First of all, don't post so many details! The information you posted could be considered an admission and used against you in court -- assuming the DA could really establish you were the person who posted this.
Penal Code 245(a)(1) is assault with a deadly weapon, or by means likely to cause great bodily injury. It can be charged as a misdemeanor with up to a year in jail, or a felony with a maximum sentence of four years in prison. If the person personally uses a weapon and is convicted of a felony, it is a strike under the Three Strikes law. If you were granted probation, the judge could order you to complete a one-year domestic violence program because you had a previous dating relationship with your boyfriend.
Shooting a BB gun at somebody from a distance might not qualify as assault with a deadly weapon, but using the gun as a club might.
Nobody can tell you, based on these brief facts, what will happen under these circumstances. You need an attorney. If you can't afford to hire a lawyer, the court will appoint a public defender to represent you.
People can use physical force to eject trespassers from their property, but it has to be reasonable under the circumstances, considering the danger presented by the trespasser.
You might not have a self defense claim for swinging the BB gun at him because you were the aggressor. You can use self defense against a physical threat, but you can't just start hitting somebody because they yell obscenities at you.
You're asking how serious it is? Very. You're being charged with assault with a deadly weapon (or by force likely to produce great bodily injury). As a felony, assault with a deadly weapon carries up to 4 years in state prison and is a strike. Because there are two alleged victims, you could be facing two strikes, in addition to other charges stemming from this incident.
Self-defense and defense of property, with the right to eject a trespasser, are all potential defenses, but the viability of each depends on more specific facts, including what the alleged victims say happened in the police reports.
Do NOT discuss this with anyone else except a criminal defense attorney in a confidential setting. That means no texts to friends about this, no myspace posts, no facebook, no twitter - NOTHING. Anything you say about this can only hurt you. Trust me. Keep quiet and get a lawyer ASAP to start defending you.
I've been doing criminal litigation for 35 years and there is no question in my mind that a BB gun shot at a person from a distance is an assault by means likely to cause great bodily injury (Sec. 245 (a) (1). BB guns are not extremely accurate. That could easily have blinded one of them. So a 245 (a) (1) seems a strong charge here. I agree with my colleagues. Keep you lip zipped except to your lawyer and get a good one. This is a case where the challenge may not be saving you from a criminal conviction but keeping you out of prison.
You didn't say, but would I be really way off base if I guessed you were pretty drunk? You might want to think about AA if so, because this is Wayyyy beyond normal behavior. If that WAS a factor, admitting you have a problem and seeking help at AA would likely be of some help in handling this case. Good luck.