H4895 vs. imr 4895

They are pretty close to the same on the burn rate chart with H being a little faster but the main difference is that H is an "extreme" powder so it isn't nearly if at all temp sensitive. You will want to develop a new load if you are changing from one to the other.
 
H-4895 has shorter kernels and will measure though a powder hopper a little better than IMR 4895. As vs pointed out, H-4895 is one of Hodgdon's "Extreme" powders which are less temperature sensitive than the IMR counterparts. If I have a choice, I will always choose a powder that has less temperature sensitivity over one that does not.
 
H-4895 shoots the 155's better for me while the IMR-4895 shoots the 168's better. They don't seem to play well with each other though, shooting 1 behind the other, the 1st 3 or 4 shots are a bit wild then settle down.
 
I too just switched to hodgdons since I couldn't find any IMR. To be safe I did a little work up and found the results to be about exactly the same for me. 42.5 gr. Pushing 168's. Gives me nice groups.
 
H-4895 has shorter kernels and will measure though a powder hopper a little better than IMR 4895. As vs pointed out, H-4895 is one of Hodgdon's "Extreme" powders which are less temperature sensitive than the IMR counterparts. If I have a choice, I will always choose a powder that has less temperature sensitivity over one that does not.

IMR 4895 had the highest velocity variation with temp in a chart of 308 powders, H 4895 was not listed but IMR 4064 showed about 1/4 the variance