You state that the caster you ordered from has a lot of LBT moulds, so I assume it is Montana Bullet Works. If so take his recommendation but get trial quantities of his or anybodys cast bullets in the future.
No lead or jacketed bullet will open up cylinder throats, so drop that concern.
The important issue is that the ammo chambers without much more resistence than the rounds weight. If you have to force them in the cylinder won't rotate freely which is hard on the mechanism at best and could cause the cylinder to lock out of alignment with the bore. Also, if the ammo has considerable cylinder drag so the rim isn't against the cylinder when the hammer falls, impact on the primer can be dampened, as the cartridge moves forward, enough that you'll get a misfire. Which in itself isn't dangerous if you wait a half minute or so before rolling the cylinder to the next chamber, in case it hangfires.
A good rule of thumb is: The larger cast bullets are the more accurately they will shoot, IF THEY CHAMBER EASILY!
That's a rule of thumb, not a law carved into rock. The part IF THEY CHAMBER EASILY IS a law carved into rock! Don't break it!
