The reports I hear on TriStars are mixed ...but the good side of this, is they do have the 5 yr warranty. These days there seems to be more and more shotguns coming out of Turkey / with mixed results ...some ok, some not ...
Most of the other shotguns in this price range ...Mossberg, Remington, etc -- do not have the 5 yr warranty - in that "under $600 or so price range" - and many of them are probably coming out of Turkey as well.
In general - if you have no issues in the first 50 shells ....it will probably run just fine for awhile. How long is "awhile" ....nobody can really tell you ...but I think you can probably expect 5,000 - 25,000 shells thru it with no major issues hopefully.
Some of us shoot 10 boxes a yr ( 250 shells ) / some of us shoot 10 boxes a week ...so the life of that gun will depend.... Is it a great gun for the money ( not in my opinion ) - but I've said many times on this forum, if you're going to spend under $ 600 for a semi-auto or an O/U at least consider the TriStar because it has the 5 yr warranty.
In my opinion - in the semi-auto market ---- Beretta has dominated it for a long time ( 390 and 391 series ) ...with Winchester, Browning, etc mixing in there in the gas operated guns. Benelli has dominated the inertia guns for a long time... but all of these guns are in the $ 1,000 - $2,000 price ranges. But in my opinion / these other guns are going to give you a life of 100,000 shells pretty easily ...maybe more ...
So its a trade off .../but if the gun fits you ..(so it hits where you look) you've accomplished 95% of the issue ...and as long as its trouble free ....just enjoy it and have some fun with it. I wish you "good shooting"...
My hunch is the extra " O ring" in the box ....is there as a replacement / if you break one cleaning the gun. By the way ....keep it clean and well lubed ...( every time you shoot it - inside the action especially ...not just the barrel ..) and it will extend its life / and improve its reliability !!