Rule 1 of airsoft: Never. Use. 0.12s
EVER.
It might seem to you that we're snobs, but it's simply not worth damaging a £100+ gun to save a few pennies. Never go for any lighter than .20.
Thou weight is not the only issue when deciding what bbs to use. Always go for recognised brands (and stay away from bulldog, imo). Also dont use the few bbs that comes with the gun. The problem is your barrel is very tight, so if your bbs, that are being blasted at approx 1 joule thru it, are not perfect in shape and size (way beyond what your eye can catch), it will be damaged, and not unlikely also jammed.
Blasters,madbull,excel,guarder,g&g,marui,systema,marushin,src, or whatever is sold at your local airsoft skirmish site will all be fine thou. (creds to joshcowin for the list, also its from a thread posted just a few days ago, about just that, bulldog bbs:
http://www.airsoft-forums.co.uk/index.php/topic/14762-bulldog-ammo-any-good/ )
For your .12 bulldogs, and the bbs that will come with your new gun. Use them with your DE m83 for plinking at home. Have fun, just dont let them anywhere near your proper airsoft gun
Have a look around the forums, there's plenty to learn just from browsing