Have a 12 gauge Mossberg Maverick Model 88 shotgun
When firing it will not hit the shell hard enough to fire. The shell will be dented but will not fire
Thought maybe I got a box of bad ammo, but tried different kinds with same results
Are there any adjustable parts on the inside or am I better off just taking it to a local gunsmith?
My guess would be the firing pin spring is worn out. Could be from storing in the cocked and ready position. That can weaken a spring over time.
I’d start with a good lubricant in the firing pin area.
Apparently this is a problem plaguing this model. It's stopping in the "half-cock" not full cock. YouTube has a video, or you can carry it to a gunsmith for repair.
Mine had broken firing pin
Squid, if the gun is older and you have used it in dusty conditions, it could simply have fine dust collected in the firing pin housing. I'm not familiar with this model, but if you can remove the firing pin and spring, cut the fuzzy end of a Q-tip and use the middle part of the Q-tip as a ram rod to push any collected dust out. I always shot lube through mine thinking it would wash it out, but I was wrong. Last year, I had mallards and black ducks hanging in my face and click...click with the same issue. Did what I suggested with a Q-tip and ran 20 years of fine dust from Canada & dove fields out of mine. Back to 100% and when I take the firing pin and spring out, I now run a Q-tip through on every cleaning. A pipe cleaner will likely work on normal cleanings. I ran about an inch log worm of black dirt out of mine. Simply collects on the walls and eventually binds up the spring.