571.030. 1. A person commits the crime of unlawful use of weapons if he or she knowingly:
(1) Carries concealed upon or about his or her person a knife, a firearm, a blackjack or any other weapon readily capable of lethal use;
571.030.3. Subdivisions (1), (5), (8), and (10) of subsection 1 of this section do not apply when the actor is transporting such weapons in a nonfunctioning state or in an unloaded state when ammunition is not readily accessible or when such weapons are not readily accessible.
You have 7 choices:
1)Keep the firearm in the vehicle, in any fashion you desire, but keep the ammunition separate (ammunition not accessible) (Also be mindful open open carry/open display laws).
2)Keep the firearm in a nonfunctioning state when ammunition present. (field stripped/ broken down, etc..)
3)Keep the firearm in a functioning state, but outside the passenger compartment (not readily accessible)
4)Obtain a CCW permit from the state of Maine.
5)Wait until you are 21 and obtain a Missouri permit
6)Push for state OC preemption and get it down to 18.
7)Be mindful of the jurisdiction you are in, and open carry/display.
Keep a copy of RSMo 571.080 and 18 USC 922(x) just in case you are questioned by a peace officer.
Transfer of concealable firearms.
571.080. A person commits the crime of transfer of a concealable firearm if such person violates 18 U.S.C. Section 922(b) or 18 U.S.C. Section 922(x).
18 USC 922(x)(1) It shall be unlawful for a person to sell, deliver, or
otherwise transfer to a person who the transferor knows or has
reasonable cause to believe is a juvenile -
(A) a handgun; or
(B) ammunition that is suitable for use only in a handgun
18 USC 922(x)(5) For purposes of this subsection, the term "juvenile" means a
person who is less than 18 years of age.