The Iowa DNR has FUBAR'd on the details of their "pistol" & "rifle" many times, so it's not a surprise. I submitted suggestions almost yearly since they introduced these weapons. Adding the "other" to the "straight-walled", negates the "straight" requirement. I'm not sure if the ".500" was purposeful, to limit the bullet diameter or just another dumb mistake of not knowing the difference between .50 CALIBER & what they wrote. Back in the day, I would explain details in emails to them & included my office info with daytime phone number, adding I'd be glad to answer technical specifications relating to firearms & ammunition. As you can guess, no response. Likely they collaborate with other states or agencies rather than SMEs.
Next thing I'd consider is whether a DNR enforcement officer will be pulling bullets or slugging your bore to measure with a calibrated micrometer to determine compliance - LOL. Or, will be just admire your items & say, yup.. 50 caliber - great rifle.
Regards,
Slim
Preparing the 1876 for duty of holding deer accountable for their many trepidations!!