I know San Ramon! I grew up in Walnut Creek before headding to the peninsula. There are actually more and more fields in this area, but most of them have really odd rules for snipers. I haven't been to Area 48 but their field looks interesting. That field is pretty tied up in politics though (land is owned by a San Jose city council member).
If you don't mind driving a little further there is ARC Airsoft. (Paintball field that is expanded and used for airsoft on a regular basis) http://www.arcairsoft.com/
I'm not sure how old you guy(s) are, but ARC and A48 have the same issue; "12 year-olds". Now, I'm sure that since you're both here, even if you're young youre more mature than most of these kids out there. Between the "speed queen" fillipinos that cheat all day long and the 12 year olds that cry because my $150 ASR took them out with their $1200 ARES or Right-hook; there is only so much that a guy can take.
Now, there are two fields around that are very no B.S.
The first is ORD: http://www.cqbchallenge.com/
18+ great field. Better for DMRs than BASRs probably, it can be tight at times, but there are some great nests. The only down side is that they have pretty standard sniper AoE, that is only granted on a case by case basis once you have a rep at the field. But a well tuned 400fps DMR can do some serious work.
And then theres Vacavillian airsoft: http://civilianarmy.ning.com/events/vacavillians-airsoft-field-2
Private field with DMR and BASR rules. They have games every weekend. I have yet to make it up there, but I troll their forums and they seem like a very no BS group of guys with some good games going on.