Semi rimmed modern revolver rounds?

I have wondered for quite a while now but never thought to post it on here. Why are modern centrefire revolver rounds that were traditionally rimmed case designs eg. .38 Special now semi-rimmed? What adavantage does it give?

Which .38 Special revolver round is now semi rimmed ?

Most modern revolver rounds have a shallow “extractor groove” above the rim, whereas older ones have the traditional straight case then rim design. Do you have any examples of modern commercial .38 Special, .357 Mag, .44 Mag etc. in you collection? They will more than likely have this case design.

Falcon–That “extractor groove” as you called it is actually a “undercut”. It has nothing to do with making the case a “Semi-Rimmed” case. It is put there as part of the design for headspacing. With the way modern cases are made, there can be a slight “hump” at the juncture of the case wall and the head. This “hump” prevents the proper seating of the head in the cylinder of a revolver. So, a slight undercut is made to remove the “hump” and thereby allowing complete seating of the head.

Thanks, I knew there would be a reason for it.


You see this undercut feature on many rifle cartridges too, for example the .303" cartridges made in the U.S. under contract for Britain. It may be easier for the case maker than achieving a neat internal corner on the cartridge but can give some problems with the extractor on some guns. I avoid these types of cases when reloading.



What those guys said.

There IS a semi-rimmed 38 Special cartridge that was loaded in the late 20th Century for use by the AMU (Army Marksmanship Unit). It has a very long and deep extractor groove very much like the 45 ACP and is easy to recognize. In theory anyway, it is supposed to function more reliably in the auto pistols, although the pistols seemed to work OK before the AMU came along.

It can be found with several different headstamps.


I recently fired a Rossi Puma, which is a reproduction of the Winchester Model 1892. It was chambered in .45 Colt which is a very non-original chambering. The ammunition was also very non-original. The cartridges were made by CBC and also Black Hills for the Cowboy Action Shooters and has flat tip cast bullets to suit the tubular magazine. I also noted the prominent groove cut above the rim, making these rounds appear semi-rimmed. In my opinion, the groove is required for cartridges used in rifles. The very narrow rim of the .45 Colt probably would not provide enough purchase for the extractor. The original reason that the semi-rimmed case was developed was to provide cleareance for rimmed cartridges stacked in a vertical magazine, so while the groove is functional for the M1892 action, it is not serving the purpose of a true semi-rimmed cartridge.
Now here is my question: These extractor cuts were originally a manufacturing short cut that has found a functional need,at least for the .45 Colt, so are the ammo manufactures stanadardizing the deminsions?


CIP and SAAMI spec the extractor grooves but how closely the manufacturers may follow them I have no idea.


I am not sure if that groove above the rim of rimmed cartridges is necessary for rifles or not, or if that anything at all to do with why it is there. It predates the popularity of .45 Colt Rifles somewhat. It certainly is not needed with original Winchester calibers, like .44-40, in Model 92 Winchesters, or any other lever-action rifle. The groove on that caliber is a fairly modern thing. I have a round headstamped REM-UMC 44-40 WIN that doesn’t have it. Not sure exactly when it was made, but it isn’t ancient. Actually, I have a full box that came out of SFPD stock and was the last ammunition, to my knowledge, that they purchased for their Colt Lightning Rifles, most of which were traded in on AR-15s, so that wasn’t ancient history since they last had them. I keep the box because I have one of the “SFP” serialized rifles from the Department. The box is the green and red box with single “Red Ball” on top, index 5744.

The original .45 Colt, especially those with ballon-head cases, had an even smaller rim than modern .45 Colt does, and it was thinner. I am sure they probably tried making rifles for that straight-sided case and between the thin rim and case walls sticking in dirty chambers, found it a poor choice for a rifle cartridge. I think it is a poor choice today as well, although it is popular with Cowboy Action Shooters. I shoot .44-40s myself, a vastly superior cartridge to the .45 Colt in a rifle.

I have a Marlin in .45 Colt because once in awhile I like to shoot my long-barrel .45 Colt SAAs in CAS just for a change, and don’t like to mix calibers. I don’t like the gun a lot (nothing wrong with a Marlin - I shot an original 94 Marlin made in 1894 in .44-40 for a year and a half in CAS and it is a cracker-jack rifle. I only stopped using it because I couldn’t find an original tang sight for it). With normal cowboy loads (I am not talking about the spit-ball gamer loads some shoot) the cases don’t obturate well and there is always a whiff of gas in the face every shot. Very annoying for use in a speed sport. The .44-40 loads well light or heavy! I looked at a case chambered in my Marlin, and can’t see where the narrow groove would have any bearing on extraction (or not extracting well). Just another view on it. Could be wrong. I know my share about guns, but an not mechanically minded nor a gunsmith.

