There are the few of us that have known for a long time that sabotless works. Bob Parker built a custom 45cal sidelock to shoot his BEs sabotless years ago.

Tom Post at SwingLock makes a adjustable sizing die for sabotless enthusiasts but im not sure if he makes one for 50cals. I know he makes them for 45cal-6.5mm

Yep,.... he makes a 6.5mm smokeless ML. http://www.swinglock.net/65standardthumb.php 120gr@3400fps

What makes Sabotloader's sabotless testing kind of unique is the powder, cap ignition and caliber. Plus using a solid copper which needs a LOT of pressure for best obturation. He found a 50cal combination that can be pre-engraved enough to impart good rotation and accuracy without using a sizing die or knurling.

Finding a 500S&W bullet that can go from package to bore in a 50cal is a complete crap shoot and pretty much all dies would need to be custom made if your bore is too tight. An adjustable "full form" die works best but they are not cheap. A full form die not only sizes but also pre-engraves the bullet with a cutoff portion of your barrel.

Sabotloader
Im surprised you were able to get that much engraving without a mallet. Im guessing the 6 groove barrel helped. The base on the Barnes XPBs ive measured is also slightly smaller OD than the rest of the bullet. I can start the 45LC-225gr XPB a tiny bit in my Pacnor but it is not going any further without a LOT of effort....maybe im just too skeered to force it.

Ive heard of a few people that have shot the 50-300gr FTX sabotless and BH209 with good results. I was a bit disappointed that my Savage bore is too tight. The Rem 50-385gr CLHP looked like a perfect candidate for sabotless with BH209 and the 1-24 twist. I have a bunch of those bullets left if you ever want to try them. I think i only sent you 3 of them to check for fit.