Extreme point of impact change

[olduser]

Have my Ruger 77 in 30-06 sighted dead on at 100 yds using 150 gr. Nosler Partition with 46.0 gr. BLC-2. Velocity 2790 fps.
Tried some factory Remington 150 gr. Core Lokts. When I fired them they grouped 6.5 inches higher than my hand loads. Windage was exactly the same. Published velocity of factory load is 2910 fps. Horizontal position was exactly the same.
I know the velocity difference is 120 fps but I have been loading for about 50 years and this is the most extreme shift of point of impact for bullets of the same weight that I have seen. I would have expected the two loads to group about 3 inches apart at the most.
Anybody have a ready explanation?

 

[Unclenick]

Well, I'm wondering if you typoed your BL-(C)2 load. At 46 grains, it is 3 grains below Hodgdon's starting load for that powder in the '06, and if you used the same brass and primer they do for the Nosler BT, then you'd be at about 75% case fill and 34,800 CUP pressure and would get about 2635 fps from Hodgdon's 24-inch test barrel. Yet you are reporting higher velocity than Hodgdon has for its 49.0-grain starting load.

If your load is actually 46 grains, I would guess your velocity reading is off unless your barrel is really long. Hodgdon's data suggests you'd need a 40-inch barrel to get 2790 fps out of that load. But if you typoed and your load is actually 49 grains, then Hodgdon's data suggests a 26-inch barrel could get you to that velocity.

That said, I'm leaning toward you not having made a typo and that you got a bad chronograph reading. I've seen them off by up to about 200 fps in challenging (for the chronograph) light conditions, so being off about 150 fps is possible. The reason I'm leaning that way is a load as light as 46 grains of BL-(C)2 would be expected to impact low due to very low pressure and recoil. 6 inches seems like a lot, but the barrel time difference between that low load and the faster one is enough to allow the normal bending a barrel does under recoil to have the muzzle swing from a low angle to a high one, accounting for the difference.

Incidentally, BL-(C)2 is canister grade WC846, made at the St. Marks plant in Florida. It was originally formulated to load 303 British for our allies during WWII. It was adopted by the military here as the powder for M80 7.62 NATO ball ammunition. It is, however, a bit fast for a .30-06 ball powder, being better suited to the smaller 308, and it leaves extra space in the case.

The ball powder the military used in Cal 30 M2 Ball is WC852. That powder is sold by Hodgdon in canister grade as H380. The starting load for you Nosler BT is 53 grains, and that will mimic the military M2 Ball ballistics very closely and can be shot in the Garand. The maximum load Hodgdon lists is 59 grains, which goes over 3,000 fps from their test barrel, but it's too large a charge for the operating rod on the Garand to be happy with. From a '03 Springfield or other bolt gun it isn't a problem. The slower H380's larger charges will fill your cases better.

Note that CCI recommends magnum primers be used for both powders for most consistent ignition.

 

[olduser]

I think you nailed it by questioning my chrono reading. It has apparently gone bonkers on me. I checked some of the factory loads and it gave about 225 fps over factory published speeds. I checked some previous known loads and get screwy speeds. Time for a new one.
The 46.0 gr. load is the starting load for a 150 gr. bullet in the Lyman 49 manual and shows a velocity of 2770 fps. It has to be a typo. As you pointed out the Hodgdon site gives a starting load of 49.0 with a velocity of 2759 fps.
I think the 46.0 load is probably giving a velocity in the neighborhood of 2550 fps. The difference in the factory published speed of 2950 and what I think is closer to correct velocity of the 46 gr. load of 2550 would be about 400 fps. and could would be responsible for the majority of my POI problem.Thanks for your help.

 

[Unclenick]

Glad to help. Note that the empty space in the 30-06 can cause wide velocities variation. I once measured a 46-grain IMR4895 load under 173-grain bullets to average 80 fps difference going from tilting down and up to level so it fired with the powder forward over the bullet, as compared to tilting the muzzle up and bringing it carefully down to level for firing with the powder over the flash hole. 4895 is bulkier than BL-(C)2, so the fill of about 83% is better than your 75% fill, so where the powder is could have a significant effect on the velocity you see.

If it is cold where you are, be aware the usual alkaline batteries can loose a lot of power in colder temperatures and that could mess with your chronograph. Energizer lithium batteries are expensive, but work way better in the cold.