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Old 04-03-2011, 02:41 AM   #1
NYECOGunsmith
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Default How To Measure The Chamber Length On A Shotgun

Here’s one that wasn’t prompted by Geo, Top, or anyone else here on the board.

An old friend who moved to Arizona a few years ago has recently gotten into Cowboy Action Shooting.

In doing so, he acquired an original Winchester Model 97 12 gauge pump shotgun.
After firing it a few times, he noted to me that it “kicked” a lot harder than a modern manufactured replica of the same 12 gauge pump ( A Norinco Model 97 clone) that he had that weighed nearly half a pound less. He was wondering why this was.

Two possible reasons I told him.

The first being that the stock on the M97 had a slightly different configuration (shape) than his other gun did, and its shape tended to slightly magnify the recoil impulse.

Neither gun has a recoil pad on them by the way.

The other reason I suggested to him was the possibility that the original Model 97 had an older, short chamber in it.

Back in the late 1800’s and up until as late as the 1950’s, shotgun manufactures chambered shotguns in a variety of chamber lengths.

For example, here are some of the “shell lengths” and/or chamber lengths that were once fairly common for the 12 gauge.

2-1/4”, 2-1/2”, 2-9/16”, 2-5/8”, the “modern” 2-3/4” (came out in the early 1920’s as the new standard length shell, with the new “star” crimp, as compared to the older roll crimps), and then we have the more modern 3” Magnum and the latest, the 3-1/2” Super or Ultra Mag.

These chamber lengths were all the measurements of the FIRED shell, its total overall length in other words.

The problem with shooting a modern 2-3/4” shell in one of the older, shorter chambers is that there is no space for the crimp to open up into. Since they are longer than the older, shorter chambers, the crimp actually opens up into the bore, which is significantly smaller than the chamber diameter, and hence smaller than the outside diameter of the hull as well.

So when a modern shell is fired in the short chamber, the crimp opens into a constriction if you will, and presents a bottle neck to the shot charge and wad column that are trying to get to open air.
This raises chamber pressure, which means you feel more recoil. It can and usually does also alter the pattern to some degree. But more importantly it also stresses the barrel in the area just ahead of the chamber.

While the barrel might take this for awhile, sooner or later it will generally bulge, or just out right split just ahead of the chamber’s forward end.

This can be cured by lengthening the chamber, which can usually be done quite easily.

The only two cases where you really can’t do it are if the barrel is Damascus Steel (you really shouldn’t be shooting a gun with Damascus Steel barrel(s)!) or if the barrel walls at the chamber leade are so thin that reaming/honing them out to the nominal chamber diameter (0.0798” for a 12 Gauge) will reduce them to a thickness that just isn’t safe (0.020” at a minimum, preferably 0.025” thick) to contain the pressure of the shell.


Chamber lengths that may be found in shotguns of various vintages include the following, with the modern “standard” length of shell shown in Underlined BOLD ITALICS . Remember that the unfired shell length will be from 0.375 to 0.500 inches shorter than the fired shell length, and the stated chamber length is for the fired shell length.

Some of these may not be listed anywhere anymore, but were found on shotguns I worked on during the last 50 years or so.

10 GA, 2-7/8”, 3”, 3-1/2”.
12 GA, 2”, 2-1/4”, 2-1/2” (Pretty common on English Manufactured guns) 2-5/8”,
2-9/16”, 2-3/4”, 2-7/8”, 3”, 3-1/2”.
16 GA, 2-9/16”, 2-3/4”.
20 GA, 2-1/2”, 2-5/8”, 2-3/4”, 3”.
28 GA, 2-1/2”, 2-3/4”.
.410 Bore 2”, 2-1/2”, 3”.

To measure the chamber length we need to know the diameter () that it should be at the end of the chamber nearest the muzzle.

Here’s a list of the standard chamber diameters at that point, I’ve omitted the odd ball gauges like the 11,13,14, 24, and 32 Gauges because you are not likely to ever see one these days.

10 GA = 0.841”
12 GA = 0.798”
16 GA = 0.732”
20 GA = 0.685”
28 GA = 0.614”
.410 BORE = 0.463”

Now we just need a gauge. There are several types available, two of them commercially.
The cylindrical gauge shown below, and the flat brass gauges below it, are available from Brownell’s and other gunsmithing supply businesses.

