Daryl is correct. My m46 has an oversize groove diameter, .370".
I did have to open up the neck to accept the round loaded with the proper size bullet.
Then it occurred to me that the throat on the original chamber cast looked "different".
I then slugged the throat and discovered that when the chamber was originally reamed the throat was cut to .367" diameter. This left rifling in the throat .0015" deep per side. So I had a choice, seat the bullet deep in the case or ream the throat. It got reamed!
Tomorrow afternoon is supposed to be near 30 degrees. I'm hoping to get out and try it again at 50 yards just to confirm the initial accuracy I was able to obtain.