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Hello,
A few things to be aware of- and the first is what was already mentioned: be sure the chamber ring and the barrel/extension are fitted properly.
The chamber ring is what sets headspace with the barrel/extension.
If there is any ridge, or bump, or dip where the two meet, there is a problem.
Also, I don't know if you are aware, but Nickle Steel barrels on Winchester Model 12s were pretty much the 'norm' until around 1930-34.
You state that your barrel extension is numbered 516xxx. If it says 'nickel steel', I'd avoid the stoutest loads in it.
They moved to Winchester Proof Steel around 1930-34, IIRC, and those are supposed to have a bit more safety factor for loads.
If your barrel says Winc Proof but the extension is 516xxx, I'd bet it was a custom fit.
BTW- there is competing thoughts about using steel shot in Win Proof Steel barrels also. But, if you ARE going to do it, the general recommendation is only modern ammo and no shot larger than 4. So 4-9.5 shot, but nothing bigger. Oh- and not in FC barrels, but Modified or more open.
As to your issues- you've had some good tips that I can't improve on.
If your chamber is too larger [wallowed out], it would do this.
I will say, I've never tried to drop a used shot shell into a chamber- so I don't know if it should drop right in like a new one or not.
I have 5 model 12s, so I've had to do some troubleshooting and fitting. 2 of them needed new adjustment sleeves [and adjustments] while the other 3 needed to be adjusted with the existing sleeve.
Good luck- they are a great piece of history!
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