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Just wondering if .309 caliber Hornady XTP bullets (100 grain hollowpoint, made for .30 Luger) would work with a reduced load in a 30-30 (Marlin 336) or would I have to resize the bullet first?
I don't see any reason for resizing for the 30-30 Winchester. But with the lack of a cannelure you should likely either load single rounds directly into the chamber or try the Lee Factory Crimp to get a good hold of the bullet so it doesn't set back in the tubular magazine.
If the hollow point is larger than a primer, you can use it in a tube mag.
The lack of a canelure, may, indeed likely will give you issues. The fact that the bullet measures 309, may make up for the problem, but, I'm doubtful.
It's likely a single load proposition.
I see no reason why it can't be fired at full power.
I have used .309 bullets a couple of times in my old 30-30. Never had any problem with the loads or the pressure. Used the same loads as I would with .308 slugs. No excess copper build up but I almost always clean every 6-10 rounds anyway regardless of rifle or calibre except rimfire.
Carry on!