Slugged M&P 9mm Barrel

[BigMacMI]

So, I slugged my M&P. I had seen that the lands/grooves were opposite of each other. I did try to grasp the widest part of the slug, and was able to determine at it's widest part (basically catching the edge of 2 opposite grooves), I was getting 0.3555. If I am casting, i want 0.002 over this? That would put me real near 0.357 for a cast bullet.

I plan to powder coat, which I gather adds 0.001 to 0.002 to bullet diameter.

Getting first mold. Would you think 0.356 and then PC it?

I am hoping for the holy grail that I can shoot after coating without sizing, but will get sizer as needed.

It stands to reason 0.358 would be a bit aggressive for diameter before PC

 

[ewlyon]

in my experience powder coated bullets size pretty easily. If you are starting with a lee mold you might get one that is right at the stated 0.356 but you might get one thats a bit off the mark or see decent variation between cavities. I would suggest getting one of the lee sizers early on, but you might as well get a 0.356 mold and see if you get lucky.

 

[LiveLife]

Make sure chamber will allow rounds loaded with .357"+ sized bullets before using larger than .356" sized bullets.

More generous factory chambers will fully chamber .357"/.358" sized bullets but tighter chambers will only allow .356"/.357" sized bullets, especially with thicker walled cases.

 

[Wreck-n-Crew]

The average 9mm pistol barrel ranges from .3550" to .3555" and shoot .356" lead or coated bullets with good accuracy and no leading. My 9mm Bullets drop from .356"-.357" depending on mix. I don't need to size them before coating from either mold. Just coat, bake, then size with ease. Go ahead and get a .356" sizing die and size them anyway. Some people size before coating but I've found they size much easier after coating. YMMV

The best lube I've found for molds is 2cycle "full synthetic". I use a dropper and a Q-tip to lube the molds well keeping any out of the molds themselves. If they begin to get a little sticky during a long casting session I use the dropper to put a drop on the alignment pins and sprue bolt's.

 

[longdayjake]

M&P barrels have recently left a bit to be desired. In order for me to get good accuracy out of them, I've had to load .357 bullets. I've shot three M&P barrels recently and all of them chambered and shot the .357 bullets well. That said, I switched two of the barrels out anyway. GOTTA HAVE THREADS!!!

Make sure chamber will allow rounds loaded with .357"+ sized bullets before using larger than .356" sized bullets.

More generous factory chambers will fully chamber .357"/.358" sized bullets but tighter chambers will only allow .356"/.357" sized bullets, especially with thicker walled cases.

 

[rsrocket1]

My M&P 9 barrel and Shield 9 barrels slugged at 0.3545 and my M&P 40 barrel slugged at 0.400 so yes, S$W M&P barrels tend to be fairly tight. More importantly, the throats are also very tight so a 0.401" bullet barely fits in the throat and a 0.355" barely fits in the 9 barrels. That means if the bullet has a straight shoulder sitting above the rim, it had better be narrow enough so as not to hang up in the throat. A simple plunk test will tell you whether or not your loaded cartridge will feed reliably in the gun.

I use the Lee 401-175-TC and 356-120-TC lead bullets in my M&P's as well as the TL356-124-2R. As you can see in the picture of the molds
the max diameter of a sized bullet will sit above the case mouth if the bullet is seated to its normal length. To prevent hang ups, you need to ensure that part is narrower than the throat. The TL356-124-2R can be seated to the top of the TL grooves and will not hang up in a narrow throat because the nose of the bullet is stepped down a lot. The truncated cone bullets can also avoid hangups if you seat them deep enough.
Here's what I do to ensure easy feeding with the 40:

You still must ensure the case mouth does not hang up when doing the plunk test. I do slightly flare the case mouth prior to seating and lightly taper crimp the mouth but I make sure the case mouth will catch on the step between the chamber and the throat because that's where the headspace takes place.

The problem with 9mm is there is so little room in the case to begin with that a deep seated bullet can send pressures beyond SAAMI specs with fast powders. Slower powders have a little more margin of safety so long as you don't run into problems like too much powder and not enough space left in the case.

 

[BigMacMI]

Good info. I was more concerned about the bullet not being big enough! I like the 124g out of the MP currently. I guess I will have to pick one up and see how it goes. I would like to be able to make minor with the cast bullet... so hopefully don't have to lose too much case volume.