This tag is for images sufficiently similar to the canon weapon in the anime (sketch, post #797684). Some artists deviate from the original design, make modifications, or replace the weapon entirely. When this tag seems inappropriate, use rifle for long-barreled guns or with rifled bores, or musket if it resembles a musket and has a smooth bore. If it looks like a rifled musket, then use both rifle and musket.
If the firing mechanism of the gun looks like a flintlock, then add this tag as well. However, avoid this tag as much as possible especially if you are unfamiliar with different gunlocks.