The Lee Navy is indeed a straight pull actioned rifle, which locks at the rear. The bolt handle pivots down to force the rear of the bolt into a recess in the receiver, much like the Model 99 Savage system. Pulling the bolt handle to the rear pivots the handle up, which in turn brings the rear of the bolt upwards and disengages it from the receiver.

It is evidently a very strong action, since I am told that Winchester kept one permanently set up to test .220 Swift ammunition.

There was a story in circulation on the web about a Lee Navy blowing up and killing the shooter. A witness said that among the debris on the bench rest was a fired cartridge case which showed indications of great pressure. Evidently this warning was not enough to prevent the shooter from continuing to fire.

I suspect that the cause was insufficient neck clearance. The shooter was almost certainly using formed cases, since original Lee Navy cases are collector's items. Failure to allow for enough neck clearance to release the bullet can cause the powder charge to detonate, and no conventional action is strong enough to hold up under the force of a detonation. Mistakenly firing 8X57JS cartridges in Springfield rifles after World War I was the cause of a number of blow ups of this nature.

I believe the main problem with the cartridge was barrel life. The smokeless powders of the day were extremely erosive and the barrels could simply not hold up under exposure to this kind of treatment. Metal fouling must have played an important role also. The rifle was adopted at the dawn of the metal jacketed bullet era, and quite a bit of experience and experimentation was needed to arrive at the proper material for bullet jackets. The .30-'06 was notoriously bad in this respect, and years went by before the problem was solved.