Can you put 20 gram bbs in 12 gram gun?

yes, you can. the .20 and .12 mean .20 and .12 grams it doesnt really matter but the .20s usually go straighter whereas the .12s being lighter curve up. if your gun says .12 bbs its usually because its weaker. .20 bbs wont wreck it but wont go as far in a .12 gun