A .458"-diameter right cylinder displaces 41.66 grains of water per inch of length/height. If Bob loads his .45-70 LT 0.3" longer than his .45-70 Govt. load with a .458-caliber bullet, he has added (0.3 X 41.66) = 12.50 grains of water in effective capacity. H335 to fit in that extra space will weigh more than that since it is denser than water.
At that length of COL, Bob has reached the end of his throat in the .45-70 LT. With the .45-70 LT+, using a .458 WM throat, further effective case capacity increase as well as greater pressure let-off are possible. With the excellent accuracy record of the SAAMI .458 WM, what is to lose ? .
Sir Ron;
Some clarification: I'm not 100% sure that the throat was extended by only 0.30". It may have been a bit more. I cite that number because that's what my gunsmith said, in his words (because I sent him some dummies): "How long do you want me to go, it's ABOUT .30" now." I told him that should be enough. I've never actually measured it. I simply wanted to be able to load the long bullets to 0.25" into the case without being jammed into the rifling, with a bit of "freebore". That worked for the 450 AF and 500 Hor. But later, I actually seated the 450 X-Bullet longer than 3.34" as in .458 WM without a hitch. I used 75 grains of H335 for the 500 and 75.5 for the 450... that's all i could get in without undue compression and still seat bullets to 0.25" If the throat had been 0.75", I still couldn't have loaded more H335, and the pressure would have been less with less MV.
In my CZ550 in .458 that came later, I used the extra "free bore" and long action to seat bullets "long", which was not strange to me as I'd done that with several rifles that permitted it. But to that point I'd been somewhat influenced by Finn Aagaard who had done a test of five rifles in .458 Win, including his own with a 23" barrel, using factory ammo from Federal, Winchester and Remington. These were 500s. The riffle that gave best results across the board in MV was his own semi-custom with a short throat and 23" barrel. For example: From the Federal 500gr he got 2188 fps from their 500, and he accounted it to the short throat, which he didn't previously know was in his new .458 barrel that had replaced the "finished" .423". He said the long throat was "a mistake". He also said he didn't experience any signs of "high pressure" from his rifle.
Of course, in a .458 with an action that permits it, seating bullets longer than SAAMI is the right way to go, as in my CZ or Ruger No.1 to take advantage of the chamber and leade. That's all been settled now and I'd want any .458 in my possession to have the "long throat" allowing more room for the best powders without undue PSI.
