[Hectic]

CA stock cylender doesnt mean anything they make cylenders in all sizes (read port location) as an example a jg m4 can come with a 3/4 or a 4/5 port tho most will always have a 363 mm inner swapping to a 430 or 460 in those cylenders will increase the fps. If they both have the same pot location on the cylenders then you should expect simaler results between the different barrel lengths. Conversly if they have different cylenders going to an inner that is longer then the volume wouldnt really decrease fps but it would increase stress on the mechbox at the cylender head due to the airbreak effect the bb has while its in the barrel once the bb exits if the piston isnt fully forward it will slam forawrd the rest of the way alot harder then if the bb was still in tye barrel.
At any rate i wasnt sure if this was your issue i just thought id suggest it incase it mayne was causing the problem. Not that its a huge problem just tryin to figure why the fps difference.

Another thought is if its the exact spring in the two guns or just the same spring and rating as they could vary from batch to batch or even be misslabled say a 130 in a 120 package or what have you
Edit i re read and see its the same spring u are using