I find N312 and N318 between N310 and N320 in some burn rate charts. But both are absent from Vihtavuori's own current burn rate chart. Neither are they mentioned in my 1995 Vihtavuori Oy hardbound manual, so I suspect they are obsolete and discontinued and have been so for some time. I still find the Scot Brigadier rifle powders in some burn rate charts, too, even though their plant burned down in the 90's and was never rebuilt. Good powders, but long gone. On the Internet, information never dies completely.
Anyway, age alone is a good reason to establish loads for N318 and use it up. Powder gradually deteriorates with age and some deteriorate a lot faster than others. You'll want to be careful when you open these to look for red dust and acrid odor, just to be sure they haven't already gone too far.
I would start by writing Vihtavuori. They are part of Lapua, now, and the address would be [email protected]
. They may have old data available. If you send them any lot numbers you have, they may also be able to tell you how old the powders is.
If you can't get any information that way and the powder looks good, I would take a magnum revolver and load target loads using N310 data as a starting point. If you have any N310, a chronograph would be useful to help make a comparison. If the N310 shoots bullets faster, you can almost certainly bring these slightly slower powders up to the maximum velocity that N310 produces in your particular gun with that same particular bullet.
Beyond that, you just have to follow the usual load workup practice of watching for pressure signs