I just bought a 6" HB (Ejector Rod Shroud - no full underlug) for a 629. It has usual roll-stamped markings and RR front sight. It also has the cross-cut at top of threaded shank for the crosspin, so I assume this is an older (pre-1982) bbl. It has approx. 1:20" twist rifling. (haven't measured it yet, but lands appear to go around close to 1/4 to 1/3 of the bore from breach to muzzle, making it somewhere around 1:18" to 1:24" twist. I expected it to be the industry standard 1:38". I actually really like this, but it raises a question for me: When did S&W change rifling twists, if ever??

Standard twist rate for ALL S&W revolvers except .22 is 1:18 3/4". That said, X-Frames may be exceptions too.

Thanks, Alk8944. Yes, you're right about the X-frames, I know the 460's (at least--not sure about the 500) have gain-twist rifling.

I find it strange that S&W would use the same rifling twist regardless of caliber. Doesn't follow SAAMI (which isn't mandatory, but it's wise, in this litigious era). It also can't be most efficient with the wide range of bullet weights across so many calibers. I'm sure they studied the problem and noted that the length to caliber ratio (sectional density) for the range of "normal" bullet weights in most calibers is roughly the same, and picked an appropriate twist, but I'm surprised it would apply across so many calibers from 32 to 45 (or higher, depending on the 500's twist).

And, they've done it this way forever?

Most 44 Mag rifles, Ruger, Marlin and Winchester use a 1 in 38" twist.
As does the 444 Marlin.

Which IMHO has always been a mistake.

That is one reason their accuracy seems to fall off past @75 yards.

They all had Micro Groove rifling as well.

The new Marlins with Ballard rifling might have a different twist now, I do not know.

I believe Win switched to a faster rate before they shut down, but I know Marlin switched to 1:20 on their 44s in the early 2000s.

I was pleasantly surprised when I looked down that older S&W tube!

MMA10,

It hasn't anything to do with litigation. It started "way back when" when S&W was using Sine-Bar rifling equipment. They simply used the same set-up for all calibers except for changing the rifling head to accommodate different calibers. Basically, yes, they have done it that was "forever".

There is a second reason, and that is because most standard-for-caliber bullets for .32/.38/.44 etc. have very similar sectional density which is what dictates twist rate, not bullet weight or diameter.

.312 120 gr, SD .175
.358 158 gr, SD .178
.430 246 gr, SD .190

Note that the original bullet weight for .32-20 was 115-120 gr, not the currently common 100 gr. Within reason, the issue with twist is being fast enough for the bullet and velocity being used. Lighter bullets are fine with a twist for a heavier (higher SD) bullet.

So far as the 1:38 Marlin twist is concerned, it works fine for the standard weight for the .44 Magnum, 240 gr. The stability issue arises when heavy-for-caliber bullets, more than 290 gr, are used. This is why Marlin/Remington stopped at 265 gr. in the heavy .444 Marlin that used the same 1:38 twist.