This weapon used the then novel Hoch-Niederdruck or "high-low pressure" system, where a conventional charge generated high pressures in the breech that was bled into the barrel through a series of ports until it generated enough pressure to shear the pin that was holding the projectile - just under 8000 psi - thereby firing it at considerable velocity, around 1700 feet per second.
Pneumatic launchers naturally have lower pressures so the high-low system is not really relevant to spudguns, but I think the shear pin mechanism is an interesting alternative to the burst disk, here is one way it could be configured:
It has to be said that it's rather more complex than a burst disk, and indeed the ubiquitous 40mm grenade launcher cartridge that works on the same principle does exactly this, with a copper cup separating the high and low pressure chambers, bursting through the ports when sufficiently high pressure was generated. The shear pin is therefore perhaps not the most practical alternative, especially as it relies on an airtight projectile, but I thought the concept was worth mentioning.
It's effectively a variation of a detent that retains the projectile by friction until there is sufficient pressure to release it:
"yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " 
