Some jurisdictions, most notably California, require monometal bullets for hunting. They’re also proving to be extremely accurate and effective, but the legal thing drives most of their sales.
Pure lead has a Brinell Hardness Number (BHN) of about 5. Bullets made of it are soft and will foul at velocities much above 750 fps. “Hard cast” is an alloy of roughly 90% lead and the rest a mix of tin and antimony. Definitions of hard cast have changed over time, but today it’s typically a BHN over 18. A lot of cast bullets are also an alloy of those elements but with a BHN below 18.
Top speed for hard cast depends on how well the bullet fits the specific gun, but 2,000 fps is probably about right.
Gas cutting happens when the gas generated by the burning powder slips past the base of the bullet and scores the side of the bullet parallel to the rifling. It ruins accuracy and makes a mess of lead fouling inside the gun, so hard cast bullets often have a gas check, which is a copper cup that fits on the bottom of the bullet to resist gas cutting. This can raise the usable velocity of a hard cast bullet to about 2,500 fps.
Velocity, plus the massive range in bore/throat/leade dimensions essentially rules them out for the 7.62x39 and 5.56 NATO. You could do it if you wanted, as long as you kept velocity below about 2,500 fps.
Buffalo Bore is in the business of selling ammo. That will always color the information they provide. The line about fouling polygonal barrels goes back to an early issue with Glocks. People who didn’t understand how to fit a cast bullet to a Glock barrel got a lot of leading, which ruined accuracy. Glock issued a blanket recommendation against using cast, which stands to this day. People who fit cast bullets to Glock barrels don’t have these problems.
Every bullet fouls something. The bullet lube required to get cast bullets to work properly creates a significant amount of smoke, which leaves a greasy film on the gun.
You can go down any number of rabbit holes when it comes to cast bullets. If that’s your thing, then head over to
https://castboolits.gunloads.com/.
Also, @
Outpost75 could answer a lot of your questions.
Okie John