It is bound to happen at some point. You are carrying your handgun lawfully on a Class-A LTC on your daily errands when you have to go somewhere that you can’t take the handgun. Let’s say your child’s school calls – your son is throwing up and needs to be picked up within the hour. Massachusetts General Laws ch. 269, §10(j) clearly says that you can’t carry a firearm (or other dangerous weapon) on your person in a building or on the grounds of a school (including colleges) without the written authorization of certain school officials. In the wake of Newtown, it is not worth taking chances with this statute.
So what can you do? Unfortunately, the answer is not clear.
Chapter 140, § 131C concerns weapons in a motor vehicle. You can carry a loaded firearm on your person, under your direct control on a Class A permit. Someone with a Class B permit can transport a firearm unloaded and contained within the locked trunk or in a locked case or other secure container. Arguably, if you make a brief stop you are still transporting the firearm, and you could place it unloaded in the trunk (if you have one) or in a locked case or container. (The ammunition, apparently, does not need to be in its own locked container, but it must be separate from the firearm.) Trigger locks and cable locks do not count! A locked glove compartment might count, but the trunk or a locked case/container is a better option.
The other possibility is Chapter 140, § 131L which concerns weapons being stored by the owner. To comply with 131L, you need to secure the firearm in a locked container or equip it with a tamper resistant mechanical lock or other safety device. (§ 131L does not say the firearm has to be unloaded.) What is an appropriate locked container under this section – according to the Court, “the container must not merely be locked, but securely locked … [i.e.,] maintained in [a] locked container[] in a way that will deter all but the most persistent from gaining access.” Com. v. Parzick, 64 Mass. App. Ct. 846, 850 (2005). Use something sturdy with a reasonable lock.
The only case interpreting this area, so far, is Com. v. Reyes, 464 Mass. 245 (2013). In Reyes, the defendant was a corrections officer who carried his firearm to work one day at the house of correction. When he asked for a key for a gun locker, he was told the lockers were all full. He put the handgun, including a loaded magazine, in the glove compartment of his car, parked in an employee lot directly in front of the entrance, and went to work. It is not clear whether the glove compartment was itself locked. Corrections officials asked to search his car and found the firearm. He was arrested, and convicted of violating both G. L. c. 140, § 131C (a) (carrying statute), and unlawfully storing a firearm (after leaving it in his motor vehicle) in violation of G. L. c. 140, § 131L (a) and (b) (storage statute). Reyes was granted a new trial after his appeal because the trial judge did not properly define “locked container” for the jury.
The Court said that leaving a firearm in an unattended vehicle while at work is storage under G.L. ch. 140 § 131L. A secured container “must be capable of being unlocked only by means of a key, combination, or other similar means.” The locked passenger compartment is NOT a secure container. The Court specifically mentions a locked vehicle trunk as a possible secured container.
As to the glove box, the Court says it might suffice under § 131L “depending on the particular factual circumstances including the nature of the locking mechanism, whether the motor vehicle was also locked and alarmed, and ultimately whether in the circumstances it was adequate to ‘deter all but the most persistent from gaining access.’”

The Court discusses § 131C, but does not explain whether § 131L applies to all instances of a firearm left unattended in a vehicle, or to only situations where the firearm has been left unattended for a long period. Reyes was convicted under both statutes, implying that both applied to him.

Bottom line:
The safest process is to put the firearm, unloaded, in a secured case or container when you have to leave it unattended. This complies with both 131C and 131L. If you have a trunk, you can keep a lockable storage box in it in case of an emergency. (If you use a proper storage container, you don’t have to secure the container to the vehicle – see Com. v. Lojiko, 77 Mass. App. Ct. 82 (2010)). While the locked trunk will qualify, a container within the trunk will protect your firearm from dings and scrapes, and inadvertent discovery if you have to open your trunk while it is still there.