I've been using an expander (came with the 3 die RCBS set) to open up the case mouth just enough to allow the base of the projectile to fit in to avoid any lead shaving issues. I have not been applying the roll crimp (? form the same RCBS die set) after seating the projectile - it is still a firm fit.

My CZ has a long throat, so maybe without the crimp, the primer discharge is enough to expand the powder space (until the projectile gets stuck in the start of rifling) so that the powder is blown around in this space and prevented from igniting, or blown out?

Trouble with this theory is that my filler load with AR2205 gave me the stuck projectile but the filler and powder stayed where they were! When I pulled the filler out (it was partially stuck to the inside case wall by a small compressed mass of powder) most of the powder was as good as new. There was a small mass of compressed powder that was yellow in colour – I presume it looked like this from the primer flame ie partial burning.

So, in this case, if the powder ignition was disrupted by physical displacement, it must have been a pressure wave that stopped it but pushed the projectile into the rifling (sort of along the lines of what Tatume wrote but powder will burn under normal air pressure so it must have been a sudden change in pressure, or transient wave, that blew it out). I was thinking that a crimp may offer enough resistance to let the primer flame do its job before the projectile starts on its way.

This is making me think too much. I just wanted to shoot cheap reduced loads.

Okay, I’ll increase the load with AR2205, get rid of the filler (I’ve now also heard horror stories of fillers causing rings in pistol-cartridge rifle chambers) and add a bit of a crimp. I’ll likely also use magnum primers. And then if these projectiles lead the barrel, I’ll change to Westcastings.

I’m not keen on casting my own. I have cast before and I don’t get any enjoyment out of it – also pushed more for time these days.