The issue is the 624 cylinder holes themselves. They are big/over sized/whatever you want to call them. Not only are they over sized, they are over sized a long way.

I respect everyone's input that posted in this thread. There's a lot of knowledge posted here. Can the cylinder issues be overcome??? Absolutely. I've owned/shot 624's in the past. It took some tinkering to get the right balance between the bullet diameter/alloy/lube. But then again I cast/swage/make my own bullets. The OP does not have these tooling/casting capabilities at his disposal.

The real issue:
The cylinder is 1.7" long
The 44spl case/the part that goes into the cylinder is 1.1" long (case body only/minus the case rim).
Where that 1.1" long case rests/ends in the 1.7" cylinder is .451"+ in diameter.
The ends of the cylinders are .432" in diameter.
The taper of the cylinder starts @ .451+ where the bullet leaves the case, then has .6" of free bore that shrinks down to .432".

Hence my using a .432" hollow based bullet that stuck out of the case .5".

Couple that with the bbl swaging a .432" bullet down 3/1000th's (.432" cylinder throats/.429" bbl). I've never had any luck with accuracy when a bullet got swaged down more the 3/1000th's in a bbl. The .432"/.429 is right on the edge of failure.

Anything under pressure goes to the least point of resistance. The undersized bullets in those .432" holes are not only going forward, they are going sideways/bouncing back and forth in the looonnnnggg freebore of the cylinders.

You need a:
.432" bullet
A bullet with a +/- 8bhn alloy
A bullet with a long body (wc/hbwc) that's basically a .432" cylinder.

You are never going to find any real accuracy with that pistol unless you either use jacketed bullets or a soft bullet sized to .432".

At least that's what I've found after owing a couple different 624's over 20+ year period.

Good luck