Okay.. by a rubber band, do you mean something like this;
http://www.fire-support.co.uk/product/layl...26-socom-hicapa ?
If so, then it's the hop rubber that has fallen out. You would need to find what parts your gun takes and buy a rubber that fits it.
To install, you need to get the inner barrel out, which means separating the body of the gun from the barrel/bolt housing, I'm guessing it's a few screws to undo, as it is on most sniper rifles.
Then you may need to unscrew the trigger mechanism and the mag release and maybe another screw or two, also remove any flash hider or suppressor you have on the end. After this, you should be able to gently push the inner barrel out the gun, along with the hop unit.
Take the barrel out the hop unit, which may require s clip or screw of some sort holding it in place. Then put the hop rubber on the end of the barrel following the grooves and then put the barrel and hop rubber back in the hop unit.
Re-assemble your gun.
Look on YouTube & Google for guides on how to disassemble your gun and do research on what upgrade parts it takes.
If this isn't the rubber band that has fallen out, a picture or description would be helpful.
Also, where do you do these 'private airsoft matches' with your mates?
