So you are probably charging your 1500mAh battery for about... hmm let's do a quick calculation since I don't know your charger - I'll assume 250mA output below:
1500mAh / 250mA = 6h (min), and the max being 120% of that (or 1.2 x 6h) = 7.2h (do not exceed this!)
So the new battery will be 3800mAh / 250mA = 15.2h (min) and 18.24h (max)
I sure hope you have a 500mA charger :)
You can also get a quick charger like this one
http://www.hitguns.com/Smart-Universal-C
not bad for $30 and will charge any 7.2v~12v 1100mAh+ battery in just over an hour (I have that one, it's great). The only thing it lacks is a discharge cycle for nicad type packs. Get what you pay for :)
cheersWill changing the mAH of the battery in your airsoft gun mess up your gun?no, it just increases the power of the battery and the gun. it increases the rpm. but don't buy a 4200 mah battery your gun will go boom