You've nailed it Rocketman. The problem is I know all this already. I'm trying to find someone who has removed unslotted endcaps or seen it done. I've already made and fitted new hingepins for guns that allowed me to unscrew the endcap and then the pin. The thing is that over the years I've seen a number of Brit 2" chambered boxlocks that I really liked except for being off-face. Of course, wouldn't you know, unslotted endcaps. I suspect these guns are more prone to looseness due to their geometry, ie. wide chambers and narrow, light frames. Yeah, you could shim or build up the hook but what do you have then? A gun that's prone to looseness with a half-assed repair. Far better to fit a new pin you know is round and perhaps a replaceable insert in the hook if the gun is worth it. That's what I think would be the best solution.
