I would like to replace the 4" barrel with a 4.25" barrel
in my MP9 M2 Compact 4".
It seems to fit just fine.

However, I saw another forum says the barrels are flared and will not lock up with the slide (maybe P.C. guns).

Another oddity. Seller measured the barrel as 4.22". I believe they forgot to include the bullet case in the measurement. But sure enough, the barrel sticks out only .22" in front of the slide?

S&W weirdly lists most M&P full size barrels as 4.25" and LE SKU M&P full size barrels as 4.22." Both are over Canada's 105mm length minimum, so I'm not sure why.

The barrels on the 1.0 are "flared" (a slightly larger O.D. at the muzzle). The 2.0's are straight.

As far as the "difference" in length, MGW lists the length as 4.22" (4 11/15" in the description, quite amusing).

Smith & Wesson M&P 40 M2.0 Barrel, 4.22": MGW

Now I have to ask, why?

.. and MGW shows M&P 9 M2.0 Compact Barrel, as 4.07".

Went to the range and the 4.22 fired fine.
Although it shoots significantly higher than the 4".

Now I have new questions. Is there a way to 'adjust' fixed sights (or do shooters just make a mental note)?
(assuming the barrel is not bent?)


And, should I change the RSA spring weight?

Attached Images

20240405 MP9 M2.1 4.22 IMG_5267[1].jpg (147.8 KB, 11 views)

Cleaned the new barrel after my trip to the range and noticed two things:
Original 4.0" barrel has concentric circles on the crown vs a flat face on the 4.2"
The number 29 or 62 is stamped on the 4.2". I assume it means nothing.

Attached Images

	20240413 concentric crown 4.0 inch barrel IMG_5345[1].jpg (91.3 KB, 18 views)
	20240413 number 29 or 62 4.2 inch barrel IMG_5345[1].jpg (149.4 KB, 13 views)

Official barrel length is measured with a rod down the barrel from muzzle to breech face. See what that gets you.

You can drift a fixed sight in its dovetail to correct windage but to change elevation you would have to modify or replace one or the other. Dawson Precision sells front sights in a wide range of heights. You are striking high so you need a TALLER front sight. Dawson shows the math for figuring out how much taller.
Smith & Wesson(R), Gun Sights


Sight Calculator

Those concentric circles on the muzzle are just where the lathe tool was starting to dull (probably a very small chip in the tool) when the barrel was faced to length.