Shotgun chamber recess??

[montveil]

I have a 410 shotgun and it has a "recessed" 3" chamber--What I mean is the chamber is larger than the bore. None of my other 12 ga have this feature.

What is this type of chamber called?
Why is it made that way?
How does it affect performance?

Thanks for any help

[Graybeard]

All chambers once upon a time were larger than the bore. The bore was the diameter of the inside not the outside of the case. These days most all 12s and many 20s are back bored or more correctly called "over bored". When they are the bore might be larger than the chamber tho that is not how it used to be.

There should be a forcing cone smoothing the transistion.