I have an old Ithaca 20 gauge double, and I'm concerned about whether it is chambered for 2 1/2 or 2 3/4" shells.

Were it a 2 1/2" chamber, would I be able to close the action on a fired 2 3/4" shell? how about an unfired 3" shell?

I don't have a gauge. Do you have any other tricks or tips for measuring it?

I did wrap electrical tape around a magic marker until the diameter measured .684", inserted it to refusal into another 20 gauge I know to be 2 3/4", and marked it. It will insert all the way to that mark in the Ithaca as well.

Is this definitive enough to assume it is a 2 3/4" chamber, or should I spring for a gauge??

A shotguns chamber length is measure from the breech face. If you have a good flat stainless ruler, you can insert it into the chamber region and feel it contact the forward edge of the chamber. A very simple process then to read the ruler. It must be remembered that a "shells" length is measured as a fired case. In other words, a 2 3/4" loaded shell actually only measures@ 2 1/2", and measures 2 3/4" when fired. Same with about any shell. A 3" magnum, loaded will only be @ 2 3/4" unfired, and will measure 3" when fired. So many think that because they can fit an unfired 3" magnum shell into a gun it must be a 3" chamber. Not so.

I would suspect that the action would close on an unfired 2 3/4 shell and might on a fired case. the problem is when the longer unfolds in the shorter chamber when fired and that is where you will get the high pressure. I'm not sure if there ever was a 2 1/2 inch 20 gauge shell but if there was it would be a reloading only proposition now. a loaded 20 gauge 2 3/4 shell is probably going to measure 2 1/2 inches long or so the chamber would measure maybe 2 3/4 inches in depth. you would have to look up the actual specs for more exact numbers

If you can give a better idea of the model, Ithaca's are hard because they were named for their designers and this is not marked on the gun. SKB -Ithaca models are usually 3". The NID were mostly 2 3/4". The Flues and Lewis are most likely 2 1/2 but could have been bored out and that could be dangerous. Probably Win AA's or Rem STS will be ok, but 2 1/2 RST would be the best bet. You really need to find a gunsmith that knows doubles and have them look at it. Most times old guns are safe and great to shoot, and I do a lot, just get it checked. It's hard to replace certain body parts. Send me contact info and i'll try to help. Mark

Thanks. i believe it is a Flues model.

Serial #202xxx. 26" barrels. 3 position safety.

We discussed this earlier in another thread. Another member stated that 2 1/2" was standard at the time, but I read somewhere else that while not real common, they could be had at that time with 2 3/4" chambers.

I want to shoot it, and have shot it before I learned that it may have short chambers. I do not want to blow it up, and certainly don't wish to blow me up.

How could you tell if it's been bored out to 2 3/4", or if it's factory, assuming it is 2 3/4? I don't know who I could get to check it out locally.

According to Best Guns by Michael McIntosh in 1924 Ithaca established standard boring 2 3/4 chambers in 12 and 20 ga. Do you know the year your gun was made? Larry.....oops I posted too late. The ser. # puts your gun at 1910.

Brownell's gauges do not seem to be too expensive. SHOTGUN CHAMBER GAUGE | Brownells

I do not think this is right. Unlike the chamber of a .40 S&W that has a lip at the end, in a shotgun after the chamber is a forcing cone to the barrel. I think you want to find out how far you can go in to reach the correct internal 20 gauge diameter, which I think is 0.685”

This, from the OP, sounds good:

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If I understood what the OP was trying to describe in his attempt to measure the existing chamber, I am guessing that the chambers originally were for 2 1/2" cartridges. IF this shotgun was made in 1910, the metallurgy is less stout than modern day shotguns. Given production variances in those days the metallurgical strength could be considerably less. I wouldn't shoot modern 2 3/4" shells in it even if the chambers were lengthened by a professional without further tests. Dies and trimmers are available to cut down fired 2 3/4" hulls to the 2 1/2" length. Then one could load very low powered shotshells specifically for that shotgun. IMHO, it is not worth the trouble. Old shotguns should become valued "wall hangers" at a much faster rate than old handguns. Over the years, there has been a much bigger jump in working chamber pressures in shotshells than most other types of cartridges. I am gently suggesting that you should appreciate this shotgun for what it is, 'an valued family heirloom'. Display it yourself or sell it to someone that is a collector and not a user.

