I may have missed it but I always size my bullets to fit the cylinder throats not the bore as the throats are usually or should be slightly larger in dia. If the bullets you have slip through the cylinder throats easily a larger dia. bullet is in order.
Yep.
Bullets sized to fit SNUG to the throats at low pressure shouldnt need lube at all, assuming the gun finish isnt rough. Your bullets are too small and too hard to bump up to make a sealed base at your loading pressure. Go softer for hardness (9-8 BHN) or use a bullet snug fitted to the throats. .454-456 in your case. Id be using a bullet that will push through with some force if staying with hard bullets or barely slip with soft 8-9 BHN.
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The sooting on one side of the case is from TiteGroup being position insensitive. Very often, even with loads beyond 20,000 psi. it will still exhibit case fouling on one side and even case obturation down one side; its especially noticeable with carbide dies.
Remington bulk 255's are .454 and soft as well. Try them as a building block to good accuracy without leading and go from there. The trick with lead bullets is leaving a tapered path for the bullet from case mouth to muzzle, always having the bullet base sealed and supported by steel. Lube and hardness are secondary factors to the all important seal/fit.
