Legitimate defense can be not only a right but a grave duty for one who is responsible for the lives of others. The defense of the common good requires that an unjust aggressor be rendered unable to cause harm. For this reason, those who legitimately hold authority also have the right to use arms to repel aggressors against the civil community entrusted to their responsibility.

• CCC 2265

Here in the states, we the people are recognized as holding legitimate authority to keep and bear arms, so such a position would be in contradiction with extant Church teaching.

Also, Papal infallibility isn't something the Pope can just "activate" whenever he wants. It's only recognized when the reigning pontiff is making explicit statements on faith or morals, and only in particular circumstances.