I have an 870 and I was wondering about barrel porting.  There is a local gunsmith that will do it fairly inexpensively.  How effective is it in reducing recoil?  Are there downsides to porting a barrel?  My barrel is 18".

I have a ported Mossberg and I don't think the porting is very effective at all with 3" magnums. I put a Blackhawk stock on it and that really helped, but then the safety isn't in the best position as its on the top of the action. With a trigger guard mounted safety like the 870 has I'd be much better off. I didn't think of that at the time but I'll live with poor safety position rather than brutal recoil.

I have a vang comped barrel.  I dont think the comped part did much but make it looke purdy, I got a good deal on it new.

I've always heard it this way:

Benefits of porting work by way of pressure. The higher pressure you're working with, the more pressure the porting has to use (without getting into all the details of porting in and of itself).

That being the case, shotguns run at a relatively low pressure (we're not talking comped nine major here). To be fair, I don't own a comped/ported shotgun (yet), but wonder if we're getting into that point on the curve they call diminishing returns.

I had a 590A1 that came ported from the factory. Porting is useless.