MSP,I don't recall ever shooting any factory ammo in my 44 mag,but it has shot a veiw 1 1/2" with the 300 gr. XTP's with a max load of H110.The talk over the 1 in 38 twist reminded me of what the Speer reloading manuel said about the 444 Marlin they made there test with. They started to not test there 300 gr. bullet,but tested it anyway,and it turned out the most accurate bullet they tested.I think that the 444 speed cranked up to the proper RPM's to stabilizie the bullet ( 2250 plus) .My 44 mag also liked the 240 Rem. bullet ( the one they use in the 444 ammo).With the loose bore,they seem to shoot best with fullbore loads or a little above.
Maybe Warf,Granpa Mike,and the other's will tell what there's done before they reamed it.But for a young boy shooting a 45 70 I would use a 300 gr. bullet an about 12 grs. of Uniqne. Digger