I have a Mossberg 500. When i have a shell in it and squeeze the trigger it doesn't go off the first time. It might take 2 or 3 squeezes till it fires. If i put a shell that has been already fired in it, it will fire everytime. Also, if i dry fire it, it will fire everytime. So i'm not sure when it will go off. It's a pump if that helps any. Can anyone give me some advice before i take it to the gunsmith?

Call Mossberg and tell them your problem. They may have a clue why the gun is doing that. I have no idea, can't see why it would make a difference unfired vs fired shell, etc.

Is it a new gun? If it is an older one, there may be worn parts, or it could be dirt inside the bolt or around the firing pin spring.

Tear it down completely, clean all parts and inspect for broken or worn parts, the reassemble. If it still acts up, I wouldn't use it until Mossberg or a gunsmith can diagnose and correct the problem.

it's a 30 year old gun. it was my fathers. So you know how much it means to get it working out right. This is the gun i strted out with as a kid. I just took it back out in the field after not using it for maybe 15 years. I'm making it my slug gun.

where are you from. is there a good gun smith around you i never heard of this happening before

I have the Mossberg 500 Trophy slugester in 20ga, my friend has it in 12ga., he just had the same problem with his as your having. Took it to the gunsmith, what the problem is, is the forend isnt locking all the way, there is a gap, he got it fix coat him $42 and its working perfect now.