There are some differences in language between certain combination weapon traits that I'm having trouble with.
The sweep trait uses one form of language:
"When you attack with this weapon, you gain a +1 circumstance bonus to your attack roll if you already attempted to attack a different target this turn using this weapon."
The forceful trait, another:
"This weapon becomes more dangerous as you build momentum. When you attack with it more than once on your turn, the second attack gains a circumstance bonus to damage equal to the number of weapon damage dice..."
And the kickback trait, yet another:
"A kickback weapon deals 1 additional damage with all attacks."
My gut says that none of these traits should apply outside of the mode in which the trait exists. The RAW language, though, makes me think there's more nuance to it. Sweep, especially, feels like it should apply regardless of mode: it's still the same weapon, even if only one mode bears the sweep trait. Forceful seems like it should apply, but it doesn't have the same "this weapon" word usage. Kickback seems to disagree with both, implying that the kickback damage only applies when the weapon is in ranged mode - it's only a kickback weapon if it can kick.
I haven't been able to find anything in the forums addressing this issue so far. Am I missing something? Is there a good RAW way to rule this?
If you're asking whether you can benefit from Sweep when shooting your Axe Musket, no. The musket form of the weapon does not have the Sweep Trait.
If you're asking whether you can shoot your axe musket, switch to melee mode, and then benefit from Sweep with your second attack, that's slightly murkier and I've seen GMs rule both ways.
Much more the second interpretation, and the murkiness is the problem. Nearly the same with forceful - you've already attacked with the weapon in ranged mode, why wouldn't the damage bonus apply for a follow-up melee Strike? It also doesn't make sense that kickback would apply in any application of melee mode. The fact that these have sort of similar language without any real RAW clarification is causing me problems. "Go with my gut" works at my table, and it's sort of doable to see/implement what seems to be the RAI, but "murky" is spot on and what's driving this issue.
Now that I'm looking into it more, the language on the agile trait is a much clearer expression of how the agile trait works for combination weapons:
"The multiple attack penalty you take with this weapon on the second attack on your turn is –4 instead of –5..."
Which at least makes that trait function cleanly, but I'm not sure whether that should imply the other traits work when doing a ranged 1st, melee 2nd turn or not.