Destabilised bullets tumbling and punching through the target sideways. It rips a kinda oblong hole through the target rather than punching a clean circle. I'd been shooting and was improving slowly (I'm new to handguns) and then became rapidly worse again. When I looked at the target up close I was seeing holes shaped like the bullets seen from the side. I fired a few rounds at very close range to confirm my suspicions.
When I got the gun home to clean it, I eventually ended up having to tap a cleaning rod through the bore to push out long slivers of lead that had filled the rifling, before I could finish cleaning normally. Don't know whether it's the ammo or the gun, but I guess I'll see next time I take it out.