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Old January 29, 2013, 01:35 PM   #6
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Join Date: October 20, 2007
Location: Richardson, TX
Posts: 7,269
It's not a dumb question at all. Since I like explaining things thoroughly, let me try to address most of the major issues.

There are two primary issues that must be addressed when using a rimless auto pistol cartridge in a revolver: headspace and ejection.

Since the OP's post count is low, I'd like to start with a definition. Headspace is most simply defined as the clearance between the breech face and the cartridge case head. Excessive headspace may cause the case to rupture upon firing, but some headspace is always necessary for reliable functioning; in the case of a revolver, the clearance allows the cylinder to rotate freely, particularly once the cylinder begins to heat up and expand from sustained firing.

In most 9mm and .45ACP revolvers, the headspace is determined the same way as with an equivalent automatic: a shoulder is machined inside the chamber, and the case mouth sits on the shoulder. This is why single-action "convertible" multi-caliber revolvers like the Blackhawk use interchangeable cylinders; the auto pistol cylinder has the shoulders machined at the appropriate depth.

OTOH the Colt Model 1917 and the new S&W Governor lack shoulders at .45ACP depth and rely on the moon clips to provide appropriate headspace; on the multi-caliber Governor, this is done primarily because it's relatively difficult to design and manufacture a swing-out cylinder revolver with an interchangeable cylinder to allow the longer .45 Colt round to be fired.

Contrary to some misinformation on the Web, S&W 9mm and N frame .45ACP revolvers do NOT use the moon clip for headspace; it's determined by the shoulders in the chambers. These guns can be safely fired without moon clips, but the empties would have to be manually punched or picked out one at a time with a tool (more below).

The second issue is ejection, which primarily affects swing-out cylinder revolvers that use an ejector star that would normally engage the case rims. On a Colt SAA-type single-action such as the Blackhawk, this is a non-issue, because these guns normally use a plunger-type ejector rod that punches the cases out of the cylinder one at a time through a loading door in the recoil shield.

The original way to handle the ejection problem in a swing-out cylinder revolver was the moon clip; the ejector star simply acts on the clip. Moon clips also allow really fast reloading IF the shooter has a cache of moonclipped rounds at ready. However, moon clips have significant disadvantages- (a) they can cause functioning problems if they're bent, (b) the process of placing the loaded rounds in the clips and (particularly) removing the empties afterwards can be tedious, and (c) the clips may be bent if this process is performed sloppily.

There are two ways to avoid the problems associated with moon clips.

The "new school" way is to design a special ejector star with some sort of tab mechanism to "catch" the extraction groove and eject the rounds. This was used in the S&W K frame 9mm Model 547, and has since been adopted in the new Charter Arms auto pistol revolvers. (I'm not sure about the recent Taurus guns.)

The "old school" way is to use a special cartridge case that has a rim for ejection but is otherwise identical to an auto pistol case. The oldest example is the .45 Auto Rim or .45AR cartridge, which was designed for use in S&W and Colt M1917 revolvers, and remains popular for use in S&W's with the exception of the Governor. (It's set up for .45 Colt rims and therefore is not compatible with .45AR.) Another example is 9mm Federal, which was intended to do the same with 9mm Luger, but was essentially stillborn because Charter Arms went belly-up right when it was introduced and never mass-produced the revolver in which it was intended to be used.
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