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Chamber Dimensions for 9MM and 45 ACP Pistols


Friends, there is much confusion in the shooting world regarding the history of chamber dimensions for 9MM and 45 ACP chambered firearms. As of this writing, (2018) the 9MM cartridge is very old. Developed in 1908 in Germany as a strictly military cartridge, original chamber dimensions were designed around the use of an FMJ, round nosed bullet. This original loading of a military FMJ-RN bullet used a bullet that came to full diameter (it’s ogive) right at the case mouth, so original chambers were short, which means the rifling’s started right at the case mouth. The same is true for the 45 ACP pistol, which was developed and turned loose by Colt Manufacturing Company in 1911, in the model 1911 pistol for our military.

 The 45 ACP, being an American invention (thank you John M Browning) was popular in the US right from the gate, but the 9MM started to see more usage in the US during the 1950’s but was still strictly a military type cartridge using FMJ-RN bullets. Today, modern ammo and firearm manufacturers (some of them) understand that by moving the bullet ogive forward of the case mouth, more case capacity is created in that tiny casing, which lowers pressures and allows for more propellant to be added to the cartridge, to increase velocity, but at lower pressures—it’s a win-win situation. Unfortunately, (there always has to be a tradeoff) some pistol designs are still using the original short chamber dimensions. When you try to chamber an “ogive forward” designed bullet into a chamber that is short, designed for 100+ -year-old ammo, the ogive of the bullet will hit the rifling’s before the slide gets into battery and the cartridge simply will not chamber.

NOTE: When using/testing any ammo in any semi-auto pistol, PLEASE let the slide SLAM home from the most rearward position. Far too many folks have the bad habit of lowering the slide by hand and wondering why it is not cycling or chambering properly. Treat/handle your pistol, in the same manner, it would operate if it was being fired, as that is how it is designed to be handled, which means that slide slams home form all the way back……….Hand Cycling Ammo in Semi Autos as a Means of Reliability Testing


America, being a “Gun Culture” generally redesigns older cartridges to make them useful for more than simple military applications… such as happened with the 9MM and 45 ACP in the last 35 years. These days, in America, you can buy very effective JHP bullet designs, many of which are loaded to +P or even +P+ pressures, but to safely fling bullets at much higher velocity out of the tiny 9MM or 45 ACP casing, the ogive of the bullet must be moved forward to create more case capacity. This works perfectly until you try to use such a modern bullet in a chamber designed for military FMJ-RN bullets.

As one example, our very popular 9MM OUTDOORSMAN load features a heavy bullet, with the ogive, moved forward so we can gain extraordinary velocities with that heavy bullet, but at only +P (not +P+) pressures. I personally tested the daylights out of this load, before we released it to the US market. Once we released this load to millions of shooters in the general shooting public we discovered that there are some versions/models of pistols still being made in the US or sold in the US that are using the original chamber dimensions from 1908.  We have discovered that the Ruger All American, one HK model (that we know of) CZ’s and a couple other makes/models, are using original short chambers… Kimber makes several models with what is termed a “match grade” chamber, which simply means a tighter and shorter than normal chamber…….and yes this can lead to an inherently more accurate pistol, but the cost is that any ammo that is not strictly in spec with a match grade chamber, will not chamber, especially if the pistol gets very dirty, like it would in some combat situations. This short chamber situation generally happens when the chief engineer of a gun making company plugs in original chamber dimensions into the design of a modern pistol. When engineering is done from drawings, instead of being done from a prolific end-user standpoint, this is what happens….. I believe that all gun design engineers, should be prolific end users of the product they design, but that simply is not the case.

If you go to our website for the 9MM OUTDOORSMAN load (Item 24L) you will see my test results with all the handguns I used to develop the 9MM OUTDOORSMAN. That load works well in those models and makes of pistols and many others I did not have at the time. The same is true with our 45 ACP +P “Outdoorsman.

Now, because humans are sloppy and assumptive, please understand that what I am writing here about the 9MM and 45 ACP, their characteristics and history DOES NOT APPLY to other cartridges, cart blanc… it applies to the 9MM and 45 ACP (ACP = Automatic Colt Pistol) for the intents of this short write up.

Hopefully, this short essay will clear up questions about why some 9MM and 45 ACP ammo chambers easily in modern guns and some does not.  We at Buffalo Bore appreciate your business. God Bless.


As a modern-day example of how effectively even a 9MM load can be redesigned, click HERE to see an NRA article where Phil Shoemaker, a professional guide in Alaska, recently used our 9MM OUTDOORSMAN load to stop an attacking grizzly.

Tim Sundles