The third one down, the silver dowel with the brass donut on the end, and the additional brass donuts below it, is my own design.




All three types can be made at home if you are handy with tools, and have the ability to both measure and cut/machine to a tolerance of 0.001” or so. Admittedly, the cylindrical gauge and my design both require a lathe to accomplish easily, but the flat brass gauges can be made at home out of sheet brass or sheet aluminum. You could cut them close to the proper dimension, then file carefully to an exact fit.

With any of the three, the proper size gauge is inserted into the bore until it stops.
The you mark where the gauge exits the chamber, remove it, and measure from the front end to your mark. No need for precise measurement here, accuracy to 0.1 (one tenth) of an inch will suffice.

The cylindrical gauge and my own design are marked at the common, modern chamber lengths with Green paint, and at the older chamber lengths with red. This just makes it quick and easy to tell if a chamber needs to be lengthened. If the gauge goes in and stops at or very near a red line, the chamber is too short. If it goes in to or slightly past the green, its good to go.

So what to do if it’s too short, the gun is not highly collectible (because altering the chamber on such a gun would drastically reduce it’s value ) and you want to shoot it?

Take it to a gunsmith familiar with the chamber lengthening process, who has the appropriate reamers.
He can ream the chamber to the modern dimension, then ream the forcing cone at the end of the chamber nearest the muzzle, to ensure it retains adequate length, and then finally polish the chamber to remove any tool marks and ensure easy extraction and cleaning.

One final note on shotgun chambers. If you have a shotgun with a 3 or 3-1/2” chamber in 12 Gauge, and frequently shoot the standard 2-3/4” shells in it, you might want to pattern the gun with your favorite skeet, trap, or hunting loads. The excessive jump the wad and shot column has to make when a short shell is fired in the really long chamber sometimes (not always!) disrupts the patterning. Occasionally it makes it better, but not very often.


Oh, and my buddy’s original ’97 turned out to have the original, shorter 2-5/8 inch chamber in it. He had it reamed to the modern 2-3/4” chamber, the forcing cone lengthened, and the chamber polished. He also had the action polished and is now quite happy with the way the “old dog” as he calls it performs, and it no longer kicks him harder than the lighter, modern version he uses as a backup gun.

Hope this helps. As always, your results may vary, if you catch any typo's, wrong info, etc. please let me know!
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Old 04-03-2011, 05:37 AM   #2
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I have an 1898 Belgium Comet double barrel 12 ga, I have no idea if it is damascus barreled, or even if it is safe to shoot. Who could I take it to that can tell me more about it?
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Old 04-03-2011, 05:57 AM   #3
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Steve, wow! I never knew about that.

What could happen, other than the obvious possible disaster, if someone fired a slug in the scenario of a shorter chamber?
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Old 04-03-2011, 11:48 AM   #4
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I thoroughly scanned your post for errors, ommissions and factual data inconsistencies and came up with the following results: 0.

From a personal analysis as I read throuh the post. memories of ancient shotgunning articles read the last century started to appear in the minds eye on chamber legnth. it's kinda funny how you aquire information and then it is packed away in the dusty corners of your skull if you do not use it regularly.

+1 Steve (the NYECOGunsmith) on excellent infomation past on to the unworthy of such knowledge.
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Old 04-03-2011, 04:04 PM   #5
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Quote:
Originally Posted by desertw0lf View Post
I have an 1898 Belgium Comet double barrel 12 ga, I have no idea if it is damascus barreled, or even if it is safe to shoot. Who could I take it to that can tell me more about it?
Your shotgun was made by Henri Pieper, founder of the Eclipse Company that made shotguns in Belgium back in the late 1800's. There is a very good chance that it has pattern welded (Damascus) steel barrels.

The folks at Rifle Dynamics should be able to look at it and tell you whether or not the barrel is Damascus steel, or the more modern homogeneous (all one piece) steel.

Or try Matt Babb.

If a Gunsmith doesn't spot any of the telltale marks, there is a "acid" test that can be done on an inconspicuous spot where it won't show. You basically put a drop of mild acid on the steel in a hidden location, let it eat down a few thousands of an inch, then neutralize it. Polish the spot gently, and look to see if you can now see the pattern weld, or if it still appears to be all one piece of steel.

There are signs you can look for yourself on any shotgun of that vintage. Post 1900 or there abouts, most shotguns made in this country had modern one piece steel barrels.

