[Jmzzl]

Mine wasn't all that bad from the factory, but I added a Mako tactical pump this past October, and it wobbles pretty bad. It may be a pet peeve to some, but I don't really mind it all that much.

[fragout]

Pumps rattle, but Mossbergs rattle the most. I used many 590's in the Army, great shotgun. But it's heavy and rattles more than a 870 for sure.
No big deal really, didn't rattle enough on patrol to matter. A 590 will last untill the end of the earth, but the just don't feel as tight as an 870.

[1smoothredneck]

LOTS of rounds through 500s in my time, and I can't recall a single problem.
Rem 870 is fine, but I prefer the 500 for ergonomics, and value to boot.
Be safe

[tebklr]

My Mossberg 500s rattle like two skeletons gettin' it on in a metal trash can. They also run flawlessly. Never a hiccup.

My 870 does not rattle.

I like them all.

[Southpaw69]

My 500 rattles a bit, i just went and shook it...i never noticed before.

So, for shaking use the 870 is the clear winner.

For shooting, they are both fine. I got the 500 because i'm a lefty and the safety is in a better place for me.

[TxGun]

Non-issue. I have 20 or so pump guns of all kinds. Most rattle somewhat. The Mossberg might rattle a bit more than most, but it's as reliable as any of them. Worry about something else because this isn't worth the time.