Shot Selection
The answer to "who uses shot instead of slugs for the poppers" is "anyone who does not want to be disqualified from the match for a safety violation."
I'll be shooting the Harvard match this weekend.
Slugs are not used on poppers at three gun matches, and it's rare to find any use of steel with slugs in USPSA/IPSC shotgun stages, although the 18 round slug stage w/6 steel in Rochester, NY two weekends ago was a pleasant exception.
Shot used is typically 6-7-8-9, with #4 for long shots on steel in some matches. Harvard is limiting shooters to shot size of with 7 or smaller or 6 or smaller (I forgot which), so there is no need to pack any #4s. I'll be using 7 1/2's for the steel shooting, and Federal Tactical (marketing for "wimp load") slugs for the slug stage.