Pure honesty, 9mm in a .40

Bah, seen it done by accident several times, and occasionally on purpose. Brush the bore to remove the loose powder, and keep shooting.

Done it before also. Since the 9mm round is smaller the same pressure wouldn’t build up. Shouldn’t cause any problems. Like the edwardware posted clean the bore and shoot it

You are not the first and are unlikely to be the last one to do it.

Saw two guys shooting a new Glock 22 complaining about "that cheap Blazer ammo" jamming and having poor accuracy and "every case splits", then they realized they were shooting 9mm in a .40S&W.

JohnhenrySTL wrote:
My question is would you continue to fire this firearm as normal?

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Clean it thoroughly and use it as normal.

Do I need to have a gun smith check it?

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No need.

I realize this was a horrible mistake.

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Yes.

And this is the art that calls for action on your part. You need to sit down and go over your reloading procedures, your storage procedures and your range procedures to determine how this happened - and more importantly how t make sure it doesn't happen again

I managed to shoot a .380 round out of a 9mm sig sp2022. The gun shot it and cycled fine. I just noticed the difference in the recoil and sound and stopped shooting. The empty case landed on the table right beside the gun ( no other case landed on the table all day.). The bad thing is, at that time I didn’t own a .380 and I don’t reload. So that .380 round was in a box of factory 9mm ammunition.

I've seen .44 magnums mistakenly fired in .45 Colt revolvers.....TWICE. Thankfully both forty fives were modern and sustained no damage.

I've seen .380's fired in nines a few times.

All sorts of foul ups...Thankfully a 9mm in a .40 isn't a big deal either.

I doubt the wrong cartridge got into a box at the factory. More likely some bored gun shop employee was playing around or showing a customer the difference and got the wrong round in the box. No harm done, but FWIW, that is one thing gun and cartridge designers watch out for, a mixup that could be dangerous.

Jim

At least it wasn't a 300 Blackout in a 5.56.
That hasn't happened to me but the possibility of that switcheroo is enough to lead me to only bring one or the other when I go to the range.
Here's an article from the NRA on this mishap.

Some guns/calibers are built with these scenarios in mind. It's not SOP, but the occasional mishap is going to happen. Relax, learn from it and have fun.

Jim K said:

I doubt the wrong cartridge got into a box at the factory. More likely some bored gun shop employee was playing around or showing a customer the difference and got the wrong round in the box. No harm done, but FWIW, that is one thing gun and cartridge designers watch out for, a mixup that could be dangerous.

Jim

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John got his reloads mixed at the range bench.

A suggestion to the OP, if you're trying out different loads. Maybe use different colored sharpies for quick cartridge ID.

25 years ago when I was 19 I managed to fire a .40 out of a Swenson 1911. I had been shooting a buddies .40, we boxed up his remaining rds and started shooting my 1911. There apparently was some loose .40 rds still on the bench and I wasn't paying attention and they are roughly the same size.

Didn't hurt the gun any, but I had to clean the split brass out of the chamber.

Since then whenever I go to the range I generally stick to one caliber for each group (rifles and pistols). The few times I do take more than one caliber I pay particular attention to the pistol that I'm bringing out initially and make sure that the caliber matches that pistol. Then when switching I clear the bench completely of the pistol and ammunition that goes with it and then bring out the new pistol that I'll be shooting and I pay particular attention to what ammo I'm bringing out of the range bag for it.

So it only happened that one time. Maybe you could institute the same sort of checks/system.

CLP said:

At least it wasn't a 300 Blackout in a 5.56.

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This is why, in my opinion, 300 BO should never have been industrialized.

The first rule of good cartridge design is that thou shalt not produce a cartridge that will chamber in a weapon, and then grenade it. Some exceptions may apply where old weapons (ie Colt SAA in 45 Colt) can't stand modern ammo, but I think it's irresponsible to produce ammo that will feed, fire, and destroy a modern weapon, especially something as common as a 5.56 AR.

