I picked up my M&P 9 FS two weeks ago, but haven't got to the range yet. What round is better to use during the break-in period, 115 grain or 124 grain?
And how many rounds constitute the break-in period?
Thanks.

I don't think it matters, whichever one shoots best in your pistol. By the time you get a box of each though the gun, you'll know if it's dependable or not.

Any factory load will be fine.
Standard pressure might be better.

There is no specified break in time nor specific break in load.

I always clean, inspect, and lubricate a new gun, and disassemble and clean the magazines. I have found some unpleasant surprises, like gobs of lube, no lube, and mag springs backwards.

Then I shoot the new gun using my standard range loads, minor power 124gr is what I use, or use any good commercial load. I'd avoid bargain ammo and gimmicks.
Clean and inspect after each shooting session, and note any problems, marking a mag that acts up.
You may find, as I usually do, that the gun is A-OK from day one!

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Science plus Art

I use 115 but mostly because I like the less recoil feeling.


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I've only used 115 ammo in my MP 9's with no issues; and to be honest cant say i have noticed any change or improvement in a gun after any "break in" period.
Now, me improving as I get used to it? Sure, but that's another story lol

IMO, handguns should not NEED a break in period. All of my handguns (all S&W except my Ruger SR1911) have operated fine out of the box, provided they are cleaned and lubricated prior to the first range session. My Shield 45 was the only pistol I've taken to the range without doing this, due to time restraints from purchase till time range closed. And still, it operated flawlessly out of the box. Proper care, cleaning and maintainance are key in a pistol operating as it should, especially anything that is carried for SD. Any standard loading should be fine in your new M&P 9, some bullet weights may shooter better/tighter groups than others. Grab a few boxes and give them a try :-)

Doesn't really matter which.

I don't buy into break-in. Modern guns don't need them.i just shoot. You can call it break in or wear and tear

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I think after a 100 rounds or so my trigger got a little smoother and the gun was smoother/easier to rack. But it has worked without issue from the word go.

A critic could argue that I just got use to the trigger and it didn't change just my perception of it, they might be right...

I use 115 all the time.

Use what you plan to shoot and/or carry. Get used to the gun from your first shot. In fact, your first shot may be your best.

BOTH!

If the gun needed to be broken in it wouldn't matter how much the bullet weighs...

Put 9mm ammo in the gun out of box of factory ammo. Shoot it. Have fun.

It matters not, what grain you choose.

There is no break-in period. Clean and lube as per the manual, shoot and enjoy.

I think many people agree that the recoil springs are really stiff when the gun is new. They loosen up some over time. While those springs are stiff, it takes more energy (recoil energy) to operate them properly. So, while the gun is new, using ammo with more energy is good. Using ammo with less recoil is not good. Its not really the weight of the bullet that matters. Its the recoil energy.

Just in case you haven't got the message yet; there is no break-in period.

Shoot whatever load you want. Have fun with the gun and come back and tell us how much fun you've had.

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Whichever is cheaper.

What the other guys said....no break in required. Best bet is to spend some time reading the Owner's manual, and note that is says it has a Slide Stop, not a slide release. Most new shooters complain that they can't release the slide using the Slide Stop...that's because it isn't intended to be used that way. That's probably the first complaint made about the gun, then complaints about the trigger. The trigger gets better with use.

Enjoy your new gun!

I have not even shot my M&P 9 FS yet, just got it, but I am too excited and I just have to carry it. I am carrying 127gr +P+ in it, and I will shoot that as my defense load, so I will start it off right from the beginning. I don't really believe in break-in period with today's guns.

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I do believe in always shooting good ammo. None of the cheap **** they sell for $10 for like 100 rounds. Stick to good name brand ammo for the life of the firearm. Clean it after every trip to the range.

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I prefer to use 115 grain...

Get yourself a couple boxes of 115 grain FMJ ammo and head to the range. After 100 rounds you will be broken in, after the 1st shot your pistol will be good to go.
Get out there and have some fun with it. Its a great pistol!

The only thing that happens during break in of a pistol is those parts that the builder didn't take the time to stone and polish so they mate together properly, wear in by rubbing together. If manufacturers spent more time fitting parts vs slapping two parts together there would not be any break in at all. Unfortunately old world craftsmanship is gone forever in the mass production of firearms and most everything else these days.

