This is unfortunately typical of the misinformation going around that has no basis in OK law. People coming into CHL classes will actually sit and argue even with the text of the statute in front of them.
1. There is no restriction how you carry the ammunition when it is not in the gun. You can pile the front seat with boxes of ammo if you want.
2. DO NOT hide the gun behind/under the seat, because it is illegal in OK to conceal the gun in the passenger compartment, loaded or unloaded. Either the gun or the case containing it must be wholly or partially visible.
3. The CHL exempts the holder from 2 prohibitions: CONCEALING the handgun, and LOADING the gun(magazine only for long guns).
What about someone without a CHL having the magazine loaded but out of the gun on a rifle?