Vertigo, I'm no smith, but try stripping everything down and see if there's something impeding movement inside the receiver. Grunge buildup usually takes longer than two sessions, but who knows?
Is it under warranty?
Zebulon, I'm still no smith(G). My personal favorites include Frankenstein, the one I built from parts. However, your question seems to be more geared towards "Serious" shotguns. So....
I put both of my "Serious" 870s together as good defensive shotguns and slug shooters for deer hunting. One has the original 30" Full barrel cut to 18" and a little, Lyman peep sight with ramped front sight, a two shot magazine extension and a long forcing cone. Trigger's under 4 lbs, and this one came like that. Owned it since Ike was president. This one has a 6 shot Side Saddle cut to 5 to clear the peep base.
The other I put together in the late 80s or very early 90s. The receiver started out on an Arlington Va Cruiser gun.I bought it to replace the earlier one for hunting, deciding it should serve simply as a dedicated homeowner's insurance tool. This one has a 3 shot extension, 20" barrel, Williams peep sight, long cone, and it took a smith and $40 or so to get a sub 4 lb pull. No S/S on this one, as a hunting tool it gets toted lots. Weight on this runs close to 9 lbs, the other half a lb more.
Both have black painted wood, sling studs, and good pads. Neither has a light yet. Both are quite smooth, and have an enviable record of performance.
Neither is state of the art, but good functional pieces that can serve my family for decades.
I'm no smith, but most of the mods and addons done on these do not need a smith. Exceptions were the cones, one trigger job and the D&T for the peeps.
IOW, if you can cross the street on the green light and use a screwdriver, you can do as well. I'm a klutz with most tools.
As for stocks....
My 870TB Trap has a LOP of very close to 15". I'm on the larger side, and find the standard stocks on my "Serious" guns a hair on the short side. That's all to the good. Resetting the position of the front hand on the forearm can compensate for a short stock. Getting a too long stock in position can take too long.
The 20 Gauge YE here for the kids has a Youth stock of about 13" LOP. I do not shoot my best with it, but I could pass a police qualifier with it. The 14" stocks come closer than any other length for GP use for me, and I could operate them under heavy winter clothing or armor. Have, in fact.
Hogue's overbuilt synthetic stocks come with a Cadet variant that's getting good reviews, but before that try...
Use a thick piece of old leather strap as a spacer. Add 1/4" to the short stock, replace pad, and try it out. Pachmyer Decellerators are outstanding, affordable and come in thicknesses from 3/4-1 1/4". Get the right one after figuring what works both for you and go aheadon.
If I were putting together another "Serious" 870 on a limited budget, I'd....
Get a beater police turnin or other short 870 of 38-42" long.
Get the stock fitting and the trigger under a 4 lb clean pull. For "Serious" use, if you can mount the thing easily under stress and your thumb doesn't bump your schnozz under recoil, it's close enough to work with.
Add studs for a sling.
Add a two shot extension. For most folks this seems the best compromise between capacity and front end weight.
Work polish it until smooth by BA/UU/R. After some time, I'd add anything that would aid accomplishing the mission, from a long cone to a light to upgraded sights. Add one thing at a time(If at all!) and shoot it in generously before making other changes.
If loaded with money, add a Vang barrel or similar.
FWIW, most of the shotgun cognizant professionals I've known have "Serious" shotguns setup along these lines. The bellsnwhistles and Ninja/Tac stuff are relatively scarce...
HTH, sing out if I missed anything.
