Things you will need assuming you have a JTAG/RGH ready to go.
Download Mod Tool and extract them into the Mod Tool Plugin Folder. Start Mod Tool ,hit the Plugin Button and Start the Plugin you want. Enter your XBOX360 IP into the Textbox and Press Connect.
open Mod Tool and connect, and click the dump button
as you can see in the image above you want the starting offset as (D9000000) and the Dump length as (FFFFFF) Once you have your dump simply open up hxd and drag your dump in to hxd take a note at your gun ammo and clip, so say we have 15 ammo and 168 in our clip, simply convert the values to hex which would be 0x0F and 0xA8 so now you search for the following pattern “00 00 00 0F 00 00 00 A8” once you find that pattern, modify the 0F or A8 for the amount you want in your clip.