Mossberg 500 and Remington 870 lockup with the barrel extension and bolt locking block. Any idea on the round counts for Winchester Model 12 and Ithaca M37 which lock up into the steel receiver?
Most of the old pre-war forged and milled parts "Tank" shotguns like the Winchester Model 12 and the John Browning designed Ithaca Model 37 apparently HAD no service life limit....They lasted almost forever so no one wore one out.
In the early 20th Century professional hunters used the Winchester Model 97 and 12 to virtually wipe out a lot of American game and they fired more ammo then they could count with no worn out guns.
Where shotguns began to have definite service lives was after WWII and the advent of shotguns made with "space age materials"; aluminum, stamped parts, and plastics.
It took some time but it was finally recognized that aluminum and plastic guns would actually wear out and would give trouble a lot sooner then the old steel tanks.
The classic case in point was Winchester and the infamous 1964 reordering of their lineup of guns.
They could no longer afford to make the old Model 12 so they came up with a new space age Model 1200.
It was serviceable for sporting use, but where the wheels came off was with the police, which until then Winchester OWNED the LE market.
Winchester sold the 1200 to the police as a totally modern gun that was better and cheaper then the old Model 12.
What the police found was that it just didn't stand up to the abuse that police guns were then getting.
Before this, police guns were literally riot guns.....They sat in racks at headquarters until there was a riot, a man hunt, or a barricaded suspect, then the shotguns were issued out.
As soon as the situation was over, the guns were put back in the racks until next time.
Under those conditions any of the older designs would last forever, and in fact a lot of police departments had shotguns bought in the 1920's that were still perfectly good.
The problem was, in the 1960's crime exploded, drugged criminals got a lot more violent and were a lot more likely to start shooting.
The police were forced to begin putting shotguns in every patrol car.
Under those circumstances the modern aluminum and stamped internals shotguns just failed to stand up to the constant handling, practice shooting, and the police night watch version "Of hold my beer and watch this" misadventures.
The Winchester Model 1200 failed, so Winchester quickly attempted some fixes. It still failed so they did a fast upgrade to a new version named the Model 1300.
It failed too, and the police made it clear they were not buying any more Winchester shotguns, and no major police agency ever bought a Winchester gun again.
The gun that did stand up was Remington's Model 870.
Remington got this one right, using a forged steel receiver with heavy duty fabricated internal parts.
The 870 won about 95% of the American law enforcement market, which it still maintains today, because it does stand up.
Virtually every local, state, and Federal agency uses the 870, and it isn't because they get a cheaper price then other brands.
It's because since 1950 the 870 has proven itself reliable and tough enough to last under heavy use.
Mossberg made a try for the police market with the Model 500, but like the other aluminum and stamped internals guns it too failed and they had to do a major re-work to the Model 590 series to get any traction in the LE market.
Mossberg advertised that "Only the Mossberg was able to pass the grueling US Government 3,000 round test" to win the contract to supply guns to the military and some Federal LE agencies.
The truth was ONLY Mossberg submitted a gun to the test.
The test was a simple pass-fail test, which if the gun passed the company could submit a bid to supply guns.
Since no American maker was able to make a shotgun cheaper then Mossberg's cast aluminum gun, there was no chance anyone else was going to get the contract, so no one submitted a gun.
The point is, under most of today's designs aluminum receivered guns simply aren't as durable as a forged steel receiver gun.