I've heard it told that people often misjudge a 2.5" chamber because they load a standard 2.75" shell in the chamber and the action closes on it. However, as we know, a standard 2.75" shell is actually closer to 2.5" when unfired and therefore should chamber without issue in a 2.5".

I've often wondered if you can therefore make a rough estimation of a standard 2.75" chamber by seeing if the action will close on a 3" shell? I don't plan on using this technique if my life depended on it, but I've just been curious if this would technically work?

Why not just measure?...Geo

With a bit of practice a 6" steel rule can be used to feel the start of the forcing cone and thus the length of the chamber.

That is how I do it.
Set the barrels in the vise, shine a light in the chamber. put the rule into chamber until you touch the forcing cone, read the rule at the outside edge of chamber.
O.M

If the gun isn't properly marked as to chamber length, it could also have a 2-1/2"(2-7/16") chamber, and a 3" shell chamering isn't a relible indicator of chamber length.

Fahgetabout using the EASY button, and measure the chamber(s).


.

I tried measuring it with the slide extension on my calipers, but I couldn't seem to feel the end of the chamber/beginning of the forcing cone.

You could take an index card, roll it up and poke it in the chamber. When you feel the forcing cone mark the place on the card where the actual chamber starts, not the end of the breech. Unroll the card and measure...Geo

Thanks Geo...I'll give it a try.

I realize this is not the answer to the question originally asked but If you have access to a lathe turn down a length of hard wood or metal to a slightly smaller diameter than the chamber.
While this gauge you are making is in the lathe cut a very thin line at 2, 2 1/2, 2 3/4 inch etc. you will then have an inexpensive gauge that will last for years and will accurately measure chambers at a glance.
Of course these gauges are commercially avalible but here is a project a person could knock out in an evening and do multiple gauges.

Please be aware that some guns were manufactured with a seamless transition into the bore. So accurately measuring chamber bore and place where diameter reduces is the only correct indicator.
If your cases are ejected with a wragged mouth , it's a good indicator that the case may be intruding into the end of the chamber once fired .

While that may work satisfactorily for the individual gun that the gauge is made for, it could not be relied upon to work for all guns of the same gauge. There are enough variations in the inside diameter of doubles to make any gauge of this type not be universally accurate. If the chamber is .001" undersized, the gauge will read 1/4" short.

Lots has been written on this in the past.

http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=466184&page=all

http://forum.foxcollectors.com/viewtopic...er+taper#p12614

SRH

So.... If I had a 2 1/2 inch gun with oversize chambers, and a "proper" gauge showed it as 2 3/4, would 2 3/4 inch shells be OK??

oldstarfire,
They would be here, we don't have proof houses. In Europe, it would have to be re-proofed and marked. It looks like it has been rechambered for 2 3/4" shells.
Mike

Yes! This is easy, cheap, and accurate. You don't even need the flashlight or a vise to do it. Set the barrels on top of a cardboard box on a table. Sit in a chair and look down the barrels while they are pointed toward a window. You can see the end of the 6" steel rule as it reaches the beginning of the forcing cone. Done this way, you can then also learn what it feels like when you find the end of the chamber. But, it can be tough to feel the start of the forcing cone when the taper is long and the transition is gradual. That's why I find it best to actually look into the chamber to see where the end of the 6" machinist ruler is. As you can see from the diagram below, a 2 3/4" or 2.750" chamber is measured from the very breech end of the barrel.

When you slip the 6" rule into the chamber, edge along the bottom of the chamber, there is no visible light under it.
As soon as the rule goes into the forcing cone it lifts from the bottom of the chamber & the light shows the gap. Slide the rule back out till the gap underneath just disappears.
O.M

Stan honestly I have not noticed any real issues.

I have that same set of gauges and also find them to be quite satisfactory.

I like to use Hosford Barrel wall thickness gauge for checking chamber length. Watch the needle closely it will change directions at the end of the chamber then read the scale on the rod and you have the chamber length. This method gives an accurate reading even with a chamber that is at the minimum or maximum of the chamber diameter spec. Its also good to know the thickness at the end of the chamber if it has been lengthened .

I use the Galazan pocket gauges and the black plugs for quick checks but they can lead you astray with a tight chamber.

The links in Stan's post offer a very clear explanation of the potential for inaccuracy with either commercial or homemade chamber gauges. If there was zero taper to a chamber, and absolutely zero tolerance in chamber dimensions of every gun you check, all would be fine. Unfortunately, that is not the case.

If your gauges lied to you because you were measuring a chamber that was very slightly out of spec, then firing 2 3/4" shells in a 2 9/16" chamber in a 16 ga. gun probably isn't going to cause any issues that you would notice. Even if firing the longer shells boosted chamber pressures by 1000 psi or so, it probably isn't something that you would feel or something that would cause immediate damage to the gun. But you would know as much about the actual chamber length as if you hadn't bothered measuring them at all.

We're all shooting guns made for thick wall paper cases. The shells we are using have thin wall plastic cases. When one of these plastic cases extends an extra quarter inch or even more, into the forcing cone, it is of little consequence. More important is the use of ammunition of a pressure level correct for your gun.

If you cut a fired 20 gauge shell in half and insert the head half into the end of a fired 12 gauge shell, you'll have a chamber gauge. A 28 gauge shell works for a 16 gauge.

This is what I have been using, for years.