A search of the site and opening the regs in PDF http://www.iowadnr.gov/law/files/huntingregs.pdf
Bottom right of page 24.
yielded only mention of Min. .24 cal no mention of mag capacity so you may wish to wait until monday to call... Main number had no answer today...
Brent
The only mag capacity restriction I am aware of in Iowa is for watefowl hunting.
Other than that I don't know of any. I usually skim through the hunting regs just prior to going home for the deer season to see if anything has changed.