Firearm Skills

Guns of ISG: The Cereberus Project

After the 2009 and 2011 ammunition shortages, it hit me that odd calibers are usually out of favor and in stock. But are multi-caliber firearms safe & reliable?

May 5, 2021


Ammunition shortages currently gripping the U.S. have sent 9mm and 5.56 prices soaring to around a buck a bang. While this level of panic might be somewhat unprecedented, the general situation isn't. We've had major ammunition shortages and panic buying of firearms over the past decade on a few occasions, and they have certainly made one thing clear: 

As the unanimity of 9mm and 5.56 have become nearly complete, less favored cartridges that have fallen out of favor can often still be found.

After 2011, I became interested in the concept of owning a firearm that could shoot more calibers than its designated chambering. This was largely inspired by the ill-fated Bushmaster MASADA, which promised interchangeable barrels, but evaporated when it was time to show some proof.

At that same time, I was beginning to realize that the carbine-based martialism I'd been raised with was an antique of the Cooperean era that espoused cliches like "your handgun is to fight your way to the rifle you never shoulda left". That sentiment just doesn't hold up in real life. Not only are gunfights in the citizens world generally over very quickly, but very few allowed the time or luxury of accessing a better weapon.

This led me to a dramatic shift away from the rifle as the "primary weapon", and towards the pistol. Regardless of its limitations, if we consider what "primary" means, "earliest in time or order", the primary weapon for most of us "Tier None" operators is the pistol... and as it turns out, pistols lend themselves to multi-caliber in ways rifles just can't.

Thus, Project Cerberus was conceived, and the hunt for a pistol that could shoot 9mm, .357 Sig, or .40 Smith and Wesson began.

Please note, as you read the following, if you're not familiar with the calibers in questions, ask someone knowledgeable. We understand there are a millions of new gun owners in the U.S. right now, and I remember the confusion of trying to figure out if 9mm Lugar was the same as 9mm Parabellum, 9mm Makarov, or 9mm Largo (the first two are the same, the last two are very different).

If you're new to shooting, file this one away for later, and pay special attention to the specifics; for example we will mention .357 Sig and .357 Magnum. These are *very* different cartridges for different types of firearms. Close attention to detail must be paid when approaching this topic.

So a couple things:

1. Hit us up if you've got questions.

2. This is 100% an 'at your own risk' proposition, and deviating from the methods we tested in any way changes our advice. Doing this will void your warranty, and will be frowned upon by the manufacturer. If you decide to do this, and do it incorrectly, it could damage your firearm or cause injury.

3. This is NOT an article for a new shooter. If you're still at the "which 9mm is this?" phase, just revisit it in time when you're more experienced. Read the article in its entirety and pay attention to details.

The Autoloader

So, given that most people now-a-days opt for an autoloading sidearm, the first step was to find a platform for the project.

First of all, you can go DOWN a size in bore diameter, but not UP. If you're carrying a 9mm, you can't swap to the larger .40, or .357 Sig, however, if you're carrying a .40, you CAN swap  to the .357 Sig, or down to a 9mm.

Because of this, and because the .40S&W caliber is a more punishing cartridge than the 9mm, I wanted to start with a pistol that was built to withstand a service diet of .357 Sig or .40 S&W.

With that, I settled on the Smith and Wesson M&P platform. It was originally designed for the .40, and by the time I was looking to make the project a reality, S&W had worked out the early kinks on the M&Ps, and aftermarket support was becoming available.

While Glock produces reliable .40 caliber pistols, they were originally developed for the 9mm cartridge. This primarily affects things like the extractor and recoil spring weight, but it those things can generally be switched out without a whole lot of trouble... moreover, it's not generally necessary, but it is something to be aware of.

To be perfectly clear, we've had great results with Glocks and M&P series pistols.

An aside - It's said that some of the Gen2 Glock .40 caliber pistols cannot be converted to .357 Sig. This information came to us from David, so shout out and thank you for the extra information.

What about .22 caliber conversions?

