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I remember reading the consequences of shooting two and three quarter " shells in two and a half" chambers
Can anybody point me to something on the subject please??

SIMPLE DONT DO IT!!!!!!!!!! GUN SCRAP YOU HOSPITAL SOONER OR LATER.

Many people habitually shoot low pressure loads in short chambered guns put up in longer hulls. They don't blow up their guns, nor do they end up in the hospital.
Its the "LOAD" that's much more important than the exact length of the hull. No shell should be fired which is long enough the "Loaded" shell is forced into the forcing cone as this can retard the opening of the crimp creating a pressure spike in the chamber.
Both Gunmakers & Proof Houses have sanctioned the use of shells with a loaded length shorter than the chamber, but which will upon firing when the crimp opens the end of the hull will lap into the forcing cone a bit.
Most factory 2 3/4" shells are loaded to a higher pressure level than should be used in the shorter chambered guns. It is however the load, not the length, which eliminates their use.

Sherman Bell, in article titled "Finding Out for Myself" Part V "Long Shells in Short Chambers" (Double Gun Journal, Winter 2001) said that with loads that are sensible in a light 2 1/2 inch gun, chamber pressures were only slightly increased by using 2 3/4 inch shells. He felt there was no reason, related to safety, to lengthen an original 2 1/2 inch chambered gun to shoot 2 3/4 shells, as long as the pressure of the 2 3/4 inch load you use is the same as the pressure of a 2 1/2 inch load.
Most turn-of-the-century U.S. maker's guns had chambers slightly shorter than the shells for which they were intended; 2 5/8" for 2 3/4" 12g. I had a 20g Parker with chambers close to 2 3/8" and recoil was markedly increased with 2 3/4" target loads.

Since 2001 the availability of factory loaded 2 1/2" shells, and components (if powder can be found) has increased markedly so it is much easier to use short shells today.

I seem to remember the article I read was in a shotgun magazine type of book, an American publication, it had a few articles about guns and so on
I just want to read about what it does...... not about loads, and so on

The DGJ Winter 2001 back issue is available
http://www.doublegunshop.com/doublegunjournal_backissues.htm

The best information hat I have ever seen on this subject was reported by Major Gerald Burrard in his 3 volume book "The Modern Shotgun". This was thoroughly studied by the British Gunmakers & ammunition loaders well over a half century ago. The studies actually started with the introduction of the fold/pie crimp hull in the late 1930's which resulted in a shorter length of the "loaded" shell than a shell with a roll crimp. They were finalised immediately following WWII prior to the 1950's.
Burrard didn't just depend on what He could "Find Out for Himself" but worked very closely with the Pro's in the field & reported their findings.
Note that if the gun has a step or a short angle rather than a Cone Proper then a shell which opens longer than the chamber length should not be used as the end would then lap up into the bore itself rather than in the cone. This condition is very rarely found & mostly on very old guns.

Thanks for the answers, but the book or mag that I read wasn`t all that old, middle 80`s?? or so

Don't take everything you read in the gun rags as gospel. There are some folks that try to pass themselves off as experts that quite frankly don't know what they are talking about. Burrard, Bell/Armbrust and Hause are my go to guys on the subject of shotgun barrel strengths and limitations.

The CIP, proof authority for members states that include most major shotgun manufacturing countries, state clearly that 67mm shells may be fired in 65mm chambers. But specifically warn against using other long shells in short chambers. Sixty seven millimeters is 2 5/8 inches according to my calculations.

We should be cautious with opinions on this subject considering that some not so knowledgeable readers read these posts. The safe practice, according to CIP is not to use 70mm (2 3/4) in shorter chambers. Same goes for 3 inch and so on.