Why is the 32 acp semi-rimmed?

The .25, 32 and 38 ACP are semi-rimmed because JMB initially didn't like the idea of heaspacing off the case mouth.

He later warmed up to it with .380 ACP, after rimless was forced on him by the Army with the .45 ACP.

Rim lock in pistols is a very uncommon thing, when you look at how cartridges sit in most pistol magazines, especially those with grip magazines, you'll see why.

.32 ACP isn't very popular today because you can make a .380, or even a 9mm almost as small, and get away from the anemic terminal ballistics of .32 ACP.

John Brownings design preference. He simply did not believe heaspacing on the case mouth was the best way, so - it headspaces on the rim.

Tough to know the thinking of designers and manufacturers 121 years ago.
I am in the "Mr Browning thought that was the way to do it." camp.
1899 .32 semirim
1900 .38 semirim
1903 9mm BL semirim
1905 .25 semirim

1905 .45 rimless
1908 .380 rimless

Looks like 1905 was the cusp when Mr Browning and Colt caught up with the 9mm Luger from 1902-1903.
But he had a head start. There was a good deal of development work at DWM before the 9mm P was finalized.

It wasn't with either of mine, Browning or CZ.
Neither is .32 in Keltec or Colt.
But then I stick to FMJ which does not leave much room for fore and aft movement to get into "rimlock."

32 ACP (aka 7.65mm Browning) was a very early automatic pistol cartridge. I think it went into production in 1899 in Europe. It probably took until 1903 in the US, because there were no guns made for it here until then. At that time, there was no general agreement about how to headspace cartridges in magazine-fed guns. Magazine fed rifles mostly used the shoulder of a bottlenecked case, although some still had rims as well (e.g., 303 British), and a lot of early pistols followed suit: 7.63 Borchardt. 7.63 Mauser, 7.65 Luger, etc., are all bottlenecked.

Semi-rims allowed for a simpler cartridge case, with a larger bullet for a given case diameter. They also simplified firing chamber machining, I guess, and did not require chamber machining as precise as that needed for cartridges that chambered on the case mouth. It may not have been a great system - 38 Super was dogged for decades with reputation for poor accuracy that some have blamed on chambering on the semi-rim - but it was practical, convenient for manufacturing, and it worked. Those things are very typical of John Browning's designs, I think.

Winchester's first semi-auto rifle cartridges were straight cased (or slightly tapered) and semi-rimmed, so Browning was not alone in going down that road. Webley's .455 automatic pistol cartridge was also semi-rimmed.

BTW, I have no idea what pistol cartridge was the first to headspace on the case mouth. The first one I can think of is 9mm Luger, which dates to about 1908. That is also when 380 ACP came out, so it was probably not a new idea that year.

PS - I should have mentioned that, as I understand it, modern 38 Super pistol headspace on the case mouth, although the round is still semi-rimmed. I don't know how many years that goes back.

lysanderxiii said:

Rim lock in pistols is a very uncommon thing, when you look at how cartridges sit in most pistol magazines, especially those with grip magazines, you'll see why.

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The stagger protects from rimlock, but only if the ammunition size is consistent with the size of the magazine. It is something that many owners of .32 ACP pistols found out once hollowpoints became available in that caliber.

Monac said:

BTW, I have no idea what pistol cartridge was the first to headspace on the case mouth. The first one I can think of is 9mm Luger, which dates to about 1908. That is also when 380 ACP came out, so it was probably not a new idea that year.

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Maybe 7.63 Manlicher? It came out in 1901.
Another candidate is 8mm Steyr, also from 1901, although it didn't become popular until Roth-Steyr 1907 was introduced.

The 7.63 Mauser cartridge dates to 1896.

Whatever the original intent, users can be inventive.
I came across a Clerke First revolver, cheap zinc frame revolver for .32 S&W cartridge.
The owner had fired semi-rimmed .32 ACP in it, higher pressure than .32 S&W.
The cartridge headstamp was impressed into the zinc frame and was readable.

Remember too, in the late 1890's when these 1st gen semiauto cartridges were being developed, the machines and manufacturing techniques had not been fully fleshed out, so the quality and consistency of brass cased ammunition wasn't stellar. A rimmed or semi rimmed cartridge is less susceptible to variances in tolerance than a true rimless round that headspaces off the mouth. So at the time, I'd imagine that the semi rimmed chamberings were more reliable than their rimless counterparts even with the occasional rimlock issue. Even 9mm, being a very early rimless round, needed a tapered case (aiding in extraction) to run reliability in the Luger for which it was developed. By the time 380, and 45 rolled out, JMB was able to get straight walled, rimless rounds to run properly

Interesting discussion so far. I have one tidbit to add, and please note I don't remember where, how or when I picked this up, but I heard or read once that the state of early smokeless powder technology in the U.S. is part of the reason JMB included the semi-rim on the early cartridge designs. The available powders circa 1900 sometimes required a strong roll-crimp on the cartridge to get a good burn. Obviously a roll-crimp would interfere with using the case-mouth to reliably headspace the cartridges. By the time the .380 and .45 ACP cartridges came along, U.S. powder technology had caught up to the latest innovations from Europe.

