The barrel sounds like it might be stock (~6.08mm). If your APS-2 is the SV (Sniper Version) it comes with a 6.05mm inner barrel, all other APS-2 variants don't have that barrel from what I've read.
Also, your gun will be happiest if you clean the inner barrel often, like every 20-30 shots. Sort out if it's happy with just a dry patch run through it a few times, or a patch with a drop of oil a few times, followed by a dry patch. My M24 with 6.03mm barrel prefers the oil then dry, but my KJW M700 with 6.05mm barrel prefers straight dry patch. A lot of my shooting when not playing is done in my basement at 30ft. After a couple dozen shots, the dime or bottlecap size groups I get start getting bigger and more unpredictable, so I run the cleaning rod through then start shooting again, fixes it right away. At least you picked the best and most consistant ammo for it, that's a HUGE chunk of 'mucho improtante'.
For fun, check this out:
http://warmongers.dyndns.org/forums/viewforum.php?f=31