edit; oh and poland, the only advantage here is that you could used shop pressure (say, 10x) to get 100x-plus perfomance. This would be good for the average spudder but may be difficult to do with off-the-shelf stuff.
Although an internal detent could be used, it would get sheared off neatly unless it was really strong. It is also a bad idea because you need orings on the piston. A detent could damage them or at least keep them from sealing as they pass over.
I don't know if auto ignition temperature would be reached- certainly not by the ignition pressure alone but perhaps with the stored kinetic energy of the piston. If not, no big deal- just add some springy electrical contacts at the front of the stroke and let the piston run into them. You could hook up a couple camera caps, and the contact arcing would be good enough to ignite the mix.
Seriously, I don't want to stop anyone from attempting it, but I had this idea in like 2009 and didn't even bother to say anything because it was so over-engineered/complicated good luck!