Arthur is referencing Colbey's gun being loaded when Marlin first set everyone up to get captured by Arthur, as a show of loyalty.
It was later brought up in Avalon, page 19.
And this video is highly applicable to the game Arthur and Marlin are playing:
There are some things I'd like to bring some attention to today! A few thousand people read this comic supposedly, so maybe you guys will be able to help these other guys out. First:
Are you of half-Chinese, half-European (maybe even German) ancestry, and under (or abouts) 30 years old? This girl needs a stem cell donor, and ethnicity is important for best chance at a match. Please reach out to the Facebook poster if you'd like to be tested. You can contact me if you can't get in touch there, and I'll try to coordinate.
Third, The Sword Interval Kickstarter is well on its way to success! There are a few early bird tiers left for some really great rewards, so be sure to claim them while you can! And if you haven't read this AMAZING comic yet, do yourself a favor!