Actually PO Ackley built 3 different 300s They were called oddly enough the 300#1, 300#2 and the 300 #3 the number 3 was the largest of the three.
He was experimenting with trying to find out what it would take to overbore the cartridge and the point of diminishing returns.
There is a book on PO Ackley that talks about all three 300s.
I had the #3 300 and made my brass out of the 300 H&H brass. and as mentioned earlier the 300 Weatherby. I stopped shooting it because it became a collectors rifle because Ackley himself had built mine (It is a piece of art).
When he built the #3 he said it was to over bore and inefficient. (I wonder what he would say
about some of todays cartridges). The #1 was the shortest and had a powder capacity of just
under 70 grains. the #2 was lengthened just enough to re chamber a 30/06 without setting the chamber back. it had a powder capacity of 70 to 75 grains. The #3 has almost the same case capacity as the 300 Weatherby (around 80 grains of powder capacity).
I hope this helps you to determine which one you have.
J E CUSTOM