I did some investigating into this area a few years ago. My BPS 10ga and My new 870 supermag took a big hit in the finnish from being underwater in the flood of '98. When Gander Mountain got it's business going again they ran a special, any gun camo dipped for $150. I did some checking and found that they were sending them to Houston,Tx to a place that did it for Gun Shops all over on a huge scale. I sent mine off through Gander and about 4 months later I got them back, and I couldn't be more pleased. It is tough, and it absolutely eliminated corrosion on the external surfaces. If you have a Gander Mountain near you, check with them. For the record I'm not a big Gander fan and especially not of their fleet of 19 year old gunsmiths. But in this case a little shopping and research paid off. But I thought Mossberg 500's wouldn't accept a mag extention. Best wishes.
chas