Questions on chamber length

[Cory Rams]

So here’s something new after all these years. I just watched a YouTube video talking about a double gun and showing marks on it. Obviously there’s not enough marking on mine really tell me much. The YouTuber was talking about his 28 gauge that had a “28-70” stampeded on the receiver. He said the 70 was mm and the equivalent in inches was 2 3/4 inches for shells. Well, I have a number 65 stamped on my underside of my barrels which if I went by this logic it would tell me it’s closer to a 2.5” inch chamber and not a 2 7/8” chamber??? Back in the 90s. I had a gun Smith measure my chamber and told me it was 2 7/8” 10 gauge. I have shot RST shells 2 7/8” in it with no problems. Drops right into the chamber and shuts fine. Wondering if maybe somebody had it bored out to 2 7/8” or if there’s just enough free play in the chamber that they fit???? Does this make sense to anybody here that can chime in and tell me if that’s what the 65 stands for? Actually 65 mm is equivalent to 2.55906” so I’m guessing it is saying it’s a 2.5” chamber that’s been bored to 2 7/8” or has a lot of free bore? Just about all my hand loads are shorter since I roll crimp them with 2 7/8 and shells and cut them down. I would say most of my hand loads are 2 5/8 inch to 2 1/2 inch. The RST 2 7/8 inch factory loads fit just fine in there. I’ve shot in quite a few of them as well so I’m kind of confused now if that’s what the 65 really means. Like I said above, maybe bored it out or it has some free play?