Old age is a pain....
I'm afraid I don't recall the details very well. But I do recall hearing about a fellow, from Utah (I think), who was flying back from somewhere back east with some guns.
His flight had some kind of problem, and was detoured into Newark NJ. The airline put him up in a hotel for the night. However, since his guns (and his posession of them) was not legal in NJ, he got arrested! He wound up spending (again, IIRC) a couple days in Jersy jail before being released!
Fed law does trump state, in this case, but you will get arrested, booked, and go to jail, until the wheels of justice turn their slow course. Last I heard of the poor guy, he was suing for both damages, and to get his guns back.
Next time someone whines about how the FOPA 86 cost us closing the legal machinegun registry, and should not have been passed, remember this poor guy, and many others like him, who ultimately get cleared of charges because of that Fed law. The closing the registry was a last minute poison pill (passed on voice vote only), intended to kill the bill. We took the hit, Reagan signed it into law, to provide the greatest good for the greatest number. Prior to the FOPA, if you were crossing a restrictive state (which was allowed, provided you didn't stop), you could be arrested for having a flat tire, or stopping to eat, get gas, or to pee! AND, you were guilty! You had no defense, and no legal recourse, because you did violate the law!
All in all, the FOPA has done (and is continuing to do) more good for us than the harm it causes those who would like to enjoy affordable full auto firearms.
I'm afraid the magazine issue is not clearly spelled out, and therefore likely to be ruled against us, until specific language is obatined in law. One magazine ought to be considered part of the gun. And if it is the original size the gun was sold with, you might have a valid argument that it is covered under the term firearm as used in the law. However, you might have to make that argument in court.
The simple solution would be to have spare oversize (by the defintion of the restrictive states) shipped to you (Fed Ex, UPS, etc) where you are going, and legal. Then ship them back home when done. Costs a bit, but considering everything, its a lot cheaper than lawyers and courts.
All else being equal (and it almost never is) bigger bullets tend to work better.