I just got an 870 and would like to attach as much spare ammo to it as possible. I have a sidesaddle, and am thinking about a sling with shell loops. Can anyone recommend a quality sling that holds a decent number of 12ga shells and doesn't drop them too readily ?

Best advice is..... DON'T.

The sling full of shells swinging around make controlling the gun nearly impossible.
You swing the gun on target and attempt to shoot, and the inertia of the shells swinging around cause the gun to be pulled off target.

If you want more on-board ammo, you might look at a Speedfeed magazine stock which holds 4 rounds inside the stock in spring loaded "magazines".  You can reach back and flip extra rounds out.

Also there's the butt cuff type which is an elastic band that fits around the stock and holds more ammo.

Just remember, the more ammo you carry on-board, the slower the gun will be in action.
If you're likely to be facing a enraged biker gang you may need the ammo.  If you're more likely to be facing one or two intruders, the extra ammo will actually be a handicap.

Yeah, I wasn't sure how great the sling would work either. I also prefer single-point slings if I must use one. Problem is, I don't like the butt cuff ammo holders as they inhibit weak shoulder shooting, plus the commercial offerings are mostly crap.

Ultimately, what I want is a grab-n-go configuration where I feel confident I have enough ammo of various types for any eventualities. Maybe I'll just attach a waist ammo belt or a shell box carrier via some type of quick release connector.

Hmmmm... I feel some wecsoging coming on.

My exboss ordered a bunch of Blackhawk slings with shotgun shell loops.   THEY ARE USLESS!

The slings curl from the elastic when they have shells in them, they are HEAVY... and they are hard to get the shells out when using the shotguns.  It is hard to sling the weapon using such a sling.

If you MUST have such a thing... get a shotgun shell bandoleer (sp?)

+1 to everything that was said above.

You're better off keeping the shells on your body in say a dump bag. Mounting shells on the side of your weapon will significantly increase the weight of the shotgun.
The sling loops are the worst.

-Steve

Don't buy the one Bushmaster sells, its made out of some stiff plastic, useless.

belt without elastic.
shotgun shell belt

Quoted: My exboss ordered a bunch of Blackhawk slings with shotgun shell loops.   THEY ARE USLESS! The slings curl from the elastic when they have shells in them, they are HEAVY... and they are hard to get the shells out when using the shotguns.  It is hard to sling the weapon using such a sling.

+1.
I still have mine.
I'll add that it attaches with hooks and that that doesn't help matters any.