when I personally go with my bow + shorty, I use my parts on:
shorty level 1 > bow level 1 > shorty level 2 > bow level 2 >armour armour armour...
I personally prioritize the shorty since it is in need of the upgrades more than the bow
EDIT: also because that is the one loadout where I utilize both weapons equally, otherwise I only focus on leveling my most used weapon and maybe my other once.
Yes... I know I'm not ACP... but here is what I do.
I usually go with the bow and revolver (sometimes the shorty)... I rarely use the revolver or shorty though (only when I run out of arrows)... so I always upgrade the bow first to its fullest. Then, I upgrade the lesser used weapon (or shotgun... depending on loadout). I only ever purchase armor when everything is upgraded and I'm not using the parts to purchase shotgun armor.
I have never used any of the boosters.
I upgrade my shorty first as it needs it more. Then I upgrade my bow then shorty again then armour. Id advise that if you get the points for it to upgrade the bow twice before getting armour but Im too lazy to do that. I prioritise upgrades over armour because upgrades last all through the game whereas armour lasts a few seconds. I dont use the one use boosters but I advise you to use them because they are helpful. I hope this information helps legend and I enjoyed our chats here