When I lived in an apartment I didn't have my gun safe, my alarm system or my dogs...but I did keep my guns locked up. I went to Lowes and got 4 stainless steel eyebolts (about a 1 inch eye and a 5 inch long course threaded bolt that was about 3/8ths of an inch thick. I also got a pair of long locks (like a master pad lock but with an adjustable length shank). The last component was a coiled cable for locking bikes up. I found some studs in the back of the closet. I drilled two small holes in each stud about 3 inches apart, one above the other, and screwed the eyebolts into them. I then put the shank of the padlock down thru the top eyebolt, then the closed loop of the bike cable and down thru the bottom eyebolt. Then I slid the lock onto the shank. I slid the cable thru all the trigger guards or some other conveneient location on my guns, and then locked the other side of the cable as described above.
While not perfect, I felt it was better than just leaving them completely unsecured. Also, the small holes in the back of the closet were easily mudded over when I moved out, and no notice was ever made of it. I used two eyebolts in each stud to prevent someone from taking a screwdriver and screwing them out. Pulling them out of the wall would have taken a LOT of strength. I guess the weak part would have been the locks themselves, but there is only so much you can do.
Cost of the project was, I'm guessing 20 bucks or so, with the locks making up the bulk of that.