That's the early style,,the biggest difference aside from the receiver profile is inside.
The early style (520)uses flat springs for the mainspring, trigger spring, bolt latch, ect.
The later straight profile frame (520A) uses coil wire torsion mainspring, wire bolt latch spring, coil spring for the trigger.
I've never had a any of the above break on an early model.
I've owned probably 10 of the early models and half dozen of the late ones.
I have had to replace a few mainsprings on the 520A (coil torsion) that have just lost their power and were causing misfires.
They can be torqued open (unwound) in the opposite direction a bit to renew their strength, and that'll work for you for a while.
But they give it up shortly after the quick fix and a replacement has to be found. You have to watch what you buy as the 'new' mainspring may be just as relaxed as the one you want to replace.
Either 520 variation does wear out the ejector. Easy to get at.
A screw on the outside of the left receiver wall holds it in place in a slot inside. Two small 'hooks' are what kicks the shell out. The small space between them is for the left side extractor to ride past.
IIRC now there's two slightly different variations of the ejector.
The carrier spring breaks on occassion too. It's a small flat spring in both variations of the 520 tucked into a slot in the right rear wall of the frame. Two different types of those too.
Firing pins break occassionaly. Just fatigue from a lot of rounds I think. Dry fire does them no good and should be avoided.
I brazed one back together using the original pieces.
One I brazed on a piece of steel for a new front half, shaped it, lightly case hardened it. Both still going strong.
On very worn 520's, the bolt will come disconnected from the slide on the back stroke, leaving the bolt behind as you bring the slide forward. Not a good thing.
Easy to fix if the tip of the slide rod or the channel in the bolt it snaps into isn't damaged. A gentle upsweep curve to the slide rod is needed. You almost can't see it w/o sighting along it and you have to be careful placing it in as the slide rod is rivited into the sheet steel forend tube. It rips out easily if you try to bend the slide rod the wrong way. Many are solder/braze repaired back in place.
Starting to sound pretty bad!
Make sure the bbl to frame TD lock up is tight. The mag tube is rotated to lock the large 'nut' with the beveled lugs into the frame at the front.
They can be worn to the point were the bevel reaches their limit and no amount of tightening will take the looseness out of the TD . Shade tree fix is to peen the lugs (of course). That rarily works. You can shim the lugs and put it back together forever, never to be TD again, but it'll be tight!
You can build up the surfaces and refit,,or machine down the flat surface of the magazine nut and the bottom of the frame lug cut so the bevel can seat deeper.
The M200 weighs 6# right on the button.
I'm guessing,,but I'd say a 20ga M520 would weight just under 7#. Most of the extra weight is going to be in the bbl wall thickness.
