A friend & I were shooting guns at a long time unofficial range. Some fisherman dialed 911 on us. A county sheriff came, took their complaints, and issued us summons for "illegal discharge of firearms".
We find out from the Deputy that the area has recently been closed to firearms, but is still not posted.
Does anybody think we should fight this in court, and what would the penalty be?

Any help would be appreciated.

Yes, and genrally judges do not impose more serious punishments because you are exercising your right to a trial or hearing.

This could be serious if the summons is alleging a CRIMINAL offense. If it a "ticket" meaning a CIVIL violation it is less serious.

You should try to talk to someone at the DA's office and determine the severity of the offense charged and how the DA's office intends to proceed. They may decide not to prosecute if you let them know the circumstances and you intend to fight any charges. Then again just by contacting them it will probably make a DA read the report(s) and they just may decide that a prosecution isn't warranted.  

Many years ago I was in a minor situation with two friends. All three of us were charged with disturbing the peace.

As the one doing the 'disturbing' I went to the county attorney, who was the local prosecuter. We had a friendly discussion and I got the charges dropped against my friends.

I paid the fine, $50 IIRC, though as I think back I may have won had I fought it.

Talk to your local prosecuter and tell him what you told us. He MAY decide to drop it. If not, fight it.

As a general statement for this type of infraction the area must be properly posted.  This may not be true if it's in an area where it would be illegal to discharge a firearm under any circumstances.

I would consult with a local attorney who is familiar with the players involved and see if he can get the charge dropped.

next time shoot the fisherman

I live in the S.F. Bay Area and I have talked Leo's in my locality. I can shoot on my private property as long as I am am not endangering others. BTW the only legal issue is the noise constraints in the area. I also live within 100 yards of my neighbors.
alex
Edited to say "endangering others" is also a constraint.

I am starting to think the Deputy was BS'ing us. That is why he went from wanting to confiscate all our gear and bringing us in to letting us go with just a summons.

Either way it goes, we won't shoot there anymore.

This brouhaha has made me hate cell phones even more.