I'd say no heavier than a 50. Hornets aren't speed racers so rpm will be down a bit. I think the first response is spot on.
One more thing.
If your loading, be mindful of pressure. OLD hornets were actually done on rimfire barrel dimensions. Meaning they used the same diameter bullet as a 22lr. (.223")
New production (modern late model stuff) uses the centerfire barrels intended for .224" bullets.
If it applies, you can still use the bigger bullets in the tighter barrel (in fact, this was a trick "back in the day" when building one of these to get it to shoot better) but just be mindful that pressures are going to go up slightly for a given powder charge.
Just be careful and use good judgment.
Last edited by NesikaChad; 08-01-2008 at 09:49 AM.