2 things that cause the bbs to chop are low quality or light weight bbs, and aftermarket magazines. I recommend using heavier bbs. like .28s. For higher fps you should be using heavier bbs. if you adjust your npas for 300 or something then, .2s would be ok.
Most likely your magazines were the magazines that were made to fit the we m4 and we scar and not the ones that were made specifically for the we scar. Not to worry, there is an apparent solution. I have done it myself recently but have not fired enough rounds to see if it worked yet.
http://www.airsoftgiforum.com/viewtopic.php?f=21&t=7684
In summary your bbs are being sliced before exiting the barrel. I theorize this cuts up your hop up too. The lips of the magazine that feed the bbs do not insert high enough into the gun and on their way out make contact with the edge of the brass tube. It's a real mess and affects accuracy. The link above can explain it much better than I can.