Just in case you've forgotten...
 

found this warning on the flap of a vintage shell box:

https://i.imgur.com/sDFNPqgl.jpg

soooo

20's in a 10 are OK?

From a late 1940s/early 1950s Western Super-X 10-gauge box --

Attachment 89709

Yet they sold reloading components.

I think the only issue is 20's in a 12, which will jam up several inches in front of the chamber. I don't think any other combination will jam

I remember a woman shooter friend who accidentally tried to load her 12 gauge with her lipstick.

The main warning I needed one time only was to take the same gauge shells as the gun I take, ended up a long time ago coming home after work, grabbing a gun in the case, grabbed a box of shells, ended up in a public game area with a 20 gauge side by side and 16 gauge shells, Long time ago, no good excuse.

Dont tell me what to do!

It didn't. It told you what NOT to do :)!

How 'bout using Proof Loads??

1933 lawsuit against Remington Arms related to the burst barrel of a Baltimore Arms Co. shotgun after using a Nitro Club marked Proof Load.
https://books.google.com/books?id=7p...C&pg=PA120&lpg
https://books.google.com/books?id=7p...C&pg=PA127&lpg
The “maximum commercial load” was described at “5 long tons or 11,200 psi” (calculated long tons/sq. Inch X 2240 = PSI)
The proof load was “7 1/2 long tons or 17,800 psi”.


1936 testimony by W.A. King, Parker Gun Co. regarding a barrel burst, likely a 20g shell inserted before a 12g
https://books.google.com/books?id=jU...J&pg=PA802&lpg

Met a fellow a few years ago who was sitting on a rock right after other hunters entered the fields on opening day. When I asked him if he was OK, he told me that he had brought a 16 gauge gun but only had 12 gauge shells with him. I offered to give him some of my 16's and went to my car to get them. When I returned with the shells he was so grateful that he offered to " bird dog " for me. Just before I was about to accept his offer he asked me " what does a pheasant look like ". I graciously declined his offer, but would show him one if I was successful.

I have a M21 20 gauge two barrel set. I thought I had put the short barrels on the gun, but when I got out in the field and tried to shoot a grouse, the gun didn't fire. Long story short - I had put the 20 gauge barrels on my 12 gauge frame and never noticed the width difference. Gun locked up tight, but the firing pins were no longer centered on the primers

One of my customers had a guest in S. Texas last year that ended up with a 28 jammed in a 20. ALL BAD. Gun and left hand badly damaged. Another hunter in the rotation picked up the wrong gun, dropped 2 28 ga. into it, realized he had the wrong gun, dumped the shells out( he thought), they fell in deep grass. In his haste to get to the point, he evidently didn't confirm the barrels were clear and put the gun back. He only found 1 shell in the grass.
I have a life long habit of looking down the barrels just before stuffing shells in. It just takes a second.......

that's a good habit to have....some times some of us fail to look at what shell we are trying to put in our gun....I one time grabbed the wrong shells for the hunt just as I was going to stuff shells in the old 12 ga double I relized they were 16 ga...lucky but it happens...easy to do.... charlie

In a pinch, I have used 12 gauge shells in a ten gauge gun. I would only do it if I were out of tens.