13/16" would be considered too small at the breech end for heavy conicals based on Original Match rifles of 45 cal that were 1.2" at the breech. The majority of these match rifles had 1:20 - 1:18" twist for bullets in 500-550gr +/- weight.
Also, do you know what steel you're barrel is made of? Be careful with any conical or heavy loads with 12L14 steel.