There are several arrows that fall under the "5/16" shaft size yet have a different internal size.
The only 50 g inserts being made at this time is the aluminum inserts by Gold Tip. they are .246 in diameter and will only fit the Gold tip shafts.
If your inserts are threaded on the back side you can add 50g weight screw adapters and end up with nearly the same thing. Yes they would be about 62g when coupled with the weight of the insert, but the 12g difference should not be an issue.
The 50g brass previously made by muzzy is no longer made since they were bought out by Rage.
We stock all I made mention of (except the brass 50g) as do some others here too.
Give us a holler if I can help.
thanks, bigjim