If money is no object, then have a competent gunsmith get a Brinell Hardness test done on some part of the barrel lockup mechanism and then if acceptable, ream the chambers to 2 3/4". ............ Big Cholla

Order a flat of 2 1/2" RST shells. They will be delivered to your door and are good insurance. Mark

Here's a lot of info about shotgun chambers from Brownells.

kwselke gave the best information and for about $20 + sh you know for sure, I have a lot of tools that were only used one time but if I need them again I have them, or you could resell.

"MEASURING & RECHAMBERING SHOTGUN CHAMBERS"

http://www.brownells.com/userdocs/learn/Inst-100.pdf

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Pig Hunter got it right no matter the chamber depth. Order some low pressure loads from company like RST or Game Bore. Both sell 2 1/2 inch loads that will work very well. If the barrels are sound and on face; I wouldn't worry about the metal but your dealing with wood that is about 100 years old. Treated that stock gently.

When it comes to measuring the chamber a 6 inch steel machinist ruler works very well, maybe better than some of the plug style gauges which may bottom out early before hitting the forcing cone.

Also, on many shotguns made before WWII, chambers were cut 1/8 shorter than the fired cartridge length. So a 12 gauge 2 3/4 chambered shotgun will measure 2 5/8 inches to the forcing cone.

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I should have been more specific. I should have said, all the chambers I have tried to measure, I was able to find the length the steel ruler. I just measured an rare Winchester model 20, and the end of the chamber could easily be seen and measured. I am FAR from a shotgun guy, but enjoy the research and repair of older quality guns. Now you got me wondering. I guess I will have to start researching chambers and get my ruler out.

Several years ago (10-15), Hodgdon published a detailed shotgun reloading manual. It was on a 9x12" format and had reloading info for all lengths of hulls: 2", 2 1/2", 2 5/8", 2 3/4" and 3" . As well as the 3 1/2" for 12 and 10 gauge. The didn't include 2 9/16" 16 gauge. Some European shell are marked this, but they are short 2 5/8". The point is they have the SAMMI specs for chambers. Your gun was made several decades before the creation of SAMMI. So your chambers may be a little different. If you have a shorter chamber, reloading date is available. Hulls can be made from modern hulls, by trimming to length. (Use high quality AA or STS to start with and you wont need to replace as often.) A regular MEC 600 Jr. press can be used with a shim inserted in the crimp station to make up the length difference. Their data comes with pressure level information and you choose the lowest pressure for the powder/wad/ primer combo at the desired velocity you want. (I think 20 gauge ran about 1050-1075 back then, and your ammo would be regulated for that speed) The different lengths had to do with the amount of shot (3/4 or 7/8 oz.) and how bulky the "New" fangled smokeless powder was. You can use modern shot cup wads. If they are long, trim with scissors or extra space before crimp use a filler (I use navy or kidney beans). Only use lead or magnum shot. On Federal land/ waterfowl hunting bismuth shot (non-toxic) will be fine. NEVER USE STEEL OR TUNGSTEN SHOT in these old guns. If you call Hodgdon they may still sell the book, or they will give you load data. Lets keep those old family guns running. Enjoy your project. Ivan
PS. I load black powder for a number of guns. It is as easy as smokeless to load, it just requires a more thorough cleaning.

Bushmaster1313, questioned my earlier reply about chamber lengths. Admitting I am far from an expert on shotguns, I went on an internet search to further educate myself. Being computer ignorant I was able to find an excellent article from GUNREPORTS titled "The long and short of shotgun chambers". I have no idea how to post a link though. The well written reports and drawings affirms my first statement about being able to see and measure a chamber with a steel ruler. However by the second set of drawings, Bushmaster1313 beliefs are confirmed and mine dispelled. An excellent article describing and illustrating the different chambers and forcing cones over the years. Well worth the read.

Thanks for the help everyone!

I believe I'll take your advice and get a case of 2 1/2" shells. As light as it is, the recoil would be more pleasant, and maybe I'll be able to enjoy the gun without damaging it.

If you would move your cursor into the white line at the top of your screen where the internet address is located, and right click on it, it will give you a box that lists "Copy, Paste, etc." Left click on Copy. Come back here, and open a reply box. Right click in it and then left click on Paste, and a link to the article will appear.
I would like to read it.

Is this the one?

The Long and Short Of Shotgun Chambers

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Yes this is the article I read. Great information

On a 20ga Flues, especially that early, I'd just assume the chambers were 'short' (2 1/2") and treat the the gun as such if I was going to shoot it.

Regardless of wether someone has re-cut them to 2 3/4,,or by some chance the factory supplied them at that early date w/ 2 3/4" chambers..I'd still treat it as a 2 1/2" chambered 20ga.