Image the hulabaloo that would result from selling 200 grain 357 Maximum-pressure level ammo loaded in 38 Special length brass. Even if the headstamp said 358 DynoBuster, that would be irresponsible to offer for sale.

My handguns are limited to .45 ACP and .357 Magnum, so that hasn't been a problem for me. However, I'm always intrigued by the folks that post on the various forums about owning a bunch of Glock's, such as a G19, G23, G32, and a G38 or a G17, G22, G31, and a G37, or maybe a 9mm conversion barrel for one of their .40S&W or 357 SIG Glocks, etc. Those mags all look nearly identical. If your habit is to bring a bunch of different guns to the range, I can imagine putting the wrong mag, with the wrong caliber, in the wrong gun, wouldn't be too hard to do.

I did the same thing once with a Glock 23. Other than feeling foolish, don't worry about it. lesson learned.

I was at an indoor range a year and a half ago getting certification for concealed carry in another state.
The instructor talked about a lot of wrong cartridge incidents he had seen. One was 9mm in 40S&W. For a full year they were finding a lot of bulged 9mm brass. They finally figured out it was a local LEO that had never used anything but 9mm in his 40.

I also am guilty of this. It didn't hurt anything but, I have since changed my procedures when shooting multiple calibers.

drunkenpoacher said:

They finally figured out it was a local LEO that had never used anything but 9mm in his 40.

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Imagine his surprise the first time he fired the correct caliber ammo. . .

Also, did his badge say "Fife"?

edwardware said:

Imagine his surprise the first time he fired the correct caliber ammo. . .

Also, did his badge say "Fife"?

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Probably started complaining about snappy recoil.
More than one bullet so it wasn't Barney.

Yes, I've done it as well with the same result. Report sounded different. Projectile hit target but low and wide. Slide did not cycle fully. Case was bulged, but not split at the base and did not properly eject. Barrel and pistol (P229R) was fine.

Maybe those of us who have done this should have a special beanie that we have to wear at the range.

My grandson did it with my M&P a few months ago. It locked the gun up. He held the gun and pulled back on the slide and I tapped the swelled case back with a cleaning rod. No damage what so ever. Funny thing was the report sounded normal when he shot it. Of course we had hearing protection on and that might have been the reason for noticing no difference in the sound.

I seen it done once. I was shooting an SR9, and my buddy an SR40. Guns and mags look nearly identical. No harm done, but once is enough. Now I pay extra close attention.

edwardware said:

This is why, in my opinion, 300 BO should never have been industrialized.

The first rule of good cartridge design is that thou shalt not produce a cartridge that will chamber in a weapon, and then grenade it. Some exceptions may apply where old weapons (ie Colt SAA in 45 Colt) can't stand modern ammo, but I think it's irresponsible to produce ammo that will feed, fire, and destroy a modern weapon, especially something as common as a 5.56 AR.

Image the hulabaloo that would result from selling 200 grain 357 Maximum-pressure level ammo loaded in 38 Special length brass. Even if the headstamp said 358 DynoBuster, that would be irresponsible to offer for sale.

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There are several shells and cartridges that can be dangerously chambered and fired in guns that aren't designed for them, not just the .300 Whisper (AAC Blackout) in a 5.56.

.308 in a .270/.280 Rem, .44 mag in .45 Colt can be disastrous, and a 20 Ga will slide in and obstruct a 12 ga barrel and blow it up if another 12 is fired (hence the standard yellow 20 ga hulls). Etc. If it didn't repeatedly happen then we wouldn't see warnings to load the ammo into the arms chambered for them on every box we buy.

Just like the reloader has the responsibility to carefully assemble his loads, the onus is always on the shooter to pay attention to what he is shooting and what cartridge he is putting into the firearm. If one is easily distracted or brings out a ton of guns and cartridges to the range, shooting only one gun/caliber at a time and putting it all away before going to gun #2 will certainly help.

Yes, we are human and we all make errors... Lord knows I have made plenty through my lifetime! But taking the time to be sure before one settles down behind the gun to shoot reduces the risk to a negligible level.

Stay safe!