Doc , your description was confusing. Polygonal rifling means many sided ,typically 6 or 8 sided with rounded corners. What you are talking about is a grooved , round barrel but with grooves having sloped sides !!Various groove shapes were experimented with in the 19 century. Some for example had rounded inside corners to make it easier to clean , important in BP days.

Questions:

A polygon bore must still deform the bullet ( specifically the jacket) in order to create a gas seal right?

Are you limited in bullet construction type?

Higher gas pressure?

Schneider Rifle Barrels
1403 Red Baron
Payson, AZ 85541
Phone: (928) 474-2852
Hand-lapped, pull-buttoned barrels with superb accuracy. Conventional and 5-land Polygon (P5) rifling options. Good prices ($254/blank). David Tubb uses Gary Schneider's barrels exclusively. Enough said. (6mmBR.com)

I was introduced to Schneider barrels by the late George Fullmer, Oakland, CA BR gunsmith, who built most of my across the course match rifles. Schneider barrels in .308 caliber lasted about 6000 rounds apiece (with barrels set back and rechambered at 3000 rounds) and would still shoot possibles at 600 yards with cooperation from the operator. If the barrels had started out even longer, they might have continued through another set back and rechambering job. I think they're great and Gary is great to work with.