Vortec & 762 are right, it's not dangerous. We've always been warned by everybody to never use points in tube mags, but it turns out that this is to prevent any possibility of minor damage rather than a safety measure. There was an article testing this last year, I'll try and dredge it up. It made two points:
First, it's hard as hell to get a detonation to occur, because rimmed cartridges won't set straight end-to-end in a tube mag. They flop over so the bullet touches the side of the tube, tip touching the rim of the next round.
Second, not much happens if they go off. The author induced this artificially by using bushings to allign all rounds tip-to-primer and appying a hammer none too gently. He got a lot of smoke and fizzle, time after time. The rounds didn't ever all go, and those that did were just split along the side, because the burn was free to expand into all the empty space in the mag. The bullets, instead of being launched, were instead shoved deeper into the cases by their neighbors.
I'll post the link if I can find it.