Good question, Tom.

Most "closed bomb" burn rate charts place the IMR version slightly "faster" than the H version of 4198. The IMR seems to work OK, however, due to the much [bulkier] granule size, you may not be able to get as much into the case as is possible with H4198. But, I haven't messed with any of the IMR version for a LONG time - perhaps it is more dense (?). Most of my manuals specifically state that H may be substituted for the IMR variety , but not vice versa . . . but all of my manuals are getting really OLD! ;) Oh, and, with the current production Lots of H-4198 (extreme),  if you work up in 1/2 grain increments, until the case is FULL or, excessive pressure is reached, you'll find the Sierra DATA extremely conservative. I'd go up untill either of the latter, but if the barrel shoots better at a less than FULL case, settle for THAT. R.G.