[Unclenick]

I assume from the word "pure" you mean Pb as in lead and not PB as in plain base, nor PB as in patched bullet, nor PB the IMR powder. Lots of PB initials in shooting. I even know shooters who take PB sandwiches with them for lunch on a long match day.

Lots of alloy cast bullets have gas check bases molded in. Some argue the harder alloys need them more than soft alloys or pure lead because a hard bullet is not so easily upset by pressure back up to groove diameter after passing through a bore constriction as soft lead is, thereby leaving gaps for gas cutting to occur. A number of example of soft bullets sealing the bore well exist. Indeed, Elmer Keith developed the 44 Magnum predecessor loads for his Mode 21 S&Ws with 20:1 and later 16:1 lead:tin alloy, both of which are soft by hard cast standards. About BHN 10-12.

As for the coated bullets, the coating fills grooves well and prevents gas bypass, so you wouldn't normally need a gas check with it. That would change with high enough pressure to strip the coating off. I don't know for sure what happens pushing coated bullets to rifle velocities, as I haven't done it. I also don't know how well powder coated rifle bullets shoot in general. You need good mass symmetry for accurate rifle bullets, and any unevenness in the powder coat would potentially throw it off.