Gunbroker. It’s now to the point where it’s the only game in town really. If it’s sell-able, it it will sell on Gunbroker.
Rare is not necessarily a good thing, but it depends mainly on who made the rifle. I know Ackley built some Helldiver-chambered rifles, so if you have one of those, it’s a totally different deal.
The “Helldiver” wildcats were developed by gun writer L.R. Wallack or something close to that, I didn’t look it up, it may be Wallrack or something similar. Regardless, he was a gun writer and benchrest competitor in the 1950s to 70s and came up with several of his own wildcats including the .250 Helldiver.
I think Wallack based some of his wildcats on the now-obsolete .30-40 Kraig, not sure if this is one of them or not. I did a quick search and I cannot find any data listed for the .250, even on some sites that trade in obsolete cartridge data. Yikes. By chance do you have any data for it from old books? That would be a major asset to include with the sale and could drive increased value.
Interesting rifle. Good luck.
Grouse