The lightweight 20ga Flues is known to crack frames right at the junction of the standing breech. They just removed so much material from the frame in an effort to get an ultra light weight gun that it left it weak.
The bbls are taken down very slim on those also to save weight. Lengthening chambers pushes you out further into the lesser diameter of the tubes. Right where the breech pressure is greatest.
The goal was to produce 5#+ 20ga SxS. The weight has to dissappear from some where.

With the loads of that time (100yrs ago), they were probably OK,,but time, use, abuse and the increase in acceptable factory breech pressure of over the counter shells puts a lot of strain on the gun.
'Nitro Proved' sometime before or just after WW1 has a whole different meaning than it would today.

Shotshell chambers are tapered. There is no step at the front. Instead the imaginary end of the chamber begins to taper down to the bore size. That taper is called the forcing cone.
You can get a pretty good read on chamberlength with a steel ruler and bright light & shadow. Pushing the tip slowly up the the beginning of the forcing cone which is a dark shadow in the chamber becasue of the taper. Holdit there and read the length at the other end including the rim recess. It helps tohave another person read the length whileyou hold the scale in position otherwise you move it a tiny amount as you turn your head to read it,,or at least I do.

Another way is a quick chamber cast with something as simple as wax.
You're only needing a reading toi a fraction of an inch,,so wax, or a quick setting epoxy works well.
A cleaning rod with a tight patch in from the muzzle to just in front of the forcing cone (you want that included). Then if using wax, pour it in from the breech end of the slightly warm bbls. Let it solidify. The push the cast out with the cleaning rod carefully. It's fragile, but you only need a quick measurement toknow one way or the other.
Measure from what would be the face of the breech to the beginning of the forcing cone taper.

If using epoxy or other glue types,,grease or wax the wall of the chamber extra good or you'll have a single shot shotgun to show off.

Older guns like this usually have a very abrupt, short forcing cone. That was for the more efficient use of the cardboard and fibre wads of the time. It does not allow a longer than chamber length shell to unfold all the way though, and the wads are forced through the 'almost all the way opened' shell sticking into the forcing cone. That's the logic for the increase in pressure.

Sometimes these older guns will have the forcing cones ONLY recut. They are lengthened and the short chambers themselves are left untouched.
The theory here is that w/modern plastic shells and plastic wads, the longer, gentle taper of the new forcing cone allows the 2 3/4 shell to unfold enough & the plastic wads which are thinner than the old card and fibre wads slip through and into the bore w/o raising pressures.

Several shotgun gunsmiths offer 'lengthening forcing cones' as a service.
You still shoot low pressure loads,,the gun is still only built to handle what was standard factory ammo circa 1910.
The modification does not allow you to go wild and run most over the counter ammo through the guns as most of it is right at the top of the SAAMI allowable service pressure. That's to make sure they work in peoples autoloaders.
Special Low Pressure shotshell ammo is available from places like RST.
You can not read the pressure by the length of the brass, nor the marketing of the ammo itself (ie: Target,Dove,Quail,ect)

Any alteration allows you to use Low Pressure 2 3/4" ammo,,instead of Low Pressure 2 1/2" ammo.
Lengthing forcing cones removes the very least bbl material possible to obtain 2 3/4" shell use.
That's all it does.

Always check and make sure the chambers haven't been reamed out to some wild spec like 3". It's been done. Yes the gun MAY hold up. It may for X number of rounds, It may crack on you.

The Flues has a very fragile butt stock attachment.
The upper 'ears' of the stock right behind the frame crack and can loose a large chip there from the unsupported wood to frame design. It's very common.
There is no through bolting of the upper tang screw (under the top lever) to the trigger plate.
It's mearly a short stubby wood screw on top.
So the action flexes back and up under hard recoil and pushes those corners of the stock off quite commonly.
There's not a lot of wood to metal support inside on a Flues.

Take it easy on them, shoot in them was was originally designed for them and they can be enjoyed for what they are.

Thank you for the sage advice. I believe I will heed it. A case of 2 1/2" shells will bust clay pigeons fine, and that's all I wish to do with it anyway.

A quick, easy, inexpensive way to measure the chamber length to the taper point of the forcing cone is to get a 3" x 5" file card, roll it up small enough to fit in the chamber opening, let it expand, and gently push it in until it comes to a halt. Mark the card at the chamber face with a marker, extract and measure the length with a ruler. Bingo!

Now that's what I was looking for! I thought there would be an ingenious way to do it. Thanks.

Buy and use the 2 1/2" shells anyway. Just my opinion.

That's the plan.