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Old August 15, 2019, 10:58 AM   #1
jb1023
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M&P 9 barrel COAL variances

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.
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Old August 15, 2019, 11:39 AM   #2
74A95
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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.
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Old August 15, 2019, 07:36 PM   #3
higgite
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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.
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Old August 16, 2019, 08:18 AM   #4
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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.
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Old August 16, 2019, 09:26 AM   #5
higgite
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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.
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Old August 18, 2019, 03:42 PM   #6
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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.
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