M&P 9 barrel COAL variances

[jb1023]

I was measuring the max COAL on a few M&P's, using the plunk method, and was a bit surprised at the results of one of my barrels. I measured 5 different bullets in 4 barrels. I don't expect all to be identical but one barrel is not even close to the often published 1.169", with this or any bullet measured.

As I stated, I measured 5 different bullets but my examples below will be for the Zero Precision 125 Gr. JHP.

The longest measured COAL was from a Shield 9 with a max COAL of 1.173".
The shortest measured COAL was from a M&P9c with a max COAL of 1.097".
I understand these are not competition barrels or anything but .076" seems rather extreme to me. The next closest measured COAL was from another M&P9c with a max of 1.146".

I hope to make it to the range next weekend and will be testing various jump to see if it makes a difference in the longer COAL barrels. I'm hoping I can just load to the shortest barrel for everything and be good to go. I know in precision rifle .076" can make a huge difference so I'm curious to see if it does in pistol also, but hoping it does not. All are just for plinking, nothing over 7-10yds.

[74A95]

Yeah, the M&P9s have notoriously short throats, and a short-nosed bullet profile might mean the OAL has to be really short to fit. In one of my M&P9 barrels a Sierra 125 JHP bullet has to be below 1.023" to fit.

[higgite]

That 1.169” that you referenced is the SAAMI maximum COL for 9mm. The SAAMI minimum COL is 1.000”. As far as I know, SAAMI doesn’t specify chamber throat/leade length. That’s left up to the manufacturer. It’s not surprising that 5 different bullets plunk at COLs that span .076” in 4 different barrels. Of greater interest, I think, is the span of COLs that each style of bullet plunks at in the different barrels. That’s more of a direct barrel to barrel comparison.

FWIW, I make it a practice to load ammo that plunks in my shortest chambered firearm of each caliber that I load. Plinking at 7-10 yds, you’ll be hard pressed to tell the difference in trajectory between pistols. More likely to influence POI is whether the pistols are sighted in at the same yardage at the factory and/or have the same sight picture, i.e., 6:00 o’clock hold vs mid-target hold.

[Unclenick]

There is a standard SAAMI throat. If you look at the specifications it is in the chamber drawing under the cartridge drawing. It's a 2° taper about 0.15" long. For more detail, go past the chamber drawing to the pressure and velocity test barrel drawing in the second half of the standard. However, there is no law requiring anyone to follow the SAAMI standard. It's voluntary.

[higgite]

Yeah, I guess I could have worded that better. Thanks for correcting me, Nick.

As far as I know, SAAMI doesn’t specify throat length per se, but I’m not a SAAMI expert. I know they spec the throat taper, which can be used to calculate throat length, but that length is dependent on the diameters at each end of the taper which have their own tolerances. Test barrels have much tighter specs than production barrels, but the production barrel spec allows a pretty wide range of throat lengths, relatively speaking. That’s why I mentioned it’s important, IMHO, to plunk test each specific brand and style of bullet in all of your barrels of the same caliber so can can make an informed decision on the COL or multiple COLs that you want to load that bullet to. Thanks again for keeping me on the straight and narrow.

[Unclenick]

They give a length from the breech to the end of the throat taper of 0.9258" with a +0.012" tolerance. That tolerance is probably what throws people because the length from the breech to the case mouth seat at end of the chamber (the headspace) has a 0.022" tolerance span, and the two tolerances are independent of one another so you could have them accumulate in some guns and with a short chamber and long throat see about 0.034" difference in the throat length compared to having a long chamber and a short throat. But that's not enough to account for what the OP is seeing.

I note the CIP dimension for throat length is 3.35 mm or 0.1319" rather than our 0.1498", so a short European chamber could have a throat that is 0.0519" shorter than a long American one. But that still isn't enough to account for the 0.076" difference the OP is getting. I suspect that if he slugged the chambers he would find his tight gun's throat is on the narrow end, as well as having the CIP shorter chamber, and that his longest throat is wider. The tolerances allow up to 0.005" of diameter difference. For a bullet ogive whose radius is near tangency with the bearing surface at the point of throat contact, that could make for quite a bit of additional seating depth difference.

[jb1023]

Thank you for the info guys. Sorry, I forgot that notifications on this site go to a seldom used address, will be changing that.

I decided to just load everything to the shortest oal so there would be no issues. I tested a few in all guns and did not notice any significant difference in accuracy. I have been contemplating mapping all my chambers, just because it sounds interesting, and in this case could answer a few questions.

[44 AMP]

I was measuring the max COAL on a few M&P's, using the plunk method,...

NO, actually, you weren't. The "plunk" test doesn't determine the max COAL (Cartridge Over All Length) that will function through a firearm. It just determines what will properly fit into the chamber.

AND, the max COAL of what ever you are plunking only applies to what you are plunking, meaning the specific bullet used. Change to a different bullet with a significantly different nose profile and the max COAL of what can plunk acceptably can change significantly.

Remember what stops the round when you plunk it into the chamber. Either the front of the case headspacing on the chamber lip, OR the bullet hitting the rifling. IN either case, it is entirely possible to have different bullets stick their noses (that don't touch the rilfing) different lengths down the barrel.

Both bullets will touch the rifling at the same point, measured from the case head, but that point is NOT the tip of the bullet and COAL measures from the base of the case to the TIP of the bullet. SO, you can have identical results in a plunk test but different COALs.

IN addition to the chamber there is the rest of the gun to consider, when looking at what YOUR gun accepts as the max COAL.

The rounds have to FIT into the magazine (all the way, from top to bottom, and move smoothly), and feed into the chamber. Often a firearm's chamber will accept rounds that are too long (COAL) to feed through the mechanism.

SAAMI (Sporting Arms and Ammunition Manufacturer's Institute) is a voluntary membership group of arms and ammo makers, none of whom are under any compulsion to use SAAMI specs. It is not, in any way a regulatory authority. You will not sail off the edge of the world and be eaten by dragons if you don't rigidly comply with SAAMI specs.

This is not to imply that SAAMI specs are not good, sound guidelines worth due consideration. It just means that if you are could be a little above, or below their values without serious risks. IF, for example a pressure spec is 27,500psi and your load is 28,300psi your gun isn't going to blow up (and if it does, it would have let go with SAAMI spec as well). On the other hand, if you stuff a 100kpsi load in a gun spec'd for 30K and it blows, its not because you were beyond SAAMI spec, it's because you were beyond sanity.

COAL, as an industry standard is set so that length is expected to properly function in ALL modern firearms chambered for that cartridge. Due to the fact that there are tolerances involved in everything, if you look at enough guns you will find a few that will run fine with rounds a little longer than "spec" and some guns that won't run right unless the rounds are slightly shorter than max length spec.

If the ammo properly functions in your gun, tiny variations from industry specs are not significant. If the ammo doesn't function properly, those variations COULD be significant. Every gun is an individual about that.

[Unclenick]

Note that using the same COL with different bullet lengths changes the amount of space for the powder under the bullet. Generally, the less there is, the higher the peak pressure.

Here is an example for 45 ACP. Note that sometimes a small powder space just allows the primer to unseat the bullet before the powder builds pressure, and that can fix the increase, but it isn't good for velocity SD, and you can't count on it happening until it does.