Here is the link to the bill:
https://www.legis.iowa.gov/legislation/BillBook?ga=88&ba=hf716
In it you will see
b. A cartridge rifle that is allowed pursuant to this subsection shall be of the same caliber and use the same straight wall
or other ammunition as is allowed for use in a pistol or revolver for hunting deer as provided in subsection 5. In addition, the commission shall provide, by rule, for the use of straight wall
or other ammunition under this subsection that meets ballistics specifications similar to the requirements for straight wall
or other ammunition allowed for use in a pistol or revolver for hunting deer as provided in subsection 5.
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The second sentence is the critical one:
"In addition, the commission shall provide, by rule, for the use of straight wall
or other ammunition under this subsection that meets ballistics specifications similar to the requirements for straight wall
or other ammunition allowed for use in a pistol or revolver for hunting deer as provided in subsection 5."
This enables/requires the NRC to establish a rule that limits the cartridges used in rifles to those that have "ballistic specifications similar" to those allowed to be used in pistols and revolvers in the preceding section (5) of the law.
So the STATUTE (law) REQUIRES the NRC to make a rule that LIMITS the ammunition that may be used in the rifles that will be "allowed" under Section 6 (those of the same caliber and using the same ammunition as allowed for pistols and revolvers in Section 5) to the same "ballistic specifications" (think "ballistic performance") as the ammunition used in those handguns.
You are NOT going to be allowed to hunt with the "high caliber ammo" even if you can find a handgun to shoot its cartridge. The cartridge fired from a rifle does NOT have "similar ballistics" as when fired from a handgun.
Hopefully this helps provide some clarification for you.