Is it bad to use 0.12 grade bb's in the Crosman c11 CO2 airsoft gun?

Yes, you are probably experiencing a drop in your accuracy level. CO2 is very powerful, and when you use a light BB like .12g, youre going to shoot badly. Take a kleenex, roll it up into a ball and throw it as hard as you can at a target. After that, take the kleenex and wrap it around something heavy like a quarter. Tape it securly, and then throw it at the same target.

Its a simple analogy; Your throwing arm is the CO2. The un-weighted kleenex is the .12g BB. And the weighted kleenex is a heavier BB such as .15g or .20b BB!

So if you want better accuracy, upgrade to a heavier BB.