20-76 would indicate a 3" chamber

20-70 a 2 3/4 " chamber.

76mm and 70mm approximately equal 3" and 2 3/4" respectively.

And chamber length is measured after the shell is fired, so a 3" will drop into a 2 3/4" chamber and when fired cause dangerous and excessive pressure.

See below for an example of marking to look for on my Arrieta 871 16 ga:

 

Chorizo. Your a champ!
I just could not remember the codes.

Thanks so much sir for ALL of your fine knowledge.

Enjoy, Roger.

 

Always a pleasure to help a Gentleman.

 

Thanks Chorizo. I really appreciate that.

I must be getting old, here's a post of ME telling someone the same thing back in Feb 2007.
http://www.shotgunworld.com/bbs/viewtop ... 6+chambers

I am still a bit confused.
My Ugartechea is a 3" chamber gun.
But my Arrietas show the 20-70 markings.
a 2 3/4" chamber.

A 3" shell drops cleanly with no resistance in both the Arrietas and lays deep and flush to the chamber/barrel.

I assume shooting a 3" shell out of this gun is a big no-no. Why?

Thanks again, Roger.

 

I found this on another web site:

If A Three-inch Shell Fits In My Gun's 2 3/4-Inch Chamber, Can I Fire It Safely With The Longer Shells?
The barrel of my shotgun is marked for 2 3/4-inch shells only, yet a three-inch shell will easily slip into its chamber. Does this mean it is safe to shoot three-inch shells in my gun?

Absolutely not. The linear dimension used to identify a particular shotshell refers to the length of its fired case and not that of the loaded round. Actual length will vary slightly among various brands of shells, but those designated as 2 3/4 inches usually measure around 2 5/16 inches before firing and 2 5/8 to
2 11/16 inches after firing. Three-inch shells usually measure from 2 1/2 to 2 5/8 inches before firing and lengthen to just under three inches when fired. Since the 2 3/4-inch chamber of a shotgun will usually measure at least that long, and sometimes slightly longer, you can see how an unfired three-inch shell will drop right into it. Firing shells of excessive length for the chamber can generate dangerously high pressures for the following reason: The inside diameter of a shotgun barrel rapidly becomes smaller just forward of the front end of the chamber in an area called the forcing cone. When a three-inch shell is fired in a 2 3/4-inch chamber, its crimp section opens into the forcing cone and in doing so effectively reduces bore diameter at that point. If the shot charge and wad column are required to squeeze through this partial obstruction, chamber pressure can increase.

 

I guess that is why I will never be a commercial writer (or maybe even a technical writer) I am stingy with words.

I said:

And chamber length is measured after the shell is fired, so a 3" will drop into a 2 3/4" chamber and when fired cause dangerous and excessive pressure.

They said:

The linear dimension used to identify a particular shotshell refers to the length of its fired case and not that of the loaded round. Actual length will vary slightly among various brands of shells, but those designated as 2 3/4 inches usually measure around 2 5/16 inches before firing and 2 5/8 to
2 11/16 inches after firing. Three-inch shells usually measure from 2 1/2 to 2 5/8 inches before firing and lengthen to just under three inches when fired. Since the 2 3/4-inch chamber of a shotgun will usually measure at least that long, and sometimes slightly longer, you can see how an unfired three-inch shell will drop right into it. Firing shells of excessive length for the chamber can generate dangerously high pressures for the following reason: The inside diameter of a shotgun barrel rapidly becomes smaller just forward of the front end of the chamber in an area called the forcing cone. When a three-inch shell is fired in a 2 3/4-inch chamber, its crimp section opens into the forcing cone and in doing so effectively reduces bore diameter at that point. If the shot charge and wad column are required to squeeze through this partial obstruction, chamber pressure can increase.

 

Holy moly Chorizo.
Would you believe I missed that entirely.

You did answer my question in your first post!

I read it in a rush on an iPhone got the 20-70 info and just plain old missed it and never looked back.

My fault.

Thanks again, Roger.