I bought some of these to play around with making my own slip tips. On the far right is a standard "Muzzy" brand broadhead arrow tip, with a 9/32" OD and a #12-28 tpi (threads per inch) internal thread. I recently found the two larger tips. The middle one is a Muzzy brand bowfishing arrow tip. It's 5/16" OD, with a 1/4"-28 tpi internal thread. The one on the far left is a "Cajun Jackhammer" bowfishing tip. It's also 5/16" OD with a 1/4"-28tpi internal thread. All of these are very hard, with the Cajun Jackhammer advertised as being a 50 on the Rockwell "C" hardness scale. 17-4 stainless shafts are usually about a 44, and the Rob Allen shafts about a 47 on that same scale. They all run about $2.00 each.
They almost fit a 6mm thread (argh!). The 1/4" diameter is close enough to work, but the 28 tpi threads are just a little too far off of the 1.0 mm metric pitch threads to fit.
Anyway, the larger tips were news to me. Those Jackhammer tips seem perfect for reef hunting, so maybe someone out there can figure out a clever way to use them.