Who all makes a 5/16 bh? I know magnus, zwickey, and tusker do... are there any others?
Howard Hill has a 5/8 BH. H
My 5/16 tapered shafts fit just fine into 11/32 broadheads.
There's an 5/16" ferruled MA-3-S that 3R's carries, a rendition of the old Bodkin.
pretty sure eclipse makes some, but dont ouote me on it ;)
If you don't find what you want in 5/16, don't hesitate to use a 11/32 head. They will work just fine both glued to the shaft or on inserts. If the smaller dia is important, you can grind 1/8" off the back of the ferrule and you will have a 5/16" head. I've done it both ways many times.
blackstump broadheads make a 5/16 head. they call it the 'ironbark'. tough as nails!