I kind of used a rather primitive method of measuring the chambers and throats. I simply took a 458 Win Mag case and loaded a 450gr TSX to the max COAL of 458 Lott, which is 3.6”. Of course it would easily load into the Lott chamber, but it would also easily chamber into the Win Mag chamber as well. I then loaded another dummy round with an even longer COAL until I finally loaded one long enough to touch the end of the chamber throat. Both the Lott and Win Mag barrels took exactly the same COAL before reaching the end of the throat. So, essentially, the Lott barrel has the same long throat as the Win Mag barrel, it just has a longer chamber to accommodate the Lott case. That said, I can easily load the 458 Win Mag to the same length as 458 Lott loads and get exactly the same performance.
I actually wrote the measurements down somewhere, but I’ll have to see if I can find them. But as you have pointed out, the magazine length will stop you before the chamber will.
As can be seen in this picture, the 458 Win Mag case can be loaded to 3.660 inches and still easily chamber into the R8 458 Win Mag chamber. Even at this length, it doesn’t come close to the end of the throat as I recall. But the bullet design won’t allow you to “practically” load it any longer due to the grooves on the bullet itself.
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