I've pretty much come to the same conclusion,,,, Handloaded some spire style bullets for the 30-30 years ago... but never was brave to load 5 in the tube... but it worked ok with 2. I cannot cite the source from memory... but someone at least once got a blown up magazine
To get enough inertia to strike a primer you would probably have to strike the gun butt on pavement hard.
With the compounds used for the primer material, if the cup wasn't as hard as it is, just dropping a cartridge on the floor would cause it to fire. I've worked at alot of the military and civilian plants where primer compound is made. By itself, unless wet, a soft breeze could possibly set it off. Before being put into the cup its classed as a 1.1 explosive, an as I said before unless wet is very unstable. It can only be handled wet, and loaded into the cup's wet. Due to the design of the cup it is farely safe to handle. Caution is still needed after loading for shipment. After being loaded into the cartridge case be it factory or handloading, it becomes one unit of ammunition, and even though its still of a sensitive nature, if it was that unsafe, think of every double feed thats happened, there would lots of hurt folks walking around from that(I'm refering to double feeds where the first cartridge doesn't fire, and you perform you immediate action drill, causing another round to be stripped out of the magazine an it slamming into the one still in the breech. The compound most commonly used today is lead tri-nitro-resorcinate instead. Usually referred to as lead styphnate.