First of all, you should invest in higher quality BBs, walmart BBs tend to have seam line that can damage your hop-up rubber. Higher quality BBs like BBbastard are less likely to cause jams and will increase your accuracy by a lot.
Personally, I would try to dissemble the hop-up unit and look for sign of damage to the hop-up bucking and sleeves (the rubber thingy at the end of your inner barrel
).
http://www.airsoftcanada.com/showthread.php?p=920838
Here is a guide on how to disassemble the SIG 552.
http://www.airsofthacks.com/2007/11/...upgrade-guide/
If there is no damage to the hop-up rubber, then I would move on to the gearbox to check for compression issues. I strongly recommend that you do not open the gearbox unless you know what you are getting into
(When I opened my mp-40 gearbox for the first time, it took me 3 hours to put it back together and do an ``ok`` shim job). Your SIG 552 is a V3 mechbox, here is a video on how to open the gearbox - ATTEMPT AT YOUR ON RISK - still trying it and succeeding at fixing your own guns can save you a lot of money
http://www.mechbox.com/mechbox/v3-me...g-upgrade.html