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Interesting.....

I had my first ever malfunction with my M&P45 on my last trip to the range. There were no failures to fire, feed, or eject, but what happened was that with the slide locked back after firing the last round, inserting a new fully loaded magazine into the pistol would cause the slide to release forward into battery and chamber a round as soon as the magazine clicked into place. The chambered round was not fired. It happened twice to me, and once to my son. In each case, the pistol was pointed safely down range, and like I said, the chambered round was not fired, but it sure got my attention. The pistol continued to function normally the rest of the time.

The only think I can think of was that it must have been just barely caught on the slide lock, such that upward pressure against the underside of the slide from the top of the magazine was just enough to "lift" the slide stop notch up off the lock, allowing it to release forward into battery.

The Annoyed Man wrote:I had my first ever malfunction with my M&P45 on my last trip to the range. There were no failures to fire, feed, or eject, but what happened was that with the slide locked back after firing the last round, inserting a new fully loaded magazine into the pistol would cause the slide to release forward into battery and chamber a round as soon as the magazine clicked into place. The chambered round was not fired. It happened twice to me, and once to my son. In each case, the pistol was pointed safely down range, and like I said, the chambered round was not fired, but it sure got my attention. The pistol continued to function normally the rest of the time.

The inertia of the full mag being slammed home jars the slide back off of the slide stop just enough for the spring to pull the slide stop down and then the slide moves forward past it.

My Smith & Wesson Sigma does that. First time it startled me. Now, it does it about 49 times out of 50. I think of it as an extra feature. Faster reloads because I don't have to take the extra time to thumb the slide stop to chamber a round. Only thing I don't like about it is that it doesn't do it 100% of the time. So occasionally, I'll insert a fresh mag and start to shoot and realize the slide was still locked back. I was expecting it to go forward on it's own and it didn't. (I sometimes use this gun in competition, USPSA Production Division. I try to reload before slide lock, but sometimes you wind up there anyway.)

I've had several Glock owners tell me theirs does the same thing. A couple of police officers I've talked to who carry Sigs have also said that they do it.
I wouldn't worry about it. (You should be dang sure to keep your finger off the trigger when reloading anyway, right?)

This seems to hardly ever happen on brand new pistols, but many get that way once they're well broken in.
Only happens with pistols that have a spring loaded slide stop, so 1911s won't do that since there's nothing to pull the slide stop down.

mgood wrote:

The Annoyed Man wrote:I had my first ever malfunction with my M&P45 on my last trip to the range. There were no failures to fire, feed, or eject, but what happened was that with the slide locked back after firing the last round, inserting a new fully loaded magazine into the pistol would cause the slide to release forward into battery and chamber a round as soon as the magazine clicked into place. The chambered round was not fired. It happened twice to me, and once to my son. In each case, the pistol was pointed safely down range, and like I said, the chambered round was not fired, but it sure got my attention. The pistol continued to function normally the rest of the time.

The inertia of the full mag being slammed home jars the slide back off of the slide stop just enough for the spring to pull the slide stop down and then the slide moves forward past it.

My Smith & Wesson Sigma does that. First time it startled me. Now, it does it about 49 times out of 50. I think of it as an extra feature. Faster reloads because I don't have to take the extra time to thumb the slide stop to chamber a round.
I've had several Glock owners tell me theirs does the same thing. A couple of police officers I've talked to who carry Sigs have also said that they do it.
I wouldn't worry about it.

I figured it was something like that. As long as it doesn't fire on going into battery, I'm not worried about it for myself... but it does make me worry about it if I'm using the gun to teach someone else to shoot and they aren't exercising good trigger discipline when the slide goes forward and chambers a round. That, to me anyway, is a problem.

The Annoyed Man wrote:I figured it was something like that. As long as it doesn't fire on going into battery, I'm not worried about it for myself... but it does make me worry about it if I'm using the gun to teach someone else to shoot and they aren't exercising good trigger discipline when the slide goes forward and chambers a round. That, to me anyway, is a problem.

I edited the above post and added some stuff.

Yeah, you want to make sure your finger's off the trigger. I don't think it would fire without you first releasing the trigger enough for it to reset and then depressing the trigger again, but I wouldn't depend on that (with any gun).

At USPSA matches, where this gun has gotten a workout the past couple years, the RO watches for things like fingers in trigger guards during reloads. If you don't have your finger clearly outside the trigger guard when you reload, that's a DQ and you're done for the day. ND, even fired safely down range, will also earn a DQ and bye-bye. So I was a little concerned about it until I grew confident that I knew what it was doing and comfortable that it was not going to cause the gun to fire.

I've done that on any compact/subcompact gun I've shot. I have big hands, and a very high grip feels most natural to me, unfortunately, that doesn't work very well when your thumb is knocking the slide release.

mgood wrote:

The Annoyed Man wrote:I figured it was something like that. As long as it doesn't fire on going into battery, I'm not worried about it for myself... but it does make me worry about it if I'm using the gun to teach someone else to shoot and they aren't exercising good trigger discipline when the slide goes forward and chambers a round. That, to me anyway, is a problem.

I edited the above post and added some stuff.

Yeah, you want to make sure your finger's off the trigger. I don't think it would fire without you first releasing the trigger enough for it to reset and then depressing the trigger again, but I wouldn't depend on that (with any gun).