Just as you mentioned, the .45 Colt should be a real dog in the repeater rifle. I had wondered what Rossi had done to get that old revolver dog to learn some new lever gun tricks. As soon as I saw the extractor groove and the flat tip projectiles, the light bulb turned on and I thought “so that’s how they do it…”. I shot the Rossi against my dads Model 1892 in .38 WCF. The Rossi action wasn’t nearly as silky smooth as the Winchester, but the Winchester has nearly a hundred years of experience over the Rossi. They both functioned flawlessly.
The other half of the equation is the sloppy chamber. Spent casings from the Rossi were bulged and there was black soot on the cases from the case mouth to about halfway down the case. There wasn’t any gas leakage in the Rossi, but your reports of it from your Marlin don’t surprise me a bit. I will be the first to tell you that my earlier remarks were straight off the cuff, and I didn’t take the Rossi apart to look at the extractor set-up. I was so surprised (and pleased) that the Rossi fed and extracted as well as it did that I latched on to the “extractor groove” as the most obvious reason. I still think the groove adds to the reliability of the .45 Colt in the lever action, but what I really need to do is to see how it shoots with the old style cases. If and when I get a chance to test my hypothesis, I will revive this post and let you know, but don’t hold your breath!

Curt - I have several original 92s, but what I shoot the most is the U.S. Repeating Arms full rifle version, made in Japan. Quite frankly, it is better made tha an original Winchester, and smooth as silk right out of the box. I have two identical ones - one a backup in case something breaks. After 8 years of hard use, I think I could have saved my money on the back-up business. Haven’t had a glitch. Of course, both are .44-40.

The Rossis are o.k. if one smooths them up a bit. My Marlin .45 Colt works flawlessly and is very accurate - it just has the annoying blow-by unless you load the cases pretty much to full factory strength, which is not desireable for CAS for several reasons. I get the blackened cases as you do, which is a result of poor obturation with lighter loads. That should stop with “full” loads. It does in my Marlin.

The .45 Colt is a good cartridge - simply not so on an overall basis in rifles. It loses some of its versatility there. In revolvers, I have found it to be an accurate caliber even in pretty light loads (although I always use 250 grain bullets - I don’t care for the lighter ones, especially the 165 grain). It is only the blow-by that is annoying. In full loads, it is fine in a rifle, thanks to the changes made in the cartridge case from ballon-head to solid-head, and the resultant changes in the rim shape. I am not sure that the rim shape changes were all a result of the solid-head case. I frankly don’t know enough about this caliber, or ballon-head cases, to be sure. Still, it is a revolver cartridge, and I hold by the statement that the .44-40 is superior in rifles, at any loading level.

I tried some .45 Colts without the very thin groove above the rim, older UMC rounds, and they feed flawlessly in my Marlin, and extracted and ejected perfectly. I did not fire them, however, so can’t speak to the question of extraction and ejection of a fired case. With live rounds, the groove is certainly not needed in my Marlin. I don’t have a Rossi or other form of 92 Winchester in .45 Colt to try that. All were solid-head cases, too, with the modern rim configuration (sans the groove). I didn’t have a really early .45 Colt in my duple cartridges to try.

Here is 38 AMU to which Ray is refering. Sorry, have problems removing the price label. I am a bit dense, so is this round the same as 38 Special, meaning, may one shoot it as 38 Spl?

Vlad–Does that .38 AMU box have a Load Index Number on it anyplace? If so, what is it?

There is a code “W15E” inside the box. What does it mean?

The .38 AMU Cartridge is a good enough round. Unfortunately, it was “an interesting solution to a non-existing problem.” I shot a Colt .38 AMU Kit pistol converted with a new barrel and other work to .38 Special (normal rimmed version) wadcutter by a local gunsmith, and never had problem number one with it, other than that it was a full 6" barrel, extended slide custom gun, with full-length Bomar Rib. The problem was the weight of the slide. You had to have a specific load to work the action, but no so hot it caused lead wadcutter bullets to tip. I used it about 15 years and decided to sell it, since my scores with the .45 were more consistent and it saved me lugging around a third gun and ammo to matches. I just used the .45 in cetner-fire too.

It is no wonder, though, that the .38 AMU round was pretty short-lived. It simply offered nothing over the .38 Special, in my opinion.

The 1911 pistol does have a beefy extractor that requires cases with a large extractor groove. Both the 45ACP and the 38 Super have such a groove. But, as John said, it can be made to work with rimmed cases like the the 38 Special so the AMU cartridge addressed a problem that really didn’t exist. But I suppose the AMU took the safer route. After all, one jam or mis-feed could result in a lost match.

Below is a standard 38 Special wadcutter, a 38 AMU wadcutter and a 38 Super. Both the AMU and the Super are semi-rimmed.


Vlad–That “W15E” is most likely a Lot number. It certainly is NOT a Load Index Number. Thanks for checking for me.

That rim undercut seems to appear in the .30-40 military cartridge a few years after 1900 & I’ll bet Randy could pin it down a lot more exactly than that. JG

Vlad, you might try freezing the box. After a couple hours that price sticker should just ‘pop’ off useing a knife point.