Another thing to keep in mind from experience:
The CYMA's barrel isn't held particularly well (the only thing holding it in is the small pin that screws into the side of the rear sight block) and the design of the gun relies on the gas tube being attached to the prong that extends from the rear sight block in order to keep the front of the gun relatively rigid.
In order to install the railed gas tube you have chop off that prong (the gas tube bolts on to the barrel) which means everything forward of the rear sight block is mounted to the barrel only. This creates a very wobbly barrel/front end (i.e., there's noticeable play). Be prepared to do some serious shimming and other minor mods/hacks if you want to get your front end rigid again.
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