There may be fins forming on PB boolits but I've never detected any in recovered boolits and I've shot some .434s on my SBHs which to be sure would be oversize.

I'd think the pressure swages the excess lead elsewhere as in tightening up the grease grooves rather than allow fins to form.

There may be fins but I've never seen any.

I'm always fond of quoting Walt Melander, "Make em big. When that pressure hits the base, they'll swage down to what they want."/beagle

The CBA and other mags have had several write-ups on fins on bullet bases, both PB and checked. I have seen fins on gas checked and PP bullets before. I get them with WW hard bullets being pushed hard in the 35 Rem and PP out of pure lead though. The most interesting was a trial with moly coated bullets. Out of one batch, half were barely sized (oversize condition)and lubed and the other half were moly coated in as cast condition. These were loaded rather warmly and shot. The conventional lubed bullets had fins, the moly coated didn't. Seemingly, it has alot to do with bore friction. The lube has alot to do with alot of factors, I hope some group comes up and out with all of them someday.

I don't know if this would apply to this discussion or not, but...

When I got my 351 Winchester, I sized down some jacketed .357 bullets. First through a .355 Lee sizer, then a Lee .351 sizer. Going from 357 to 355, there was no discernable difference in the bullets except the size. After going through the 351 sizer, though, the copper jacket smeared down and left a noticable ridge at the bullet base. It cleaned off easily with a file, but the point is, it took a LOT of swaging to create that "fin." FWIW.

I can only marvel at the level of intelligence on display here. After years in the dark ages with only a couple of "casting books" to spread too little light( butI have only praise for Col. Harrison) , we now have a group of inspired individuals seeking to perfect this art. If we could but pool our knowledge/experience..... This gives me a thought for a new thread. dale