GuzziJohn:
It seems like things are going reasonably well, but I'm not sure whether the 38 special rifle thing is a true problem or a bullet quirk.
Are those 100 yard groups? The only thing I can think of is that the bullet may be destabilizing, as some short fat bullets will do beyond about 50 yards. Can you give us more detail about what velocity, distance, etc this load is used for? In the short term, try it at 50 yards and see what the groups are like. If those are 100 yard groups, then I'd chalk as a case where another bullet design should be tried. I prefer faster powders for 38 special, and like Winchester 231 the best.
Go ahead and use the nickel plated cases. I use them for .223 for high volume shooting on prairie dogs. Just be sure they are clean and properly lubed before putting them in the dies. (My reason for using them is to make them easiliy distinguishable from 221 fire ball.) After your nickel plated brass wears out, you can switch to unplated if it makes you feel better.
The arguments against nickel plated brass are:
1) More brittle and case mouths split within a couple of reloadings. My responses: so what? You've already got the brass and you might as well use it up. I've reloaded nickel plated 45ACP brass so many times that the plating has mostly worn off and the brass is visible. That's easily over 20 reloadings, which include expanding the case mouths. Service life with the 45s seems to be about the same plated vs. unplated.
2) It wears out the dies. My response: Don't hold your breath waiting for the dies to wear out. You'll be at the loading bench quite a while before you need to worry about that. And if there is a problem, just have RCBS replace the single die body. It will cost a lot less than throwing out all that good brass you have and replacing it with unplated brass.
3) The inside of the case mouth, where the bullet is seated, is abrasive. My response: Doesn't appear to be a problem. I use it for .223 and need groups smaller than 1" at 100 yards for prairie dog shooting. I get that accuracy with nickel plated brass so any additional abrasiveness is academic.