I really wish I could remember the source of this information, as I'm sure it would be interesting to go back through to see what else could be gleaned from it. Lately I've been bored of modern stuff and have been having a great time looking at and learning more about early-20th century automatic pistol development.

Monac said:

PS - I should have mentioned that, as I understand it, modern 38 Super pistol headspace on the case mouth, although the round is still semi-rimmed. I don't know how many years that goes back.

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I believe 38 Super barrels that headspaced on the case mouth began to become popular in the the late 1980's or early 1990's. It coincided with the 38 Super's rise in popularity in many of the action shooting games.

So many things happening at once, back in that day, as well noted above.

The jury was still out on whether to use rebated heel bullets, or case ID diameter bullets.
A handy middle ground is to use a semi-rim.
Which also is more tolerant of case wall irregularities--drawn gilding metal cases were also very new, too.

Also, then as now, the "gun public" could be very conservative and not welcome to "new" things. Rims were known and reliable. Rimless cases were new and no one knew if they'd work. Which makes SR a good "sales" option, too.

Which is also good if you are trying to convince hide-bound military types, too.

I am not sure if it was browning or the engineers at FN that specified a rimmed case. anyone here know?

barnetmill said:

I am not sure if it was browning or the engineers at FN that specified a rimmed case. anyone here know?

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It was Browning. He had a completed prototype pistol with ammunition when he met with a representative of FN who was visiting the United States. Browning may have worked with Winchester on the ammunition; at least, he had a history with wthem, and not with UMC, the other really big cartridge manufacturer. The FN rep was able to take the prototype to Belgium, and it greatly impressed the FN executives by firing something like 500 rounds without any problems.

Browning's previous automatic pistol cartridge pistol design, the 38 ACP also had a semi-rim, and so did the subsequent 9mm Browning Long and 25 ACP. 380 ACP and 45 ACP did not, but I think 380 couldn't, if it was going to fit the Colt 1903 pistol, and 45 ACP...did the Army design that?

There were a couple of quasi-Browning cartridges that failed to launch, like 9.8mm Colt and 9.65mm Grand Browning, but I don't know anything about them off the top of my head.

CapnMac said:

The jury was still out on whether to use rebated heel bullets, or case ID diameter bullets.

That was settled in the 1870s, with .45 Colt, .44 Russian, and .38 S&W. Centerfires with heel bullets just carried on to feed the old guns.

Which also is more tolerant of case wall irregularities--drawn gilding metal cases were also very new, too.

Gilding metal cases? Drawn brass cases came out in the 1880s; UMC had the S H head stamp for Solid Head, what we now call balloon head.

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Monac said:

Browning's previous automatic pistol cartridge pistol design, the 38 ACP also had a semi-rim,

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Previous to what? The 1899 .32 hit the market before the 1900 .38 but they had to have been parallel developments with negotiations between Browning, Colt, FN, and the US and Belgian armies, not to mention travel time by rail and ship.

It has always been my impression that Colt began work on the 38 ACP Model 1900 before FN got going on the 32 caliber Model 1899, with the delay in Colt getting the Model 1900 on the market being due to the US Army. I have not checked up on that, so I could easily be wrong. I will see what I can find out.

bersaguy said:

Remember too, in the late 1890's when these 1st gen semiauto cartridges were being developed, the machines and manufacturing techniques had not been fully fleshed out, so the quality and consistency of brass cased ammunition wasn't stellar. A rimmed or semi rimmed cartridge is less susceptible to variances in tolerance than a true rimless round that headspaces off the mouth. So at the time, I'd imagine that the semi rimmed chamberings were more reliable than their rimless counterparts even with the occasional rimlock issue. Even 9mm, being a very early rimless round, needed a tapered case (aiding in extraction) to run reliability in the Luger for which it was developed. By the time 380, and 45 rolled out, JMB was able to get straight walled, rimless rounds to run properly

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Exactly correct. When 8mm mauser was introduced in 1888 as a rimless round, the designers could already appreciate the technical superiority of the rimless design. But it was a gamble and a leap of faith in the newfound German manufacturing prowess. The Brits in the same year, and the Americans a few years later, opted for the rimmed .303 and .30-40 Krag respectively, because exact cartridge manufacturing was still in its relative infancy, and the rimmed cases offered better practical reliability, their outdated design (even at that time) notwithstanding.