It all depends on the child AND on what you expect. One thing I have always done is to go over the important rules before I go. The child MUST be paying attention to ME while I am doing it. Tv, games, books, all of it must be stopped.
I found that this did help.
As for the computers, you need to either have a very strong net-nanny program or teach your husband to not leave any computers available. in my opinion that is one that most 14yo boys (difficult child or not) would do. Expecting anything less is setting everyone up to fail. Of course, difficult children are going to do more and find worse stuff than anyone else, but that is why they are difficult children.
Does he have access to the airsoft rifle all the time? What are the rules for using it in the house? SHould it be locked up when he is not allowed to use it? (I truly do not know because we don't have one & I have not even seen one.)
The trash around the house, the mess, all of it is kind of normal. Especially the first few times. You have to work to train them (and husbands) to be okay at home alone. You need to either make difficult child clean up the airsoft pellets and all the other mess, or set an hourly rate that you would pay a cleaning service and charge him that. If he has no money then something of his must be pawned to pay the bill. If he really wants it back he will do extra chores to earn it back. Read Love and Logic Parenting for Teens, it explains how to do this very well. It is an AWESOME book for teen pcs and teen difficult children, in my opinion.
I took a Love and Logic seminar a couple of years ago. One man in his 20s was in his first few years of teaching. He was there with other teachers from his school. His mom was a teacher and was there also. It was not her first seminar.
The author who was presenting, Dr. Fay, was talking about a child refusing to clean up, then the mom pawning something to pay for a cleaning lady to come clean it. The son got mad and took something of mom's to pawn to get his item back. THe mom then called the police and pressed charges of theft. It helps set the responsibility back on the child and helps establish the authority of the parent. It also sets good life lessons and natural consequences into play.
THe young teacher burst out with "THAT is why she did it!" and everyone broke up laughing!!! His mom had been to a seminar years before, when he was a difficult child. She had tried everything else and he figured that he had her set up to let him do whatever he wanted. (As so many of our kids think we cannot make them do anything.) This really helped open his eyes when she did not back down, but enforced the consequences anyone else in the world would.
I did NOT make that up and they were not plants. In talking to them later, the mom taught with my stepMIL and knew my father in law. It let us parents see that while our kids may "hate" us at the time for this, it isn't forever and they WILL eventually learn.