Conversion Issues: Going From .40 S&W to 9mm

posted on April 18, 2019

This article, "Forty to Nine, not so Fine" appeared originally as a Tech Wisdom column in the June 2017 issue of Shooting Illustrated. To subscribe to the magazine, visit the NRA membership page here and select Shooting Illustrated as your member magazine.

With the recent move away from the .40 S&W to the 9 mm within the law-enforcement community, there are a lot of agency-owned trade-ins on the market for ridiculously low prices. I was thinking of picking up a Glock, SIG Sauer or a Smith & Wesson M&P in .40 S&W and swapping the barrel for a 9 mm to subsequently reap the benefits of less recoil and cheaper ammunition. Several of my fellow gun buffs are telling me that this conversion can’t be done with any degree of reliability. Why not?

Brian Hopkins, Atlanta, GA.

There are a lot of “depends ons” to your question. While converting .40 S&W pistols to 9 mm can be done under certain circumstances, I personally wouldn’t trust my life to a barrel-change conversion without a lot of rounds through the gun, absent any stoppages. There are several reasons for my reservations given this type of conversion.

First, the replacement barrel needs to be the same outside diameter as the original barrel, particularly at the muzzle, or accuracy will suffer because of inconsistent barrel lockup at the front of the slide. If the replacement barrel is smaller in diameter, it will not index to the same place every time the slide cycles when the gun is fired.

At the chamber end of the barrel, the barrel hood may not have consistent lateral index due to the width of the breechface being wider to accommodate the larger diameter of the .40 S&W cartridge case. Where this is a real potential problem, other than affecting accuracy, is consistent extraction. The 9 mm cartridge case is smaller in diameter than the .40 S&W. This means the extractor has to reach in a little farther to grab the rim of the case with sufficient purchase to pull the case out of the chamber and hold it against the breechface until the case is expelled by contacting the ejector as the slide cycles rearward. Consistent extraction and ejection are essential to a semi-auto’s reliability.

Many manufacturers list different ejectors for the 9 mm and .40 S&W chamberings, especially in their older model pistols. There is a reason for this. The locking lugs on the bottom of the barrel may be configured a little differently as well, which means inconsistent positioning of the barrel in the slide resulting in less-than-desirable accuracy.

If the recoil spring in the .40-caliber gun to be converted is new or at least in good shape, it is likely that it is too strong to allow proper cycling with the 9 mm, particularly when using lighter-weight, generic practice ammo.

The magazines are often overlooked for replacement when swapping calibers simply because often a person can put 9 mm ammo in a .40 S&W magazine and vice versa. The problem is, the width of the feedlips differ between 9 mm and .40 S&W magazines. When attempting to cycle 9 mm ammunition in  a .40 S&W magazine, the smaller-diameter 9 mm cartridges have a tendency to sit higher in the magazine than designed, which can cause feed-way stoppages. In some cases, the cartridges will escape past the feed lips with nothing more than the force of the magazine spring. Needless to say, this isn’t a desirable condition. All this considered, you may get lucky with just a barrel change, but the odds are against a happy ending.

If you are convinced that 9 mm is the way to go for you, my recommendation is to buy a slide, barrel and recoil spring along with some 9 mm magazines for your particular pistol to ensure the highest likelihood of reliability. If you are willing to take the chance and compromise on accuracy and reliability try the barrel swap—and see what happens. While some may disagree, I don’t see that there is a safety concern, mechanically speaking, with just a barrel swap, if the gun will function as expected. With the information provided, you have sufficient ammunition (no pun intended) to make an informed decision.

Just as a side note, keep an eye on the price of .40 S&W ammunition. With its loss in popularity among the law-enforcement agencies, there is likely to be a surplus for a while, which may provide incentives to buy and shoot .40 S&W for the foreseeable future.


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