Magazine Capacity

[Nightcrawler]

How come some magazines hold more rounds than others, when they're about the same overall length?

For example, let's compare the CZ-97B and either the Springfield or Para Ordnance double stack 1911s (they take the same 14 round magazines).

The magazines on the P14 and the CZ-97 both fit flush, save the plastic floorplate.

The CZ-97 magazines hold 10 rounds. They're not castrated as near as I can tell. Also, the grip of the CZ-97 is longer, but it doesn't feel as wide as a P14, USP45, or other double stack .45. (I believe the length of the grip has a lot to do with the massive amounts of steel in this sturdy pistol).

The P14 has a wider grip, but it's not any longer as near as I can recall (been awhile since I've handled one). Yet it's magazine holds 14 rounds.

Another example are the 9mm vs. .40 guns. Most 9mms hold 15 rounds. The Ruger P94, for example, holds 15. The Ruger P94/.40 holds 11. The Beretta 92 holds 15, and the Beretta 96 holds eleven.

However, the Sig P226 holds 15. The 226/.40 holds 12.

So, what's the deal? Are the P14/Glock21/etc. magazines wider than the CZ-97, thus allowing more rounds for a given length? Is that how that works?

Thanks. Just curious.

 

[4v50 Gary]

Even though two guns of the same caliber have the same frame size, considering that one caliber is larger than the other, it is conceivable that the bigger bullet gun will be the lesser capacity gun. Hence, the P94 Ruger in 40 S&W (Ruger never says "40 S&W" and only calls it "40 Cal") magazine will hold fewer rounds than the P85, P89, etc. Same for the Sigs.

Same applies to the M1A1 Abrams tank with the 105 mm Nato gun and the M1A2 Abrams with the 120 mm gun. The former hold about 60 rounds of 105 and the latter can carry about 40 (correct me guys on the numbers - it been years since I've read this stuff).

 

[Nightcrawler]

I think I worded my question poorly. I understand why a doublestack .40 will hold less rounds in the same size magazine than a doublestack 9x19.

What I want to know is this: since the Sig 226 and the Beretta's 9mm magazines both hold 15 rounds, they must be about the same size.

However, the Beretta .40 holds 11. The Sig .40 holds 12.

I guess my question is, how come one doublestack .40 or .45 holds less rounds than another doublstack .40 or .45 of the same (abouts) frame size?

 

[Blackhawk]

The centerline offset of the rounds in the magazine is determined by the width. The "next" round is shoved up by the one following it. If the offset is great, the magazine won't work as well as one with less offset because it takes more spring pressure to shove the rounds up for loading. The critical point is where the top round has to contact the opposite lip to be aligned for loading. The round below it not only has to push it up against the mag side, but it also has to push it into the lips.

A short, hi cap, DS mag is theoretically more trouble prone than a longer one of the same capacity because of the centerline offset of the rounds.

Now we're at why. A manufacturer may opt for a more trouble free mag by keeping it narrow so the centerline offset isn't as great.

 

[Kobun]

The way the mag spring is made also makes a difference.
Most springs, when compressed, will stack the coils on top of each others. But some will have offsett coils that stack inside each others, and therefore will compress more.
The thickness of the follower is a part of the equation.

 

[T.Stahl]

Gary, it's 55 rounds of 105mmx617 vs 40 rounds of 120mmx570, AFAIK.