M&P Pro Accuracy Problems

[Bob Hostetter]

In a discussion on another forum the reported accuracy issues of the M&P Pro in 9mm are being blamed on the barrel/slide unlocking early causing the groups to string vertically and the groups to be high relative to the point of aim. It has been suggested that the issue is made worse with lower pressure rounds because they cause the pistol to unlock earlier. The following statement was offered in response to my question as to why lower pressure rounds would cause earlier barrel/slide unlocking with everything else being equal. Does this seem to be correct?

"Chamber pressure is what keeps the M&P locked in the first place. As pressure decreases, the barrel unlocks.

Lower pressure ammo unlocks sooner because it reaches the threshold sooner.

With a gun susceptible to unlocking early -- unlocking before the projectile has fully exited the muzzle -- the pressure of the round plays an important role. A higher pressure round may provide enough "lock" for the bullet to escape while a lower pressure round may not. That's why, in the M&P's case, it's more a matter of chamber pressure than bullet weight playing a role in accuracy and targeting".

Originally Posted by Bob Hostetter

Curious as to why a low pressure round in an M&P would show greater signs of early unlocking then higher pressure rounds, and what those signs would be? I am always interested in learning something new.

[BillGarlandJr]

I have experience with M&Ps in 9mm, 40, and 45. The 45s have shown themselves to be the most accurate of the 3. That being the case, I don't think chamber pressure has anything to do with the pistol unlocking early or not. If the chamber pressure theory was true then the 45 would be the least accurate since it typically has the lowest chamber pressure. What the 45s have vs. the 9s are bigger, slower moving bullets, which in theory probably allow the barrel to remain locked up longer, resulting in better accuracy. I'm sure there are other factors in play that my simple mind can't fathom, but the above is just my thought on the matter.

Edited March 23, 2012 by BillGarlandJr

[Bob Hostetter]

it didn't sound right to me either but was hoping someone smarter then me might be able to confirm or deny.

[jar]

They're talking about varying pressure in the same problematic gun. I can't really speak to that issue one way or the other. The early unlocking problem has only been reported in the 9mm guns. The common theory is that when they designed the compact, they changed the geometry of the locking surfaces. They then changed the full size barrels also for manufacturing commonality reasons. It seems to be a tolerance stacking issue with the new design in the full sized guns. That's why some are OK and some aren't.

[superdude]

okay, someone will have to explain to me how semi-autos distinguish chamber pressure.

"As pressure decreases, the barrel unlocks." That's a true statement, but one does not cause the other, they are only correlated.

it seems that slide velocity due to the recoil impulse is what drives the slide rearward. the recoil impulse is regulated by bullet weight, powder charge weight, velocity and recoil spring weight (resistance). It seems likely that one could easily get a faster slide velocity and stronger recoil impulse just by gunpowder/bullet weight/velocity selection alone while maintaining the same pressure.

in other words, what does pressure have to do with it? are they just making a spurious relationship because of the simple correlation between pressure and when the gun unlocks during normal operation?

[Gary Stevens]

In a discussion on another forum the reported accuracy issues of the M&P Pro in 9mm are being blamed on the barrel/slide unlocking early causing the groups to string vertically and the groups to be high relative to the point of aim. It has been suggested that the issue is made worse with lower pressure rounds because they cause the pistol to unlock earlier. The following statement was offered in response to my question as to why lower pressure rounds would cause earlier barrel/slide unlocking with everything else being equal. Does this seem to be correct?

"Chamber pressure is what keeps the M&P locked in the first place. As pressure decreases, the barrel unlocks.

Lower pressure ammo unlocks sooner because it reaches the threshold sooner.

With a gun susceptible to unlocking early -- unlocking before the projectile has fully exited the muzzle -- the pressure of the round plays an important role. A higher pressure round may provide enough "lock" for the bullet to escape while a lower pressure round may not. That's why, in the M&P's case, it's more a matter of chamber pressure than bullet weight playing a role in accuracy and targeting".

( Gary) I was not very happy with the accuracy of my Pro 9mm shooting a variety of 115's. However, when I went to a 147 load at about 133 PF the groups became much better. I am going to recrown my barrel and see what happens next.

Originally Posted by Bob Hostetter

Curious as to why a low pressure round in an M&P would show greater signs of early unlocking then higher pressure rounds, and what those signs would be? I am always interested in learning something new.

Edited March 24, 2012 by Gary Stevens