What is considered a safe meplat size for a mag tube?
Meplat as I understand it is the diameter of the flat on the front of a flat nose bullet.

I'm doing a SWAG (Scientific Wild Assed Guess) that .25 (1/4)inch would be safe. Any meplat larger then the primer pocket of the case would work.
Something to consider. A buddy of mine was loading his 1873 Winchester 45 Colt with factory ammo and got a detonation. The dead soft lead of the factory bullets swaged enough to set off the primer of the cartridge. The meplat is important but the softness of alloy is a consideration.
All of my lever actions, with tube magazines are 45 caliber and the meplat is not a consideration, since the bullets are all SWCs.
Jim