SK -- I know that many people dislike it. I think there are some genuine reasons to do it:
Gun grabs -- Smith has a mountain of testimonials from police officers whose lives were saved because, while wrestling over their sidearm, they were able to eject the magazine and disable the pistol. When the BG got the gun, he had nothing but a paperweight. Reality has taught us that sometimes proper weapons retention training and techniques won't be enough. While a BG might be able to figure out how to disengage a safety, odds are he isn't walking around with a spare M&P magazine.
Administrative safety -- One benefit of a SFA pistol is that it's easy to shoot. One disadvantage is that it's easy to shoot accidentally compared to guns with longer and/or heavier trigger pulls. While a lot of gun owners and instructors like to act holier-than-thou and proclaim that they could never make a mistake, I don't consider myself infallible. The ability to deactivate the firing mechanism during administrative handling might sometimes come in handy.
Proper disassembly procedure -- I've seen too many ADs caused by people pulling the trigger to disassemble an "unloaded" gun. Well, with the mag disconnect mechanism, this is all but impossible in an M&P.
There are also some arguments against it:
On some guns, a mag disconnect safety can impair the trigger pull. This isn't the case with the M&P.
The oft-cited "ability to shoot in the middle of a reload." Sorry, I've been unable after more than a decade of searching to find a single instance where anyone has had to do this in a fight. First, most reloads during a fight occur at slidelock ... so there is no shooting possible until you put a fresh mag in the gun. Second, if you are performing some other reload (by definition, a discretionary reload instead of an immediate action reload) then you should have enough situational awareness that you aren't going to be threatened during that split second it takes to eject a mag and insert a new one. Third, if you did get surprised in the middle of that moment, your brain isn't going to stop and reverse course ... finish the reload and then finish the fight; firing one shot and then having a gun that needs a new mag and a rack to the slide is going to eat up a lot more time.
It makes dry-firing a pain in the butt. This affects both dry practice as well as the mandatory "unload & show clear" procedure used at IDPA and IPSC matches.
That's a longer explanation than is probably necessary. Basically, I see the advantages (primarily the defense against a gun grab) as outweighing the negatives (particularly the fanciful notion of a mid-reload desperation shot). If I can make the gun a little safer without any loss to practical ability, why not use a mag disconnect?
So I'll confess, my plan at present is to drop the slide, pull the trigger, and leave it at that. With hearing protection on, I'm hoping the ROs won't notice. After all, technically I've proven what they're asking ... that my gun is in a condition that is safe even if the trigger is pulled.
The alternative is that the RO will insist I insert a mag to dry fire. So I will insert a mag, pull the trigger, then remove the mag ... without checking the chamber again after the magazine was inserted. Which creates a greater chance to make a mistake and have a loaded gun, doesn't it?
You then need to holster the gun. The idea is that with the slide open and no mag, they know the gun empty. The redundant check is the "hammer down". At the class I took they said that a decocking lever (on a sig for example) isn't enough. You need to pull the trigger, they want to ensure that the gun is empty, and this is their 2nd check.
I'm fairly new at IDPA so my times don't reflect any loss I get from having a mag-disconnect. I'm not too partial on firing without a mag in anyway, but that's just me. I see the merit of having it and not having it and I think the OP pointed these out nicely. The question for people that carry is one of practicality. If a BG grabs for your gun, you're going into retention drill, either keeping it in the holster or keeping it pointed away from you if it clears the holster. You will either retain the gun or not. With the mag-release, you have the option of dropping the mag and rendering it useless to the BG and yourself until you slam it back in or replace it with your spare (you do carry a spare and practice, right?). Without the mag-release, you can deny follow-up shots by dropping the mag, but it only takes one to kill.
Comparatively speaking, how often will you actually need the ability to fire that one shot without the mag in? I think it's better to have on on a carry piece, but IDPA should allow for those with them to be modified to the same function as those without (i.e. changing the sear-block or simple removal) if they are also produced that way.
"There are two sides to every schwartz." Everything has it's good and bad sides.
Although many people complain about the mag disconnect safety, I specifically requested that my M&P have one when I bought it. There were a couple others on the shelf that did not have it but it was a big selling point for me.
I had not been out of the academy very long and was working nights for a very small town P.D. when I found myself wrestling a drunk on the side of a road for my still holstered pistol. Thankfully, my current agency is much larger and I have much better accessability to backup. But, that's a lonely feeling when it happens to you. A lot goes through your mind and I wish I would have had a mag-disconnect on the Beretta I had been carrying at that time.
YMMV and for those who are against the idea of a mag disconnect, I hope you never find yourself in a fight for your weapon. The guy who tried to take mine was quite a bit smaller than me (I'm 6'2" and 235lbs) and I was scared to no end that he was going to overpower me. I guess some lessons are only learned through personal experience. Stay safe folks.
They were VERY clear that they wanted the click at the classes I took. Those with external hammers were asked to re-do the drills when they used the de-cock levers. Might depend on the local RO I'd guess.
I very much understand the need/desire for the mag-disconnect. For me, I don't carry. So the risk of putting a non-empty mag into the gun by mistake to satisfy a RO was greater.
If not for this, or if more said they had luck with the "no click" at matches, I would have had the mag safety.
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