Here's the transportation/possession text:
"(b) Authorization.-Notwithstanding any provision of any law (including a rule or regulation) of a State or any political subdivision thereof, a person who is not prohibited by this chapter from possessing, transporting, shipping, or receiving a firearm or ammunition shall be entitled to-
"(1) transport a firearm for any lawful purpose from any place where the person may lawfully possess, carry, or transport the firearm to any other such place if, during the transportation-
"(A) the firearm is unloaded; and
"(B)(i) if the transportation is by motor vehicle-
"(I) the firearm is not directly accessible from the passenger compartment of the motor vehicle; or
"(II) if the motor vehicle is without a compartment separate from the passenger compartment, the firearm is-
"(aa) in a locked container other than the glove compartment or console; or
"(bb) secured by a secure gun storage or safety device; or
"(ii) if the transportation is by other means, the firearm is in a locked container or secured by a secure gun storage or safety device; and
"(2) transport ammunition for any lawful purpose from any place where the person may lawfully possess, carry, or transport the ammunition, to any other such place if, during the transportation-
"(A) the ammunition is not loaded into a firearm; and
"(B)(i) if the transportation is by motor vehicle-
"(I) the ammunition is not directly accessible from the passenger compartment of the motor vehicle; or
"(II) if the motor vehicle is without a compartment separate from the passenger compartment, the ammunition is in a locked container other than the glove compartment or console; or
"(ii) if the transportation is by other means, the ammunition is in a locked container.
"(c) Limitation on Arrest Authority.-A person who is transporting a firearm or ammunition may not be-
"(1) arrested for violation of any law or any rule or regulation of a State, or any political subdivision thereof, relating to the possession, transportation, or carrying of firearms or ammunition, unless there is probable cause that the transportation is not in accordance with subsection (b); or
"(2) detained for violation of any law or any rule or regulation of a State, or any political subdivision thereof, relating to the possession, transportation, or carrying of firearms or ammunition, unless there is reasonable suspicion that the transportation is not in accordance with subsection (b).".
It is the lead-in that I am not sure I understand:
"(1) transport a firearm for any lawful purpose from any place where the person may lawfully possess, carry, or transport the firearm to any other such place if, during the transportation-"
I read it to mean if you lawfully possess in MA and you are traveling to Utah and in Utah you would lawfully possess under a Utah non-resident equivalent license, then you can pass thru all of the intervening states w/immunity.