There are plenty of tutorials not only on this site but all over the web.. but anyways,
Model.mrs
1. Decompile Model.mrs and add the Elu and the Texture file for your shotgun into Model.mrs/Weapon/Shotgun.
2. Go back to Model.mrs and right click Weapon.xml and add this in the Shotgun section,
Code:
<AddWeaponElu name="blue_doni_shotty" weapon_motion_type = "4" weapon_type = "4" >
<AddBaseModel name="blue_doni_shotty" filename="model/weapon/shotgun/blue_doni_shotty.elu" />
</AddWeaponElu>
(Assuming the name for both the texture and Elu is blue_doni_shotty) Now once you've done that compile Model.mrs and you're done with this part.
Zitem.xml
1. Decompile System.mrs and open up Zitem.xml with Notepad. (Just like you did with Weapon.xml)
2. Find your Shotgun section and add this in,
Code:
<ITEM id="Item ID #"
name="Name Your Shotgun"
mesh_name="blue_doni_shotty"
totalpoint="0"
type="range"
res_sex="a"
res_level="5"
slot="range"
weapon="shotgun"
weight="15"
bt_price="200"
delay="1100"
damage="6"
ctrl_ability="60"
magazine="3"
maxbullet="15"
reloadtime="5"
slug_output="true"
gadget_id="0"
hp="0"
ap="0"
maxwt="0"
sf="0"
fr="0"
cr="0"
pr="0"
lr="0"
color="#FFFFFFFF"
image_id="2"
bullet_image_id="0"
magazine_image_id="0"
snd_reload="we_shotgunpa_reload"
snd_fire="we_shotgunpa_fire"
snd_dryfire="762arifle_dryfire"
desc="Description in-game"
effect_id="7" />
(Edit each item in red to what you want)
3. Save the Zitem, then right click > copy and paste it over the existing one in Matchserver. Now compile your new system.mrs with your new Zitem.xml and test it out.
Note: If you want the item in the Shop you'll have to add it into your Shop.xml in Matchserver (It's not to hard to figure out how) But just send yourself the item through SQL or your Admin Panel and it should be in your inventory.