There may be a local code that prohibits the OC of your pistol & rifle based on the jurisdiction. If you have a CCW endorsement some jurisdictions "permit" the OC of a firearm.
Given the context of your post I think that you are not discussing the OC of a LG as defined by the forum rules. But, I am not the final arbiter.
The statutes provided above are the ones that local governments use to infringe upon your right to OC a firearm.
Check the/your local codes.
If a cop sees you dressed as a Ruskie soldier they may check your firearms to determine their "realness." The rifle, in my humble opinion, should remain unloaded. A cop may give you a pass on the unloaded rifle because a rifle without ammo (no ammo on your person, or your friends person, could be considered "readily available) is functionally nonfunctional under RSMo 571.030 and local codes (I hope, local codes are notoriously out of date in may jurisdictions) and thus not a violation of state or local law.
If the pistol is your daily carry it will likely be loaded. Thus you will be subject to the OC of a firearm laws if any exist in the trick-or-treat jurisdiction.
Good luck and welcome to OCDO.