I had discounted SEE as a possibility because 1) 221 is such a small case and 2) Blue Dot is a pretty fast powder. Seen it happen with things like 45-70, and in particular the big magnums, but never heard of it in a very small case like the 221. I did see where ATK says not to use Blue Dot in light bullet 357 loads now - and the 357 and 221 are roughly the same size and the 221 bullet is MUCH lighter so I guess it is even a known problem.

I just sidestep the issue by using 4759, 4198, 5744 etc. in cast bullet loads.