OK, OK - "Uncle"!
OK, I agree that it is a bad idea to convert .45LC to .454 Casull. Bergmen's SAAMI dimension are pretty sobering. If I read correctly, the cartridge diameter is given in two places, 1) near the base/rim after any deformation from that is healed and 2) near the bullet crimp after any deformation from that is healed. Again, if I read correctly, the .45 LC firing chamber is bigger in diameter, as follows:
Location: ......Near Base .......... Near Crimp... Case Taper
.45 LC.......... 0.4862" ............. 0.4806" ....... 0.0056"
.454 Casull.... 0.4797" ............. 0.4779" ....... 0.0018"
Difference: ... 0.0065" ............. 0.0027" (.45LC is LARGER)
These (above) are from the bottom panel of each cartridge's SAAMI drawing. Apparently the top drawing panel for each cartridge gives a "CYL" dimension, which I take to be a nominal or average diameter. These are:
.45 LC.......... 0.4800"
.454 Casull.... 0.4775"
Difference: ... 0.0025" (.45LC is LARGER)
As I read it, all diameter dimensions are toleranced to allow for up to 0.004" larger than given. So the .45 LC chamber diameter could be as much as 0.0065" larger in diameter than a minimum-diameter .454 Casull chamber. And the minimum diameter of the cartridge casing could be even smaller than that, by how much? Another 0.004"? (That call-out of the tolerancing is an odd one, if my presumed interpretation is correct...)
OBSERVATIONS:
a) The .45 LC case taper is steeper than the .454 Casull's
b) If we take the 0.004" larger-diameter tolerance as some sort of a low-side indication of how much the case expands during explosion of the powder, then it might be reasonable to speculate that a .454 Casull case will expand to seal/grip in a .45 LC chamber. (This may strain and work-harder a .454 Casull casing faster than would happen in a true .454 Casull chamber...)
I have a .454 Casull Rossi M92. I might be able to measure what Rossi/Braztech is cutting these chamber diameters to. All I have is dial calipers, however, not really the correct measurement tool... But if it can reach into the receiver/chamber port, I will report the inside diameter of the rim-end of the chamber.