My Mossberg's mag spring is significantly weaker than when I bought it in 1981. That said- it's still working. No bulges on those-wait for it- 31 year old rounds- I've heard that story but have never seen it myself.
At the shop I used to 'smith at, we did yearly "certify-for-duty" of every firearm in a 100+ person department (and we had several departments that we did this for), so every M-16, MP-5, 870, revolvers, 3-gen Smiths came through our door each year. It was interesting, as you really got to see the strengths and weaknesses of various designs.
The 870's had tons of handling wear from going into and out of the patrol car every shift, every day. They trained with thier 870s monthly, so white plastic buffer was everywhere, the barrels were filthy, but the 870's flat out worked. About the only issue I ever saw was some shell stops that needed to be re-staked, and one broken ejector. All the magazines fed 100%, year after year when we test fired them.