I tried the 275 and 325gr Gold Dots that I have, and they also fall through. I do not have any cast bullets to try. My SRH will shoot 3" groups at 100 yds off a solid rest with a 2X scope. My best effort at 25yds is just under 1/2". You don't say how much shooting experience you have with bigger bore handguns.
If you have plenty of experience and shoot well with other kickers, perhaps it is a problem with your loads?
If you have good performance with these same loads in a different 480, then it sounds like your gun has a significant issue.
Have you had a gunsmith take a look at it? Perhaps there is an alignment issue between cylinder and barrel? Have you sent it back to Ruger for them to do a rework on it?
I had troubles with hard extraction on my 480 SRH the first year I had it. It was a pain in the butt with factory loads, requiring heavy duty pushing to get the cases out. When I started doing even moderate handloads, it became impossible. I had to bang the extractor pin with a wooden mallet multiple times to work the cases out. I saw some comments on line about the combination of rough cylinder walls and high spec chambers with thin cylinder walls allowing expansion and contraction during firing to grab the cases (why they went now to a 5 round cylinder). The fix was to replace the cylinder with one that had thicker walls with the chambers on the small side of spec. I sent it to Ruger and they replaced the cylinder and polished the chambers better, no questions asked. It gives me very little trouble now. A slight bump takes out the factory loads, and a good push takes out the handloads.
I would send the gun along with a sample 25yd target and explanation to Ruger. They might surprise you!
Either way, good luck