Glad to hear everything went well and even some of it better than expected! Means you doing it right! haha
As for your capacity, you have an advantage over the remington 870 with your 870. You can by Mag Tube Extensions that will give you up to 8 or so rounds. If you really want to dump some cash into your rig and have a hole box of ammo at your disposal, check out the X-Rail System. I forget how much it was last time I checked but it was a pretty penny. I also wouldn't consider it AT ALL for a "tactical" Home defense or self defense situation lol
Also, I noticed your title says "Remington 870" but your first post says "Mossy 870 Mariner".. lol Maybe clarify for us what you have exactly
Carry On!
Rick