I believe that with modern PCPs there is little difference in accuracy with variable barrel lengths (although I am prepared to be shot down in flames on this). For the distances most shoot at, a shorter barrel will do the job just as well as a full length one. Look at the accuracy of rifles like the BSA Ultra or WH100k
However with the shorter barrel generally comes a shorter air cylinder and, consequently, less shots per fill.
If you are shooting from a vehicle or a hide a carbine is a must, as it enables you to move the gun around quickly and efficiently without getting caught on nettin