New
New
[Black Hat walks in.] Black Hat: New headcannon:
[Cueball is sitting at his desk, using his computer.] Cueball: Yeah?
[Black Hat lifts his hat, revealing his "headcannon": a tiny cannon on the top of his head. The headcannon fires and blows up Cueball's desk, the explosion throwing Cueball backwards.] Headcannon: BOOM Cueball: Augh!

This comic strip uses a play on the homophonic relationship between " canon ", the literary term, and " cannon ", a projectile weapon. The word headcannon is a compound of "head" and "cannon".
In this strip, Black Hat tells Cueball that he has a "new headcannon". Cueball , thinking Black Hat means "headcanon" (an unjustified belief or theory about a fictional universe), inquires what Black Hat's new idea is. Instead of the expected idea or theory, Black Hat removes his hat to reveal a tiny cannon on his head which blows away Cueball and his computer desk.
While headcanon may often be ignored or dismissed as a personal theory, a headcannon would be far harder to ignore, as it is a physical object which has a notable (and in this case violent) impact on the real world.
In the title text Randall makes the spellings of these two words indistinguishable by using three consecutive "n"s to spell "headcannnon". Therefore, the title text is deliberately vague. It could be interpreted that it is easy to convince people that you have a cannon on your head, that it is easy to make people believe in a self invented headcanons, or both. Since you are choosing your own interpretation of this title text, the joke is that you are creating your own headcanon.
This comic also shows Cueball being once again distracted from his work in a manner similar to 1388: Subduction License .
In terms of a given literary series, " canon " describes a set of works that are collectively recognized by the community as having authenticity. Generally, works created or endorsed by the original author(s) are considered canonical. Not all original content is considered canon and not all canon is original content.  Sometimes creators will rewrite the canon (called a retcon ) and make things that were previously canonical non-canonical. For example, the origins of a character may be rewritten, thus invalidating the portions of the works that speak to the old origins. Other times creators will incorporate non-original content and therefore incorporate the canon of these borrowed works.
A headcanon as the name implies is a form of canon that only exists in one's mind. More specifically, a headcanon is created when a consumer watching or reading the material develops their own ideas about a fictional universe that are not actually part of the canon, perhaps developing their own backstories or experiences for characters. Some frequent examples of headcanon include relationships between characters, abilities, events following the conclusion of the work, etc. which the author or creator has not explained or included. For example, a consumer may "read between the lines" and assume that there was a previous romantic relationship between two characters where no conclusive evidence actually exists of one. Some fans who come up with particularly interesting or convincing headcanons may decide to share them with others in hopes that their idea spreads.
[Black Hat walks in.] Black Hat: New headcannon:
[Cueball is sitting at his desk, using his computer.] Cueball: Yeah?
[Black Hat lifts his hat, revealing his "headcannon": a tiny cannon on the top of his head. The headcannon fires and blows up Cueball's desk, the explosion throwing Cueball backwards.] Headcannon: BOOM Cueball: Augh!
