Round nose bullets in a tubular magazine: can a person get away with it, or is there likely to be a detonation?
With all due respect to previous answers... As one who has successfully used a particular round nose bullet in a .38 special loading in CAS since about 1985, I'll say it
depends:
1) on the hardness of the bullet;
2) how "round" the nose is, which can vary greatly;
and 3) velocity.
You also have to recognize that bullets in rimmed cartridges don't sit flat in the magazine, so even a flat nose can have a corner in contact with the primer. It is inertia that will set off primer, exacerbated by the weight of the column of cartridges in the magazine, how many times a particular primer is struck before it collapses the onto the anvil, which can be dependent also on the hardness of the primer.