Definitely no shorter than 24". Preferably 25" or 26".

My Ruger #1 in 300 H&H has a 26" tube; however the #1 action is shorter than a bolt gun, so it seems like you're carrying a 22" rifle.

Still, my handloads (and even the one or two factory loads out there, e.g., Federal 180gn Noslers) do a bit better ballistically with the 26" barrel.