That last statement doesn't necessarily hold true for foreign manufactured shotguns, there were some still being made of pattern forged (twisted, Damascus,etc.) steel up into the late 1950's.
.

Remove the forearm, and take the barrels off. Look them over carefully under a bright light with a magnifying glass.

If you see the words "Damascus Steel", "Twisted Steel", "Pattern Welded Steel", "Laminated Steel", "Belgian Laminated Steel" , "Hammer Welded", "Forge Welded" , or any words of similar meaning, it's probably pattern welded steel.

If it says "Fluid Steel", "Armory Steel", then it is a one piece, homogeneous barrel of modern manufacture and SHOULD be safe to fire with modern, smokeless powder loads AFTER it has been thoroughly checked over by a competent gunsmith.

Any of the pattern welded barrels, no matter how tight, perfect, new, etc. that they may look should NEVER be fired with modern smokeless loads, even ones that advertise "Safe for Black Powder Firearms" or words to that effect.

The pressure curve of the smokeless powder is quite different from that of the black powder, and while the ultimate peak pressure might be the same for both loads, the barrels won't long tolerate the smokeless powder's sharper rise to peak pressure.

The blackpowder loads have a much slower, gentler rise to peak pressure.

There is a way to be able to fire modern smokeless loads in a single barrel or double barrel shotgun with good, tight Damascus type barrels.

You can send the gun to Jesse Briley down in Texas and have him fit a single or set of, sub gauge tubes to the gun.
For example, if its in good shape and does have pattern welded barrels, your 12 Gauge Comet could be "tubed" or "sleeved" to handle 20 gauge, 28 gauge, or .410 bore shells quite easily.

Not inexpensively, just easily!

As for Top's question: "What could happen, other than the obvious possible disaster, if someone fired a slug in the scenario of a shorter chamber?"

Well, since the slug is going to be close to nominal bore diameter, BUT , unable to squeeze down into a longer string like a charge of shot can, it will hasten the destruction of the barrel, often with a much more catastrophic failure.

In other words: "Baby go boom, then baby go BOOOOM and shooter go hospital".
Or at least shooter go change underwear!

Rat, thanks for the fine job of Technical Editing, there will be a bonus in your pay envelope this week!
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Just because you are paranoid doesn't mean someone ISN'T OUT TO GET YOU!
Just because a complaint was never filed doesn't mean a defective parachute was never made.
I've been to three world's fairs, two rodeos and a goat roping contest, but I've never seen the likes of the stuff that goes on around here before!
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Old 04-03-2011, 06:00 PM   #6
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Thanks NYECOGunsmith for the great info. But, before this post is forgotten there is another hazard that must be cover when shooting old and sometimes new shotguns. I know because it happened to me. I purchased a double barrel from Rossi in the early 1970's and had no problems with it in many years of hunting, until 1989. I was in Arizona then and it was the last year for lead shot for duck hunting, next year would be iron shot only. By hunting buddy bought a box of iron shot to see how they compare to the leads. He gave me 2 to try. A duck flew pass, bang, bang and to my horror the ends of both barrel where badly bulged. Thankfully, a gunsmith cut the barrels down to 18 inches so I still could use the shotgun, but now they are cylinder bores. He said the bulges where really bad, and that I was lucky I still have my face. So, please if you dont know if your shotgun can handle non lead shot have a gunsmith check it out.
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Old 04-03-2011, 06:55 PM   #7
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Quote:
Originally Posted by BKMe View Post
Thanks NYECOGunsmith for the great info. But, before this post is forgotten there is another hazard that must be cover when shooting old and sometimes new shotguns. I know because it happened to me. I purchased a double barrel from Rossi in the early 1970's and had no problems with it in many years of hunting, until 1989. I was in Arizona then and it was the last year for lead shot for duck hunting, next year would be iron shot only. By hunting buddy bought a box of iron shot to see how they compare to the leads. He gave me 2 to try. A duck flew pass, bang, bang and to my horror the ends of both barrel where badly bulged. Thankfully, a gunsmith cut the barrels down to 18 inches so I still could use the shotgun, but now they are cylinder bores. He said the bulges where really bad, and that I was lucky I still have my face. So, please if you don’t know if your shotgun can handle non lead shot have a gunsmith check it out.
Good point! Thanks for bringing it up.

