I'd been thinking about trying a 1:10" barrel in my M&P 9mm 4.25" and uechikid posted that they were available at Midway. Well they were sold out in the blink of an eye. So I'd like to give you my thoughts and experiences on my barrel.

My M&P's spent bullet casing says Jan 2013 but it also says Rifling: 5 RH. After doing some research I round that this means it's a 1:18.75" right hand twist. After doing more research I found out that the following aftermarket barrel makers use the following twist rates:

Storm Lake Barrels - 1:16"

KKM Precision - 1:20"

Bar-Sto - 1:16"

I don't have any accuracy issues with my M&P but I did initially until I found the right ammo to use and that's 115 gr FMJ or JHP. The 1:10" barrel seems more suited to 147gr. I understand there was also a 1:24" factory barrel produced. Rifling is a complicated subject and I'm not an expert. My other FS handguns definitely prefer heavier 9mm and they all have a 1:10" twist but my Sig P938 works extremely well with 115 gr 9mm but of course it has a short barrel. European guns all seem to use a left hand twist too as opposed to US right hand twists. Supersonic ammo I understand also affects twist rates and I believe the reason S&W chose 1:18.75 was because that's their standard revolver twist rate. To sum up I'm happy with my barrel and believe S&W got it right first time but obviously the ammo is more of an issue than the barrel. However I would like to try the 1:24" barrel or the 1:20" KKM. Your input would be appreciated

In general, the longer the bullet and the slower it travels in the barrel, the more twist is needed. The original S&W twist in the M&P, and all the aftermarket barrel twists, are well within the theoretical twist given by various formulas such as Millers.
Miller twist rule - Wikipedia, the free encyclopedia

I shoot my pistol bullet handloads at 100 yards to confirm they are stabilized, and both 115gr and 124gr 9mm bullets loaded to minor power leave nice round holes at 100yd from my M&Ps. Associates with KKM barrels have no problem with bullet stability with 115 and 124gr bullets.

IMHO, isolated complaints of accuracy problems with 115 and 124gr bullets in M&Ps had zero to do with barrel twist rates. I won't bore you with anecdotes of shooters who bought even very expensive match barrels for M&Ps to "solve their accuracy problem" only to find it didn't.

In theory, a 147gr bullet loaded down to barely make minor power is marginally stable at twist rates around 1:18. I don't load 147gr 9mm and will leave that discussion to someone else.

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Science plus Art

S&W has always used a 1:18.75 rifling twist on anything even remotely close to .355/.358.

Supposedly, they recently went to 1:10 (fairly standard European twist for 9mm) in the 9mm. If so, probably due to whining by those shooting 147 gr bullets at air rifle velocities. The 1:18.75 did really, really well with 147s at 900 + fps.

I'm not smithing anymore, but I've never seen a 1:20 or 1:24 twist in 35 caliber pistol barrels. Come to think of it, I've never seen it in rifle barrels. I'd think you'd have to drive bullets at high warp factors to get stabilization.

I've always been fascinated at the technical stuff people use to explain their failures to apply the fundamentals of marksmanship to achieve good groups.

I have an M&P9 Compact, made in 2008 (18.75 twist rate) and 25 yard groups have always been minute of pie plate (off a sandbag), no matter what 115 grain range ammo was used. I took a chance on a Storm Lake barrel, as I was told I could return it, if not happy. After two days of testing all types of 115 grain ammo, not any better than the old barrel. Returning the Storm Lake.

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NRA R.S.O. & Life Member

Personally, I prefer the slower twist rate over the fast. Less chance of leading with hard cast bullets.

About aftermarket barrels, Bar-Sto and several other aftermarket barrel makers have made a business making replacement barrels somewhat oversize at the breech ends. This improves repeatable barrel/slide lockup and, at least theoretically, should improve accuracy.

Minor differences in rifling twist aren't really going to have a significant effect on accuracy at most pistol ranges. Improving the repeatability of barrel/slide alignment can have a significant effect on accuracy.

Flip flopping the twist rate is a smoke screen. The 18 twist has been stabilizing 158 gr bullets at 850 fps or less since about 1899, and it should surely handle a 147 at that velocity for Minor power factor.

Barrel fit is the key to accuracy.

I remember reading an article about twist rate but it pertained to ballistic performance of defensive ammo and not accuracy. A round was less affected by barriers when the same round was fired from a barrel with a higher twist rate vs. a lower twist rate. For example a round fired through a windshield showed a lower angle of deflection when it was shot out of a barrel with a higher twist rate. An other example is if a round had a tendency to turn yaw or turn backwards when passing through ballistic gel, that would be cured when fired through a barrel with a higher rate of twist. In the end the defensive ammo performed better with guns that were equipped with a barrel that had a higher rate of twist.

Now that I read "Millers Twist Rule" I completely understand!

My head is making that cartoon noise like when the character gets REALLY confused!!

I will just trust what you say, That's WAY to much math for me, although it was an interesting read.

LOL

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Criminals don't register guns.

David Sams, who builds all my Bullseye guns, made two 9mms for me both with slow twist KKM barrels. If memory serves they were slower than 1 in 20". One gun was a highly modified Model 92 Beretta for Ball Gun shooting and the other was a 1911 in 9mm for Centerfire Match. Both guns shot sub-1.5" groups at 50 yards all day long with 115 grain XTP loads using 4.2 grains of Vit N340. Since I'd been reading this forum well before him building these two guns for me I was familiar with the discussion on twist. After relating it to him all he could do is shake his head. He (and other pistol smiths who also built match grade 9mms) had arrived at his conclusions based solely on results. He just kept experimenting with twist until he got the results that he wanted. He didn't let the traditional approach to twist get in his way. Similarly, with my first M&P9 which had the 18.75" twist barrel, my accuracy was **** until I had the PC install one of their new 10" twist barrels. Accuracy with my 115 grain XTP loads and AE 115 JRN significantly improved as well. I'm now quite happy with the accuracy of all my M&Ps.

Keith