Great question - the answer is "yes". While they tend to be finicky about reliability, you should absolutely, consider getting a .22 caliber slide and barrel kit, which is principally similar, but functionally very different from the goal here.

Whereas the .22 caliber kits are more or less for training with less cost per round, the conversion barrel process is more about being able to use other full powered loads in the same pistol.

Unfortunately for me - they don't make .22 conversions for the pistol I'm working with, or I'd certainly add that to the backpack.

If your platform of choice allows for it, it's absolutely worth considering.

Barrel Swaps: What's Required?

With the platform selected, all that was required was a drop-in barrel and magazines correct for the caliber - and this is very important. 

If you try and use 9mm bullets in .40/.357 Sig magazines (the inverse isn't possible), you're going to get endless malfunctions.

While you may choose a different route, I decided to buy barrels that looked distinctly different from one another to prevent confusion, and purchased Hyve Industries +3 round Baseplates to color code the calibers.

This ensures that I know the correct color combinations for the barrel and magazines.


Finally, frame size is a consideration. I have absolutely ZERO interest in imagining scenarios where I'm looting magazines or teaming up with a heroic cop while sharing ammo to take down a pack of terrorists or something, but I *am* concerned with production quantity and availability.

With Glock and S&W leading the pack in terms of being issued to departments, I'm fairly certain that the magazines will be in production for either pistol.

With that said, we're talking situations that restrict our access to resources - like magazines - and that means I want as much latitude in selecting what magazines I can use as possible.

A subcompact, duty-frame pistol will accept the magazines from larger duty framed pistols. That means the Subcompact M&P (the MP40C, in this case, but G27 as well) can accept magazines from ANY larger handgun in ANY caliber the series is chambered in. There are three frame sizes in three calibers, so I have 9 options to choose from, if shortages kick in and I need to find magazines.

Are they Reliable?

A handgun is nothing if it isn't reliable, and machines of any and all type experience failures. Acceptable failures DO occur, and anyone who tells you that XYZ pistol 'doesn't jam' probably doesn't shoot enough. So let's discuss briefly what constitutes reliability, and how we determine if a pistol is reliable... but before we do, a quick word on consumable parts:

If you shoot high volumes, you're going to wear parts out. For this reason, it's wise to take some combination of the following precautions:

  1. Have spare recoil springs, striker assembly, extractor/ejector, etc as appropriate for your pistol, and/or;
  2. Have a spare, identical 'pre-runner' pistol you train with, and a dedicated carry pistol.

Most modern autoloaders have a lifespan that exceeds what most shooters will put it through, so really, some spare parts are probably good enough. The lifespan on some of these consumable parts is generally listed at around 5000 rounds, and that's probably a good threshold for preventative maintenance.

As often as not, when someone starts talking about how reliable their pistol is, they'll say "I've only had Y number of malfunctions in X number of rounds!" Good for them. Their results mean almost nothing, because malfunctions and reliability depend on WAY more than anecdote, to include:

  • Quality of Ammunition
  • Grip and posture
  • Service and maintenance (cleaning, lubrication, and general condition)
  • Magazine Serviceability
  • Condition of internal, consumable parts
  • Material Quality

So, when we talk about reliability, we're not going to say "it must shoot y/x rounds", because it's honestly just a wild guess at that point, because no two combinations of the person, pistol, environment, support gear, and maintenance will be the same. What we will say is that it needs to be a solid firearm in the first place, built with quality parts for rugged conditions. If it has a track record of military or police trials for service weapons, that's a great place to start.

Personally, I would NOT consider a base pistol that wasn't made specifically for the military or police for this role, as those pistols ARE held to consistent standards of use in a variety of environments.

So, given a pistol that's using factory parts within their service life, the only mechanism really left to create stoppages in a quality, modern autoloader are fouling, parts wear, or bad ammunition.

If we use this criteria, I'm hard pressed to say that I've ever encountered a stoppage while running 9mm through a correctly converted .40S&W caliber pistol that wasn't directly related to bad ammunition (verified by running it through a Glock 9mm, and having identical results), or worn parts (some slow cycling due to a well worn recoil spring).

So, reliability?


