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.