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The .45 Auto Rim

by Allan Jones   |  February 28th, 2011 1

The .45 Auto Rim (AR) may be the first cartridge developed a half-decade after the firearm for which it was intended.


At top is an as-issued .45-caliber Colt U.S. Model 1917 revolver. Below is the rare 1917 Commercial model, assembled by Colt in 1932 from unused WWI parts still in the factory.

The .45 Auto Rim (AR) may be the first cartridge developed a half-decade after the firearm for which it was intended. Peters Cartridge Company found a viable alternative to using metal clips to fire .45 Auto ammo in Colt and S&W Model 1917 military revolvers. These big sixguns came to the surplus market after World War I, but the clip system that the military used proved less practical for civilians.

The “half-moon” clips, each holding three .45 Auto cartridges for easy extraction from a DA revolver, presented little issue to the soldier–the arsenals packaged ammo already “clipped,” and saving clips for reuse was not an issue. However, clips presented a challenge when the surplus revolvers reached the civilian market. Clips were easily bent and could tie up the cylinder, and aftermarket clips varied hugely in quality and dimensions.

The Peters solution was to make a .45 Auto case with a rim. The rim is almost 50 percent thicker than the rims of the .44 Special and .45 Colt. This odd rim fills the large rear cylinder gap that is cut to accommodate the .45 Auto’s rim plus a 0.050-inch clip. Peters also opted for a lead 230-grain RN bullet, which was deemed more appropriate to revolver shooting.

There are many of these revolvers still in shooting condition, but I hear people complaining they are not very accurate. Assuming the sixgun has not been abused, it can shoot as well as any other firearm, and better than many.

The Pressure Question
The .45 Auto Rim’s pressure assignment is 3,000 CUP less than the .45 Auto. This appears to be an accommodation to the soft factory lead bullets, not the strength of the revolvers. Peters chose to load the .45 AR with a soft lead bullet with a deep, hollow base and a thin skirt to better grip the shallow rifling. The downside is that typical .45 Auto pressures can deform the skirt as the bullet leaves the muzzle and destroy the accuracy.


The heavy rim of the .45 Auto Rim cartridge (right) is the thickness of a .45 Auto’s rim (left) plus the thickness of a half-moon clip.

The revolvers were designed for standard .45 Auto, so there should be no issue with factory ammo or handloads that remain within .45 Auto pressures–18,000 CUP or 21,000 psi, depending on the test system.

Achieving Accuracy
My first .45 AR revolver was a near-mint Colt 1917 that I bought in 1972. I immediately set out to find its handloading potential. I assembled 30 rounds of cast-bullet loads that were as identical as possible except for the bullet. Half were loaded with a roundnose cast bullet profiled just like the military .45 Auto FMJ, and the other half were loaded with a 235-grain semiwadcutter (SWC) designed expressly for the .45 AR. I used a stout lead alloy in deference to the shallow rifling.

The accuracy difference was striking. At 25 feet, the cartridges loaded with roundnose bullets shot a 4-inch pattern; the semiwadcutters printed a tight cloverleaf. I saved a few rounds of each for recovery of fired bullets in our lab’s water tank. The reason for the accuracy difference was all too clear. The roundnose bullets showed evidence of tipping in the overly long cylinder throats. They did not have enough bearing surface to stay nose-on before entering the barrel throat.


A good semiwadcutter .45 Colt bullet (right) will always outshoot lead roundnose bullets in .45 Auto Rim revolvers. Remember to use that crimp groove!

This and subsequent testing told me that accuracy from a .45 AR revolver required a diet of bullets whose bearing surfaces made up a significant portion of the total bullet length. For cast bullets, the sizing diameter is also important to fill that long chamber throat. I’ve settled on a sizing diameter of 0.454 inch. These bullets are a sliding fit in the chamber throats of both my old Colt 1917 and my custom 5-inch-barreled S&W Model 25-2, and they produce amazing accuracy.

Even with similar bearing surfaces, .45-caliber semiwadcutters always outshoot other nose shapes in my revolvers. One of the tenets of Elmer Keith’s design of cast SWC bullets was to leave a piece of the bearing surface ahead of the crimp groove to act as a pilot, centering the cartridge in the cylinder throat when it is loaded. To do so, that part of the bullet has to be the same diameter as the rest of the bullet. If undersized, this important feature degrades to a cosmetic trinket.

Being a tinkerer, I seldom limit myself to one load for any of my firearms, but the .45 AR is the exception. I found a load that shot so well that I use it 95 percent of the time. It is the Keith 255-grain SWC from either the old Ideal No. 454424 mold or the RCBS No. 82050 mold, sized 0.454 inch and perched over 6.0 grains of Unique. I’ve used both CCI and Remington Large Pistol primers with equal success. I take advantage of the bullet’s crimp groove to leave the pilot section of the bullet ahead of the case. In my revolvers, this package comes in at 820 to 840 fps and is wonderfully accurate.

Odds And Ends
You can shoot .45 Auto ammo in these revolvers without the clip as long as a fast reload isn’t an issue. You can flick the spent cases out with a fingernail, and the chamber’s 90-degree headspace shoulder adequately supports the case mouth.

The .45 AR case is a survivor. I’ve yet to lose a case from too many reloadings.

All Model 1917 and commercial .45 AR revolvers have the same style of rifling as the .45 Auto–shallow. Avoid soft lead alloys when using cast bullets.

If loading jacketed bullets in the .45 AR case, choose from those for the .45 Colt. They have a crimping cannelure that prevents bullet jump and improves ballistic uniformity.

Just in the last few months, I’ve revived my interest in shotshell loads for the .45 AR. The commercial CCI and Remington .45 Auto shot loads are not designed for revolvers, so I got some Speer empty sho
t capsules for the .45 Colt. I hope to have a follow-up report soon.

The .45 Auto Rim is still a viable cartridge. Loaded properly, it combines power and accuracy with very manageable recoil. Those old 1917s are still worth shooting if in good shape, and handloading extends their versatility and accuracy potential.

  • Gene

    I recently picked up a 45 AR revolver and have found it like the 2 I have had previously to be a lot of fun. This one is NOT getting out of my hands as the others have.

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