I have experience loading for three
very long throated rifles. The first was a "P17" (Enfield model of 1917) in 30-06, and I have applied some lessons learned from it to rifles chambered in 7X57 and 6.5-06.
The P17 had a throat so long that 150 gr Spitzer bullets would not engage the rifling until they were completely clear of the cartridge neck. In doing load development I ran into several "hangfires" with the lighter loads, the phenomenon apparently disappearing with higher pressures. I think that the hangfires developed due to a sudden and severe drop in pressure when the bullet was in "limbo" (neither in the neck, nor engaged with the rifling). With higher pressures, perhaps the cartridge neck was tightly sealed against the chamber reducing the pressure loss. In any event, I changed to a longer bullet that remained in the cartridge neck when engaged with the rifling and never again experienced a hangfire. In the 7X57 I use the flat-base Hornady 175 gr RN, as 140 gr BT Spitzers are too short, and in the 6.5-06, the flat-base Lapua 155 gr "Mega", as again, the popular BT 140 gr Spitzers are too short.
I keep the Loaded OAL to Mag Length - 0.030" (as already suggested), and keep the full length of bullet in the cartridge neck. There's still a jump of about 0.100" to the rifling in these rifles, but accuracy remains pleasing. With the extra case capacity, I can go 1-2 grains above published maxes, but I don't.
I suggest:
- see how much of the bullet remains in the cartridge neck at the point where the rifling is engaged;
- if it's still in the neck, develop loads as usual; or
- if it's not still in the neck, you could look at a longer bullet that will, perhaps the Sierra 150 gr GameKing.
Sierra 140 gr "GameChanger"
Sierra 150 gr "GameKing"
