It says to deal a hit with the same gun but the gunslinger was errata'd to remove the "same gun" part. Is the cowboys suppose to be errata'd as well?As far as I can tell, no. This is a line of thinking I too have pursued, and all my research (including some con-asks of FFP) indicates that you should play it exactly as worded or - put differently - you should go get a Trun Disintegrator or a Plasma Arc.
Sidebar: Keep in mind that - a per the Official FAQ - any extra Hits done by Trick Shooting won't benefit from any +Damage, as the "extra Hits" effect doesn't replace the damage of the Hit assigned nor does it even affect the original target. Therefore, we can safely conclude that those extra Hits don't come from a To-Hit roll (they came from an triggered effect off of a normal Hit assignment) and therefore cannot benefit from +Damage.
Second Sidebar: While we're on the subject of the Plasma Arc and +Damage, I asked Jason Hill at Gen Con this year about how those two interacted (normal "Con Answers" disclaimer applies). He said that the two auto-Hits that get assigned to the target DO benefit. Because the effect replaces the damage for the assigned Hit and both of the resultant Hits will hit the target of the original Hit assignment, those two "on-target" Hits are considered to have come from the To-Hit roll assigned. Conversely, the splash Hits that emanate out from a Hit assignment do not gain the benefits of +Damage, because they are considered to fall under the Splash Damage clause of the Official FAQ.