The new rifling is done using a process that is a derivative of EDM machining, however it's a S&W developed process and they aren't providing any details on the specifics. However, I did see one posting that claimed that when the new rifling method was first demonstrated to the ATF that S&W was told that it was "too perfect" and they would have to do something to degrade it enough to allow independent "ballistic fingerprints" on fired bullets. My hunch is that S&W is not etching a small area in each barrel with some type of acid, as for why, there is one small spot in one groove when the reflections of the finish is just slightly "off". To me it looks like somebody used an eyedropper to plant a drop of acid, let it sit a moment, and then cleaned the barrel.
I've seen indications that S&W is, or has, run out of stock for the 38 caliber J frame barrels with broached rifling. Two years ago only the Perfomance Center models came with the new style rifling, now it's starting to show up in the regular production models.
As for what's different, the best description is that the new rifling looks over polished and shot out. Because that is truly how it does look. You won't find a sharp corner because lands and grooves are blended with a radius. You'll also see a surface finsh that is so smooth that it approaches melted glass. However, I have a model 620 with the new style rifling and recently mounted a handgun scope on it due to all the cloverleafs I kept putting up. At 50 YARDS I put 4 rounds into a 7/8 inch group with 3 of those 4 in a 1/2 inch cloverleaf. Then I threw a called flyer when I snatched the trigger and shot a 5/8 inch double to finish the cylinder. IMO my 620 is capable of shooting at 1 inch or less at 100 yards in the hands of someone less excitable than myself and a bit steadier. Sum it up, 3 7/8 inch barrel and capable of sub MOA accuracy means I don't have any concerns about the accuracy of the new rifling method.
However, I will also say that these barrels should ONLY be used with jacketed bullets. I tried 50 rounds of Blaser 158 gr. LRN and it created so much leading in the barrel that it took a full 8 hours of work to get it clean. I expect that in the coming years as this type of rifling becomes universal S&W will do as Glock does currently and recomend that lead bullets NOT be used in these barrels. BTW, some have claimed that they are using lead bullets in these new barrels without leading issues, however all are reloading their own ammo and currently not one of them has revealed what alloy mix or lube they are using to keep the leading at bay.
As for the potential for a velocity gain similar to what is seen in Glocks barrels with jacketed ammo, a close look at the surface finish that results from the new style rifling leads me to believe that it's likely true. If you compare the velocity results fro a 4 inch Glock and a 3.9 inch Sig Sauer in 9mm shooting the same ammo, the glock gains a 100-150 fps advantage. I would expect a similar result comparing two 4 inch S&W's with the 2 different types of rifling, however variables such as the B/C gap would have enough influence that a rather broad sampling would be required to state that as a certanty.
Bottomline, there isn't anything wrong with the new rifling as long as you are shooting jacketed bullets. If you want to cook up your own ammo and cast your own bullets, the older style rifling is a better choice.
As for these new barrels in a J frame, I really don't see any problem at all. Fact is the J frames are Defensive revolvers and I can't imagine anyone foolish enough to use LRN ammo in one if these revolvers when there are so many vastly superior choices available in jacketed ammo's.
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