Another possibility is that this barrel was simply mis-marked. Chamber length is stamped twice, once in inches and once in millimeters. The millimeter stamp is part of the proof marks and is much less likely to be wrong. Unfortunately I can’t make it out in the pictures, but it should be in read 20-75 for a magnum chamber and 20-70 for a Light Twelve.
Let’s hope. I was pretty certain I’d see some obvious differences in a magnum barrel. I seem to remember a spring loaded ejector, but there’s nothing out of the ordinary that I can see. If it works, I’m going to conclude that it was mis-marked, not that a 3” barrel will work on a 2 3/4” gun.
I would agree, I don't think a 3" barrel would work, but it looks the same as light 20 barrel to me. I guess we could go a little further. Mr. Lewis can you send me some pictures with a measurement of the ejection/feed port on the receiver? I will post here if you can. Hopefully you can test fire this thing.
Yes, it is marked 20-70. I haven't fired it but I did drop a 3" shell into the breech end and it bottomed our on the rim of the shell. Would that not indicate it is a 3" chamber, which is what is marked on the barrel?
So load port 2.587 and feed port 2.521. Thanks for sending these. Just curious what those measurements are on another light 20. Not home right now but will measure this weekend and post if someone doesn't have access before then.
Chamber length accommodates the fired length of the shell. A 3” shell isn’t 3” until the crimp unfolds.
I measured the length of an unfired 3" shell and it mic'd at 2.66" so a "3" unfired shell would fit in a 2.75" chamber. I'm about convinced that based on that test and the information posted in this thread it seems reasonable that what I have is a mis-marked barrel as to the chamber.
This isn’t the first time I’d seen this issue with 20 gauge barrels, but I’d forgotten about it. https://www.shotgunworld.com/threads/belgian-auto5-magnum-twenty-with-questions.514050/#post-4329802
Mismarked or not if it cycles 2 3/4 shells, I would be happy. I see no need for a 3" 20 ga load anyway. I had forgot the barrel extension is different, the post on SGW remined me, so I am more confident that it will cycle 2 3/4 now. Are you planning to test ?