What is a reasonable range limitation for taking an elk with a .452-inch bullet weighing 300 grains leaving the barrel at 1300 feet per second?

My 6x6 was with a .458" at 213 yards using a 350g starting at 2181fps. At impact, the calculated values are 1647fps and 2110fpe.

A 300g .452" Speer starting at 1300fps would have 1123fpe at the muzzle. Assuming a 50 yard zero, 100 yards the bullet would be down 4+" with 1153fps and 886fpe. I probably wouldn't reach out much further than that and in fact set 100 yards as my self-imposed limit for my .44 Mags with 300g bullets, which start out at 1531fps in my carbine, 1213fps in my revolver.

I suspect your limitation will be your ability to consistently keep your shots inside 6 or 8 inches from field positions. Assuming a well designed cast bullet.

As others have said, it's going to be limited to your ability to hit the target. I only mean that in a ballistic manner, not a marksmanship issue. The trajectory will be that of a rainbow. Sounds like you have a 45 Colt with some 8+" barrel. Or maybe a mild 454 Casull. You are definitely at that top end of the pressure chart if it's a 45 Colt. That recipe is putting out 1125 ft.lbs. of energy, so no worries there. I suspect if you do your part, you could theoretically take a shot to about 150yards. Personally, 100yards on an iron-sighted smoke wagon is about as far as I'm willing to go.