Why do my airsoft bb's curves to left even without wind resistance?

my airsoft is JG M4A1 400 - 410 fps all stock, half metal. please give me some good answer, i have played my airsoft twice. and my bb's just turns left. i've try the following trouble shooting but still it wont work!!! adjusting my hop-up, adjusting my sights, cleaning my barrel and lubricating some of the parts of my AEG with silicone oil, what should i do ?