Bought my first air gun (Umerex Glock 19 Gen 3) last week and have put around 300 “rounds” through it this far. I have been putting the small drop of oil on the top of he cartridge before tightening it down and now there is, what I call “smoke”, coming out the end of the barrel after each shot. It has only shown up in the last two cartridges.

Is this a problem? If so, what is the remedy/fix?

Thanks.