It's possible your spent 38s were fired in a gun with loose chambers. Have they been resized? If not, size them and try again. The Mark III revolvers are of high quality, extremely durable, and usually VERY accurate.
I've never heard of one with chambers so undersized that it was apparent from a visual.
Mark Lane to William Buckley: "Have you ever referred to Jessee Jackson as an ignoramus?"
Buckley: "If I didn't, I should have"