I’m not a lawyer so if it really matters consult one and not take my advice at face value and just another voice on a message board.
As a primer
What is legal and what is not as to civil law is rather subjective as what it really comes down to is who has the better story as to rights or damages in a court of law and copyright is only enforceable by the individual or company who owns the rights and will act in their best interests even if what’s considered fair use doctrine does apply. So no copyright infringement is not criminal law and you will not go to jail, assuming of course your not just paraphrasing.
Patents only applies to manufacture goods or as a means of functionality as to design. If you make a digital weapon that shoots digital bullets it is not manufactured or functions in the same manner as a real weapon would as to it’s intended purpose of use.
You can not copyright what is common knowledge or what is considered prior art so a mechanic of a player bumping their head on a box to activate an event is neither unique or a function that a patent would apply to a given game or as a trademark entity, like Mickey Mouse, that would be considered as an infringement that has nothing to do with either copyright or patents.
You can not copyright common words, letters, or numbers even though Apple would have you believe other wise as to the commonly used letter “I”. Can only imagine having to pay Apple a fee every time a game reviewer said “I love this game” ;). MacDonald’s gets around this as they call their letter “M” the Golden Arch and although they can not claim ownership of a letter they can copyright the font that is called a golden arch font. 
Where copyright does apply is in the completed shape and form of intellectual property and copy-right is a means of conveyance between the owner of the IP as property to another for the purpose of distribution. For example as the author J.K. Rowling “owns” as property the Harry Potter series but grants others permission the Right to Copy in other media like films or merchandise. In this case Rowling’s, and her estate, will always own the IP as property in hand which is a right that never expires.
Trademark Infringement.
You can call a digital side arm a 1911 but you can not call it a COLT 1911 but it does not take much Googling skills to come to the conclusion that there is no recorded case where a game developer was sued for use of weapon branding and the only case as an issue is EA saying they will no longer pay branding fees in their games. Really does raise the question just how much does EA pay for licensing and is it just another fee to pay as part of the day to day business of games development and not just more look at me hype?
The use of Trademark though is where most developers can get into trouble from entities that are not even related to the ideology of game development. Trademark is in fact the true identity of any company that produces goods and services and the use of their trademarks, even in a video game, would be on par of someone stealing your identity and leaving you to have to deal with the consequences. This is their identity and something as unassuming as the Red Cross means something more to someone else as it does to you.
Not all companies are Draconian though as George Lucas does allow fans to use his trademarks and works as part of the Star Wars universe where Disney will shut your 90 year old grandmother’s Disney fan site down for using a few images of Mickey Mouse. As a matter of fact there is an extension to US copyrights called “The Mickey Mouse Protection Act”. In all though it’s if you plan to distribute your works, video game or other wise, is where due diligence needs to be served to understand how nasty the free market actually is and most fail because they do not understand the difference between making something and selling it to others.
In all <insert descriptive word I can’t use> shoot as to what will happen once you and your game, product, does get noticed but Universal vs Nintendo is a good starting point as to case study as this is where it all started so don’t worry about copyright until due diligence needs to be served prior to distribution and with luck you will make a ton of bucks that you can hire someone to deal with the annoyances afterwards. After all I’m willing to bet that Epic has a few in house lawyers hanging around that knows their/this stuff better than I do. 