Depends!

It isn't common knowledge; but brand new polymer firearm frames need to, 'relax' and take something of a, 'set' before they can be trusted to perform reliably. The best way to do this is to repeatedly fire the gun. With a brand new polymer frame pistol I'd, personally, recommend breaking it in with a minimum of 500 rounds. (On pistols I've had some sort of a problem with I've gone as high as 1,000 fired rounds before carrying.)

April, 2016 issue of the NRA's premier magazine, ‘Shooting Illustrated’. On page #10 there's an article titled, ‘Plastic Fantastic’; and I quote:

I've run new guns, usually without failures with 115 gr Name brand range ball ammo. Winchester White Box and the equivalent. But a young gentleman of youth and enthusiasm who plays King of the Swampy Desert in competition has gone all 124 gr because a batch of 115 gr didn't make minor. Fortunately it wasn't a record match, just a local club, but he is all 124 since.
Geoff
Who doesn't blame him, imagine getting DQ'd for ammo before the match!

Why only "factory ammo"?

Because there are idiots that don't know how to reload properly and their rounds can cause malfunctions.

If someone can actually reload correctly and the rounds run fine then great. I would never run reloads in a SD gun, that's just not me.

In last 12 months have bought 2 brand new 9mm semi-autos, a M&P 9L Pro and a G17, and used both 115 and 124 grain ammo in the first hundred rounds through both with no difference in function. My LGS sold me some 115 grain Aguila with one of these and said something about it cycling the slide faster but I've not noticed any difference.

Think that S&W customer service probably says for all their new firearms that a break-in period of 300-400 rounds is recommended but functionally all semi-autos should be 100% out of the box except for a few minor issues like sometimes not holding open on the last round or slightly heavier trigger weight than advertised. Such minor issues usually improve as the surfaces of new parts wear in on each other.

Not just "factory".

For me it is a matter of being prone to break anything I touch (so guns tend to go back to the factory for repairs of all sorts). The owner's manuals do tend to advise you use factory ammo / saami-certified, and may advise against aluminum / steel case, no reman or reload, no +P+. Some, like HK, will tell you the warranty is effectively voided by certain ammo. It is up to you. If the gun has a failure, and you have to send it back, you will probably be asked what ammo was in use, and the answer may determine if they handle it as warranty, or expect you to pay.

I have settled on a few types of ammo that meet the specs of all the manufacturers, and actually work in the guns I have, so that I don't have to worry about being caught with a gun / ammo combo that doesn't work.

Good example - my EMP 9 will not run ZQI Nato rounds due to the very hard primers. Every other gun down to the Shield run it without issue. The EMP 9 may get sold, depends on how long those cases of ZQI last. It has been back to SA twice, they say it is in spec, and they don't like ZQI. Oh well ....

I am up to loading and shooting about 260,000 rounds since 1997. I estimate I have only purchased 5,000 factory loads in the same time frame.

With the money I have saved I could by 20 of my $500 Glocks or M&P's and maybe a handful of my 1911's and AR's.

Every pistol I have ever owned including Beretta, Glock, M&P, Walther, H&K, Sig, Kimber, Springfield and Colt have fired my reloads without fail. I trust my loads and will continue to do so.

I remember a few years ago when ammo supply dried up, the only people still shooting at the matches were those of us who reload.....ahhh, good times.

If supply of your favorite/approved ammo dries up, your guns will be paperweights.

Agreed, I reload for competition and practice but carry factory SD rounds.

There have been a lot of comments stating that modern pistols don't need a break-in peroid, and while this has largely been my experience as well, the fact remains that a lot of pistols will experience a couple glitches early on, and then "break in", and run flawlessly ever after.
IIRC, Ruger advises a thousand dry-fires with their revolvers to help them "break in", and the Rock Island's 1911 manuals tell the owner to fire a full 500 rounds, before even calling them about any issues.
But again, when I immediately run a couple hundred rounds through a new toy, I'm not so much trying to break it in, as I am just confirming 100% reliability, though I'll still use the general term "break in".