A lot of "modern manufacture" shotguns have relatively "soft" fluid steel barrels. Perfectly safe for use with lead shot, but if you have a tight choke (full, extra full, even some "modified" chokes are too tight) you can bulge the barrel(s) at the choke constriction when firing iron or steel shot through them.
Especially true if its an internal, fixed choke.

If the barrels are not labeled "Steel Shot OK/Acceptable/Proofed",etc. something that denotes that they are usable with steel shot, I never recommend using steel or tungsten or iron shot in them if they have a choke tighter than a light modified. And even with a IC (Improved Cylinder) I usually tell folks to limit the number of rounds of the harder shot being used unless the barrel specifically says its suitable for steel or iron shot.

But the best bet is to use a gun with a barrel and choke(s) that are steel shot capable.

Your cylinder bore barrels, if the barrel wall is thick enough, could be threaded for the "Thin Wall" model of chokes by Colonial Arms.

I also generally recommend that when using screw in choke tubes, you use the extended tubes instead of the flush fit. This puts the majority of the choke constriction beyond the threads (thinnest portion of the barrel at the muzzle) so that if a bulge does occur, it will most likely be in the portion of the extended tube right at, or hopefully just beyond, the muzzle crown.

Makes getting the now useless (bulged) tube out a bit easier, and there is also less likelihood of the barrel being damaged.

The early (late 60's to late 70's) Rossi side by sides had relatively soft barrels, perfectly fine for lead shot, but not the harder stuff. I used to do a bit of business during and after Duck and Goose season with hunters who used these guns, who had leaned them against a sharp edge (like the corner of a 2x4 in a duck blind for example, or the bumper of a car or truck) a bit too enthusiastically, and found that they had a dent in the barrel.

That's not too difficult to fix as long as it is not a crease, but merely a dent. A sharp crease in a shotgun barrel is a death sentence for the barrel. Replacement of the barrel is the only option at that point, depending on where the crease is on the barrel. If it is close enough to the muzzle that the barrel(s) can be cut back and still be legal, you may be able to save the barrels.

For minor dents, you can make ( if you have a lathe and mill that is) a set of ironing mandrels that let you gradually raise the dent back to flush with the surface, or buy one of the very nice Hydraulic Shotgun Barrel Dent Raisers from Brownell's.

The latter tool requires a bit of caution in its use, as it is so powerful that it can take a dent out and create a bulge if you are not careful with it.
Its a bit more difficult to do that with the drive through ironing mandrels.

I had the Hydraulic tool in the shop, but generally used the mandrels first just because I'm a sissy.
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Just because you are paranoid doesn't mean someone ISN'T OUT TO GET YOU!
Just because a complaint was never filed doesn't mean a defective parachute was never made.
I've been to three world's fairs, two rodeos and a goat roping contest, but I've never seen the likes of the stuff that goes on around here before!
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Old 04-04-2011, 02:36 AM   #8
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thanks for the shiney nickel Steve!
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*There isn't a place called Black Mesa, NV. It's a fictitious Government research facility in a 1998 PC video game released by Sierra Studios named Half-Life. In which this facility closely resembles the aura of covert experimentation that surrounds the Nevada Testsite and the hero loosely based on Bob Lazar.
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Old 01-15-2013, 04:41 AM   #9
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Default measuring chamber lengths

I have been checking a few of my old Winchesters both 97s and model 12 for chamber lengths and was getting totally confused. I found this site and thought I had it all figured out. I made a tool and started checking and nothing was making sense. I have a model 12 made in 1939 marked 2 2/4 " chambers that was only measuring abit above 2 1/2" inches. I was measuring the chamber by removing the barrel from the receiver, WRONG. I finally figured out the correct chamber length is from the bolt face, not the end of the barrel. and tool will not work with the barrel installed!!
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Old 01-15-2013, 06:39 AM   #10
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Originally Posted by snoopy View Post
I have been checking a few of my old Winchesters both 97s and model 12 for chamber lengths and was getting totally confused. I found this site and thought I had it all figured out. I made a tool and started checking and nothing was making sense. I have a model 12 made in 1939 marked 2 2/4 " chambers that was only measuring abit above 2 1/2" inches. I was measuring the chamber by removing the barrel from the receiver, WRONG. I finally figured out the correct chamber length is from the bolt face, not the end of the barrel. and tool will not work with the barrel installed!!
Not trying to clown you but isn't 2 2/4" the same as 2 1/2", or is that just a typo?
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