Polygonal rifling in a rifle?

There is no correlation between velocity and accuracy. Velocity increases range and decreases bullet drop and drift, but all other things being equal, this would really only be noticed by long distance shooters. The accuracy advantage of a polygonal barrel is that they maintain their off the shelf accuracy longer. Accuracy does not degrade over usage as quickly. Polygonal barrels are really only beneficial to those that that shoot thousands of rounds through their rifle and expect it to maintain its inherent accuracy---such as a competition shooter might experience.

One reason that polygonal rifling is not used more often is that that it requires a more expensive manufacturing processing.

Rather that trying to rewrite in my own words, I am going to quote an excellent article on the subject, with citation below:

"To understand why it’s generally not a good ideal to shoot non-jacketed lead bullets out of barrels with polygonal rifling, it’s first necessary to understand how a bullet passes down the bore. All bullets are slightly larger in diameter than their nominal caliber, which is defined as the distance between opposing lands. The bullets are larger in order to provide the "extra" metal that gets squeezed into the rifling by the propellant’s expanding gases as the bullet travels down the bore; this extra metal fills in the grooves to provide a purchase for the rifling, thus allowing it to spin the bullet down the bore. Conventional rifling, developed when lead bullets were the norm and muzzle velocities somewhat lower, has grooves deep enough to accommodate the buildup of lead deposits caused by the friction between bore and bullet. Regular cleaning removes the deposits before they become constrictive.

However, because more of the bullet’s bearing surface is in contact with the bore in polygonally rifled barrels, lead bullets, especially when pushed at high velocities, are literally squirted down the barrel, "smearing" the bore with a lead veneer. As this veneer builds up and fills in the gap that normally exists between the bore and bullet, it causes pressure from the expanding gases (now less able to pass through the gap) to increase dangerously. This is not a problem with jacketed bullets because the jacket material is a copper alloy that’s much harder than lead and resists shedding."

In addition to what "Ben" said, there's also the simple fact that a rifle barrel is much longer than a handgun barrel. That means that the process for polygonal rifling would have to be carried out for a longer duration for every barrel. I don't know if it's a more expensive or more difficult process, but button rifling and other forms of rifling have worked fine for well over a hundred years; why move to a different process?

It lets the barrel last longer? Well, tell that to the thousands of gun owners who have rifles that have lasted for thousands upon thousands of rounds of ammunition. If it caused the barrel to last 10% longer, it would last until halfway through your grandchildren's lifespan, instead of just through their childhood. Seems like a waste, to me.

I have never seen that velocity has anything to do with accuracy. Most hand loaders load below the maximum velocity and find that it is more accurate. I know Blackhole Weaponry is big on polygonal rifling for ARs, but I have never tried it.

It is safe to shoot lead out of a polygonal rifled gun, it is just dirty (same with traditional rifling).

As to why more guns don't use polygonal rifling? Well, the biggest reasons are tradition (if it ain't broke, why fix it?) and the advantages to polygonal rifling are insignificant if they exist at all. That means that whatever claims you hear are more hand-waving and woo (superstition) than they are observable fact.

It went away in long barrels quite some time ago because it's hard to do well and offers no advantage.