Is it Safe?

Pic is unrelated, but converting your pistol is similar to loading your own rounds; you have to be switched on and paying close attention or it could have consequences. Done correctly, it allows you greater control over your platform.

In a word - yes, but only if you stick to a disciplined approach. Any time you fire a round through a firearm there's risk attached. If you start mixing and matching things and forget what's what - then no, it's not safe and it increases the risk of an out of battery discharge, or putting a round through a barrel it's improperly sized for.

These are major considerations, and if you're not able to essentially treat this process as a very serious, potentially dangerous combination, you shouldn't do it... no different from handloading your own rounds/shooting handloads.

You shouldn't be rushing through or reconfiguring calibers while under time pressure or duress (kinda obvious), so please think about that before deciding on doing a conversion barrel.

Further, I'd be really surprised if there wasn't a bunch of 'boo-hiss!" from dudes who think glowey donuts are cool, or who's chief qualification is getting certs from the NRA. Not that long ago it was taboo to work from the holster or perform single handed reloads. Schopenhauer was right, and we've never been afraid of presenting violently opposed truths.

So all shooting sports are dangerous. As with anything, there are calculated risks.
Done correctly, this is literally no different than shooting any normal firearm.

It mucks with forensics and profit margins, so naturally people will be like "Don't do that!"

What you'll need

To recap, if you want to shoot 9mm or .357 Sig through a .40S&W caliber pistol, you'll need the following:

  1. A duty framed pistol made from quality materials in serviceable condition. The only recommendations we will make are Glock, Sig, and Smith and Wesson M&P pistols.
  2. A conversion barrel from a reliable manufacturer for the SPECIFIC caliber you intend to shoot (9mm and .357 Sig are the only options for drop in barrels). I personally preferred Storm Lake, and have used their products pretty extensively, but there are many excellent manufacturers out there. I believe Storm Lake may now be out of business.
  3. Proper Magazines for the caliber(s) you intend to shoot. Often, .40/.357 are the same.
  4. Recoil Spring appropriate to the pistol, frame, and caliber. This isn't absolutely mandatory, but will help longevity and recoil management.

As mentioned, I've got a few boxes of .40 caliber through my pistol, a S&W M&P40C, and the remaining balance of around 20k rounds over the last 7 years has all been 9mm, and it is trouble free using good quality brass ammunition and correct magazines... further, these results have been typical with the Glocks in the ISG crew, too. John S. of Blackfox IRT conservatively estimates 12k through his, Mike W. has run about a thousand through his. Neither have had problems with reliability.

Be warned: it may void your warranty, but done right, it's perfectly safe, and it increases your odds of being able to find or scavenge some working cartridges in an emergency, while retaining the reliability you'd expect from a defensive handgun. The bottom line with using conversion barrels and reliability is generally this:

If your pistol was safe and reliable before, it will be safe and reliable using a conversion barrel and proper magazines.

While there may be others with differing opinions, I *would not* try and do this with a Taurus, Kimber, or another manufacturer who's been known to use lower quality MIM parts in their guns.

Don't do this is you're unwilling to accept the responsibility and liability, and this is for informational purposes only.

The Revolver

As we've mentioned before, the Revolver might not get much mainstream love, but it's not obsolete. It's still got a place and some standing, not only for familiarity, but one of its strengths is the lack of requiring a magazine, and in the case of a .357 Mag, the ability to chamber the .38 Special and .38+P cartridges.

Like the .40-to-9mm conversion, this is a one way street. If you have a .357 Magnum, you can load up on .38 Specials... but if you have a .38 Special, for the love of God don't try and run .357 Magnum through it.

Unlike the .40-to-9mm you don't have to worry about changing anything on the revolver. You simply drop .38 Special or .38 Special+P in and you're good to go. This is an obvious advantage for a few reasons. Not only are revolvers reasonably non-threatening politically, they're very prevalent around the world. So while the political winds change direction every few years, it will be pretty difficult to make a case for banning guns like revolvers. If that were to happen, it's likely that most non-government security forces would end up relegated to the Revolver.

