Barn job workaround?
 

Do you think this is accurate (enough)? I went through my socket set and found the widest one that would fit in the barrels. Attached an extension to make it longer, slid it in, made a sharpie mark on the extension where the edge of the barrel was, and measured it. I got 2 7/8”, so I should be safe to shoot with a 2 3/4 shell? Make sense? I guess you could also drop a 3” shell in and if <=1/4” is still sticking out….

Just buy the brass CSMC gauge and be done with it. It works, it gives you all gauges and is reasonably priced.
The only thing on it that may not always be accurate is the choke gauge.

thanks for the rapid reply! My only personal problems with getting the brass gauge are I don’t see myself ever needing to do this again, and I’d like to shoot this weekend. My concern with the steel socket was damaging something, but I was gentle. :)

When use a pencil or a dowel.

Yep, I also did it with an unsharpened pencil and flashlight and measured >=2 3/4" that way too.

Unfortunately Bill is a Fox collector and declined to add the 410 chamber on the original, well Herschel did. Bill great tool in the 1970's and still is.

I tried the cut the brass off an unfired shell and measure the remainder with a strip made from an old credit card. easy as can be and accurate enough to know . love this site!!

Gents, just buy the dam brass gauge. It's short money and you will find yourselves using it from time to time and on your buddies guns too. It's short money

Gun Evaluation Kit
 

As Phil stated, a gun evaluation kit is very helpful. I made this kit years ago when the components were considerably cheaper. It has saved many mistakes over the years. It can be carried in a pocket and is lightweight. I could have doubled my money many times over the years if I had wanted to. The contents are as follows;
1 Chamber gauge
2 Bore gauge
3 Bore light
4 Tape Measure
5 Magnifying Glass

This kit is used along with a list of pre established checks to help prevent mistakes. My son calls it my screw prevention kit.

Tom, your kit looks a lot like mine; screw prevention kit is a good name! I've since replaced the 6" rule and the round choke gauge with a brass folding chamber/choke gauge. It has saved me a few times, or at least helped me to determine what I was willing to pay! I like your tin...

https://i.imgur.com/17YxlTKl.jpg