I have been working up a load for a 7.7 jap rifle and have got it shooting 1/2" groups with IMR 4064. But as the groups get smaller with heavier charges there then comes a partial falttened primer with CCI. Going on with Federal 210 primers and heavier loads the groups begins to spread. I need to change powders but which way should I go, slower or faster.

What size (grain weight) bullet are you using?

I am shooting the Serria 180 grain pro hunter. At 42 grains of IMR 4064 and a CCI 250 primer the groups have been 3/8 at times. But the CCI primer starts to flatten. Going on with Federal primers does not help the group even at 45 grains. Was wondering what powder I could go to to get better grouping at a lower load.

Why are you using the magnum power CCI 250 primer? It's not needed in that size cartridge. Why not try the CCI 200?

I used the 250 as a test along with Federal 210, Winchester WLP, CCI 200. It shot the best group of all. That is why I am trying to determin what powder to go to and use a federal 210 or a CCI 200 and get the same results.

If you are truly shooting 1/2 inch with a 7.7 jap you have no problems. Be happy with that as it is one in a thousand.

No what you mean about one in a thousand. This rifle was brought back from Japan by my father in law in 1945. It was found in a box with a service rifle and had been used very little. I had the barrel checked out and it is in perfect shape, must have been made early from american steel. 44 grains of IMR 4064 with a federal 210 primer shoots almost as well as the 42 grains with the 250 primer. May try some IMR 4895. But yeah it is shooting well.