Having the ability to shoot both .357 Magnum and .38 Special/+P is an advantage.


So the next most logical step is "can I do this with my rifle?"

Sorry to say that at present, there's no good way of doing this, with the possible exception of AR-15 uppers. If you run an AR, going mutli-caliber is pretty simple, but at present, the practicality isn't quite on the same level as it is with the handgun... Most AR uppers are either in less common, more expensive calibers, or pistol calibers you'd be better off saving for your handguns.

I remember when I was a kid, there was some Fuddlore that the M14 (chambered in 7.62x51) was designed to shoot the smaller, Soviet cartridge (7.62x39), which no... it was not, and don't do it. With rifles, stick to the cartridges they were intended to shoot. Not only are you dealing with much higher chamber pressures, but rifles are just an entirely different animal.

Short version: don't bother if it's not a swappable AR upper, and even then, eh.


2020 has brought us a lot of verification in the form of a long, slow burning Type III catastrophe - COVID19 and its attending debacles. As we discuss in Understanding Emergencies, situations like this disrupt or fundamentally change our lives for indefinite periods of time, and they create smaller scale situations in which *other* types of emergencies are more likely; for example, protests leading to riots, which lead to shootings.

So, this isn't theoretical or academic anymore. We're living in a Type III, and it may resolve soon, or it may persist for a couple years, as did the Spanish Flu.

It's entirely reasonable to think that you may be forced into unusual situations during this time. Involuntary movement, quarantine, or large scale civil unrest is -again- happening right now, even if the media outrage has shifted from the rioting to the mass shootings, as the political winds blow.

When we addressed the myth of the bug out bag, situations like this were what we had in mind. You either have time to pack up and leave voluntarily, or like the fires that swept the west coast - you have 15 minutes to get your family and pets to the car and leave forever and anything you leave behind is ash.

With that in mind, you may not be able to bring several thousand rounds of ammunition, and while you'll probably not need it, it's good to know that while you're moving, you have more options if you do.

For that reason, it makes some sense to make conversion kits a part of your second line (sustainment) kit. It doesn't take up a lot of space, doesn't add a lot of weight, and adds a significant amount of versatility.


As the 2021 Apocalypse Bingo continues to pull the strings of the panic buyers in the gun and ammunition market, it may make sense to grab a .40S&W pistol with 9mm and .357 Sig barrels, and a .357 Magnum Revolver, and train on them. As of yesterday, my local shop had .38 special and .40 S&W in stock, but not a single round of 9mm or .45ACP.

5.56mm was up over a dollar a round, and 7.62x39 was hovering right around .60 cents a round.

Even guys who have planned ahead are glancing sideways at their ammunition inventory and wondering how much is prudent to shoot.

If you run a conversion, you may not be running your favorite, most cost effective cartridge, and I'll readily admit that I prefer 9mm to anything on the market, but when it comes down to it, I can usually find a box of .40 or .357 Sig. If I can't, I can almost always trade some 9mm for them in a way that's favorable to me.

As with most contentious topics, or those that go against the mainstream of the tactical world, someone out there will almost certainly hem and haw about how "but but but, muh owner's manual". That's fine. All truth passes through three stages, and we're used to the ridicule becoming "violently opposed" turning the corner and becoming "accepted as self-evident". So, when you piss people off confronting them about whether G10 Sharpies are stupid or not, it's just their way of saying "you're right".

As long as you don't do something stupid - like assume that because this works when you take a .40S&W or .357 Sig, and add 9mm magazines and barrel, that you can Dremel out your LCP and run 9mm through a .380, you'll be perfectly fine.

Now, as a final word, some gun company exec is going to go all C:\normie\start outrage.exe over this further banishing ISG to the dark corners of the internet, but our prediction is this: Eventually they'll all be offering this, just like RMR cuts from the factory. It would be a nothing upsale to offer a couple magazines, barrel, and recoil spring, and I hope that instead of shouting the idea down, it'll be recognized as what it is: a safe, utilitarian way of having a duty gun that can fire multiple common calibers.

Be smart.


Published May 3 2021

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