you need to link your .rul file from the config file in your user folder. Then the game will look at your file
It looks like your are using the same sprites, and all make sure you call the right sprites, if you want to add new sprites you will need to add those to the game also.
Also if you want to add more content, you need to know if you are able to research your weapon, manufacture it, and listed on the UFOpedia.
Once you have done that you have to make sure that the language file has the strings you are listing STR_MYPISTOL defined as well as STR_MYPISTOL_UFOPEDIA
if you do not want the STR_MYPISTOL_CLIP to show on the ufopedia you need to set that value to false. just check other items that are not listed in the pedia.
If i forget something someone more experienced will point it out