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The only diffrence between the J2's and Series5 is milling. Unless you are getting a pump version then the J2's have auto-shot (you can hold the trigger down and just pump the gun and every time the pump arm fully closes it will fire) and the Series5 you need to (pump, pull the tirger, let the trigger go, pump....).
When I was looking at guns I picked CCM because a lot of people said they were better then wgp but if you want the '03 body get the sniper.
Kyle