This should help:
Only upgrades my upgraded G19 has (yes, I have a stock one too with only an enhanced recoil spring/guide in it to help give longevity using propane) is a 6.04mm tightbore. It has a metal mag catch, enhanced hammer spring (for hot weather reliability) and metal slide, but only the tightbore increases the fps (I went from 305fps to 341fps using 0.20g BBs). Yes, the best I've found has been 0.30g BBs in a GBB (my KSC MK23 with no upgrades using propane regularly gets 150ft using 0.28g to 0.30g BBs, ist just has a long inner barrel and shoots only around 315fps stock............ but I recently put in a tightbore..............
And BBS, I am so gonna grab a bag of those in the next couple months. Heck, might as well make it two bags. Will let you know.