A couple of things, JR.
First, I assume you meant Accurate 2460. If so, QL shows a max charge of that at 64 grains and 32k PSI (the max spec for the .45-70). And, velocity is only estimated at 2260, NOT 2660 fps (which is what you'd need t propel a 300 grain bullet out with a ME of 4700). BTW - that's with the case lengthened to 2.4" and the COAL set at 2.85", out of a 26" barrel.
So, I'll assume that you're loading WAY over max. That, or your math is off.
But, JFSAG, let's assume that you could chuck a 300 grain .458" JHP at 2660. BC of the Hornady version is 0.197, so let's use that as a basis.
A quick look at JBM shows (for a 5" vital zone) a 241 yard zero, and 280 yard MPBR. Retained energy at 300 is 1477 ft/lbs, and you're at -2.7MOA from your zero. At 350, you're down to 1200 ft/lbs, and at -5.4MOA. 400 yards, you've got 988 ft/lbs, and you're -8.6MOA
Assuming the same pressure required to get you to 2660 via the 2.4" case, and using the 2.1" case to acheive the same COAL of 2.85" with an RCBS 45-405 cast to 425 grains (standard, in QL), it will have a nominal BC of about .303. Let's use that...
Initial starting velocity would be somewhere around 2225. Zero range is 221 yards, with an MPBR of 260 yards. 20 whole yards difference in favor of the 300 grain slug. Hmmm... Oh, btw - the energy level would be the same.
Retained energy at 300 is 2140 ft/lbs, and drop is -4.0MOA. At 350, it's retaining 1868 ft/lbs, and at -6.9MOA... 400, still 1635 ft/lbs, and -10.1MOA.
Start them both on even zero range footing by zeroing at 230 yards (10 yards close for your 300; 10 yards long for the 425), and they are dead even on trajectory, with the 425 carrying considerably more energy, momentum, and mass at any range.
Oh, and that RCBS slug is not even that good for BC; there are considerably better...
Damned facts, anyway.
