[InnerVision]

My dad has an old .410 pump shotgun, and it sometimes will fire when a new shell is chambered.

What causes this?
Is it easy to fix?
Is it expensive to fix?

[Brian Williams]

As you said it is an old shotgun, many old pump shotguns did not have disconnectors. This allows for holding back the trigger and just pumping and makes for a great combat shotgun. Not one for the rabbit field unless you know to let go with your trigger finger after each shot.
Check while you have your finger on the trigger and pulled to the rear, pump the gun thru a cycle to reset it and; if it goes off every time it does not have a disconnector built in or it is completely worn off and malfunctions all the time.. If it goes off every once in a while while it is pumped and the trigger is held to the rear, the disconnector is worn to the point of malfunction.

No disconnector built in, you will just have to live with it, too expensive to add one. Disconnector failed completely or malfunctioning, a good cleaning or some new parts.

Try giving it a good cleaning and then retry the pump while holding the trigger back.

[mswestfall]

BW is correct, this could be the cause. It also could be a broken part. A qualified gunsmith can help with this.