I haven't tested it yet, but I've been meaning to try it with the gun unloaded and an empty magazine. With the slide locked back, you would think that an empty mag would be just as likely to drop the slide and would a full mag. Anyway, I'm going to try it that way with my finger pulling on the trigger to see what happens. It shouldn't fire, as releasing the slide into battery with the trigger pulled is part of a normal post-cleaning function test; but given that the slide shouldn't be dropped at all unless deliberately done by the shooter, wisdom would seem to dictate making sure.

The Annoyed Man wrote:

mgood wrote:

The Annoyed Man wrote:I figured it was something like that. As long as it doesn't fire on going into battery, I'm not worried about it for myself... but it does make me worry about it if I'm using the gun to teach someone else to shoot and they aren't exercising good trigger discipline when the slide goes forward and chambers a round. That, to me anyway, is a problem.

I edited the above post and added some stuff.

Yeah, you want to make sure your finger's off the trigger. I don't think it would fire without you first releasing the trigger enough for it to reset and then depressing the trigger again, but I wouldn't depend on that (with any gun).

I haven't tested it yet, but I've been meaning to try it with the gun unloaded and an empty magazine. With the slide locked back, you would think that an empty mag would be just as likely to drop the slide and would a full mag. Anyway, I'm going to try it that way with my finger pulling on the trigger to see what happens. It shouldn't fire, as releasing the slide into battery with the trigger pulled is part of a normal post-cleaning function test; but given that the slide shouldn't be dropped at all unless deliberately done by the shooter, wisdom would seem to dictate making sure.

An empty mag will not allow the slide to be released. A partially charged or fully charged one will.

Remember the little doohickey (looks like a ledge below the top of the follower and only on the side the slide hold open lever is on) on the side of the magazine follower. It holds upward spring tension against the slide hold open lever, which then does exactly what its name suggests.

When at least one round is in the magazine, this part of the follower is inside the magazine body and does not touch the slide release lever, which then stays out of the way of the slide.

To see this work, lock the slide open, insert an empty magazine, pull the slide back, and let go. It will be held back by the slide release lever in the release lever notch in the slide and won't go into battery.

Try the same thing with a snap cap or dummy round in the mag. This time the round will be fed into the chamber and the slide will go fully into battery.

Please don't try this little Mr. Wizard experiment with a live round.

Excaliber wrote:Please don't try this little Mr. Wizard experiment with a live round.

Not bloody likely!

Of course an empty mag wouldn't work.... and apparently my brain wasn't working either. I don't have any snap caps, but I can seat a bullet into an empty unprimed case and use that I suppose.

Every semi-auto handgun I own has done this from time to time. A hard slam of the magazine into the mag well will occasionally jolt the slide stop allowing the slide to close. Glocks seem to do this most often, but even the venerable 1911 has been known to do it. Keep your muzzle in a safe direction at all times.

edit: TAM - - - Take the follower and spring out of a mag and try that.

The Annoyed Man wrote:

Excaliber wrote:Please don't try this little Mr. Wizard experiment with a live round.

Not bloody likely!

Of course an empty mag wouldn't work.... and apparently my brain wasn't working either. I don't have any snap caps, but I can seat a bullet into an empty unprimed case and use that I suppose.

That'll work.

If you go that route, I suggest drilling a hole in the case to create an easily verifiable way to make sure you're working with dummy loads instead of live rounds. Without this or a similar precaution, getting live ones mixed in with supposedly inert ones is easier than one might think over time.

I use the orange plastic dummy rounds myself to avoid this issue.

My HK P30 does the same thing. I consider a hidden feature, faster reloads.

The Annoyed Man wrote:

Excaliber wrote:Please don't try this little Mr. Wizard experiment with a live round.

Not bloody likely!

Of course an empty mag wouldn't work.... and apparently my brain wasn't working either. I don't have any snap caps, but I can seat a bullet into an empty unprimed case and use that I suppose.

You could also just take the spring & follower out of the mag.

The Annoyed Man wrote:Of course an empty mag wouldn't work.... and apparently my brain wasn't working either. I don't have any snap caps, but I can seat a bullet into an empty unprimed case and use that I suppose.

I have some dummy rounds in .45 that a friend made for me. (I don't reload.) They're just the case and the bullet with no primer or powder. I use them to practice reloads since a full magazine feels quite a bit different than empty mags. (Be VERY sure to keep these seperate from your live ammo. You don't want to load a live round in the pistol when practicing dry firing at home. And you don't want to have a dummy round in the pistol when you really need to shoot something either. Maybe use a different kind of bullet, so they look distinctly different and/or paint them with a Sharpie or something. Keeping them in a seperate room from the live ammo wouldn't be a bad idea.)

I don't have any in .40, but I have .40 caliber snap caps. I've tried the test with the snap caps. They're much lighter. I really have to slam the heck out of the mag to get the slide to go forward that way. Just not as much weight to jar the gun as you have with a loaded mag.

Pawpaw wrote:You could also just take the spring & follower out of the mag.

True.

mgood wrote:(Be VERY sure to keep these seperate from your live ammo. You don't want to load a live round in the pistol when practicing dry firing at home. And you don't want to have a dummy round in the pistol when you really need to shoot something either. Maybe use a different kind of bullet, so they look distinctly different and/or paint them with a Sharpie or something. Keeping them in a seperate room from the live ammo wouldn't be a bad idea.)

I have reloading gear for several calibers, including .45 ACP, and I have some semi-wadcutter bullets that I don't use, so seating some bullets in unprimed cases is not a problem for me.

I don't know if other M&P's regularly do it, but it's definitely a known "feature" of the M&P Pro among those in IDPA. My M&P Pro will do it consistently as long as I insert the mag with moderate amount of force.