Here is an ASM that will make (almost) any item of your choice function as an Arms Scroll.
This is similar to Brendor’s ASM, however it’s in a nice neat EA file, making it easy to install.
It will only increase a single weapon rank like in FE9/10/11/12 rather than all the weapon ranks such as in the 3DS games
It will base the weapon rank to be increased off the first item in your inventory
The item ID of the Arms Scroll must be above 0x5B
It will not increase your weapon rank if the weapon rank is A, and you already have an S rank in something else.
If wanted, it may be possible to make a version similar to the 3DS games.
reply soon with any bugs or issues.