Using an old toothbrush that has no residue on it can be a great way to clean rust or debris off of the ornamental features on the outside of pistols or revolvers, and some of the decorative smaller areas on long guns like rifles and shotguns. A toothbrush is not large enough to be able to reach each area of the barrel in a long gun which is why a bore brush or nylon cleaning brush is always going to be better than a toothbrush for long guns.
Pro-tip: Do not attach a toothbrush to a stick or piece of metal to clean the barrel of a rifle or shotgun as it could fall off and bristles can come loose and create a hazard in the firearm.
You should not use a toothbrush on any of the areas inside the firearm or that are used for firing the gun. This includes the inside of the barrel, the chambers, firing mechanisms, and the magazines.
A toothbrush is not designed for cleaning a gun, it’s for removing plaque and buildup off your teeth. Depending on what the bristles are made of they could fall into the chamber or barrel, melt against it under heat, or clog the barrel leading to multiple types of issues when using the firearm. If thread or cloth gets inside, especially in a magazine, it could jam it and create a very dangerous situation.
One other issue you may run into if you use a toothbrush to clean or polish a gun is the bristles are softer and more bendable than a typical nylon brush. This may increase the amount of time you spend cleaning your firearms. Some people find the process soothing and as a way to relax, so this could be a great option for them. But remember, the toothbrush is not designed for cleaning a gun, so you may end up doing damage to it.
Pro-tip: If you do use a toothbrush to clean your gun, gun oils and lubricants tend to be thicker than a WD-40, so opt for a thicker lubricant so the looser bristles of the toothbrush can support it better.
It’s always a good idea to use products designed specifically for cleaning your firearms like a bore brush made from nylon, bronze, brass, or other material designed for removing excess powder and cleaning your guns. It isn’t a good idea to clean your gun with a toothbrush, but you may be able to polish outside and ornamental features with one.