Just because it’s on the internet doesn’t make it true.
S&W standard 9mm barrel twist rate was 1-18.75 or 1-16, (depending on who you talk to.) Five grove, cut rifling is still the standard I believe for all S&W barrels.
I understand on the post 2011 M&P barrels they’ve switched to a 1-24 to try and get better accuracy with 115-124 gr. bullets.
(longer bullets require faster twist’s and will tumble in slow twist barrels. Shorter bullets don’t seem to mind faster twists, so manufactures normally use a twist to stabilize the heaviest bullet their customer may use.)
Most firearm manufactures use the old Greenhill formula to determine twist rate. It’s a mathematical formula taking into account the bore diameter, bullet length and velocity to come up with an “ideal” twist rate. You can use this formula for almost any caliber from 17 caliber to artillery and you’ll find that whatever twist rate comes up, that’s pretty close to what all the manufactures have been making for years.
Except 9mm Parabellum. Using this formula, the “ideal” twist rate for a 9mm with a 147 gr. bullet would be 1-22” yet manufactures can’t agree on what’s an ideal twist rate. Most American companies use a 1-16” but european companies use much faster twist rates. Glock, Sig and H&K use 1-9 to 1-10 and I’m told the original P-08 Lugers used a 1-7 twist.
Looking at aftermarket match barrels, we see there are basically two choices. 1-16” to stabilize even 160 grain lead bullets and 1-24” for folks building open class race guns that will only see 100-125 gr. bullets.
Jeff