams spring and follower I was getting 19 rounds. In 9mm it's 23 with the same Dawson base pad and grams followers andsprings, 22 with the grams spring and follower and factory base pad and 21 for the factory. None of my tubes are tuned