Determining Chamber size?

Lead Loader said:
Just measure it? 2.75" or 3"
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It's not that simple. A gun chambered for 2 1/2" shells will measure 2 3/4" in the chamber. Chambered for 2 3/4" measures 3.0".

The chamber is bored 1/4" longer than the (longest) shells the gun was built to shoot. This is so that the shell crimp can fully open without opening into and constricting the forcing cone.
 
While some guns have some leeway in chamber length, a 2 1/2" shell does not have a 2 3/4" chamber and a 2 3/4" shell does not have a 3" chamber. Measure a 2 3/4" shell before firing. It will only be around 2 1/2" and will open to around 2 3/4" after firing. A 3" shell will not measure 3" before firing. It will measure around 3" after firing. A shell that is marked as 2 1/2" will only be around 2 1/4" or less before firing. So, a chamber that measures 2 3/4" will shoot a 2 3/4" shell as it is designed for it and a chamber that measures 3" is designed for a shell that is marked 3" etc. Simplified, the length that is marked on a shell is the length after it is fired.
 
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Shotgun chambers go as follows
If you have a 3" gun and measure a 3" unfired round
It measures 2 3/4"
It opens to 3" with crimp not passing the forcing come of the chamber
If you have a 2 3/4" gun, your factory rounds are 2 1/2" long
Unfired, 2 3/4" opened
2 1/2" guns have a 2-2-1/4"
Un opened depending if roll crimped or star
Roll crimps on older ammo tend to be 1/4" and slight less
If anyone has doubts I entertain conversation
How many of you guys buy 2 1/2" shells to shoot in your
Pump 12 gauge for ducks lol that's chambered for 2 3/4"
Che k at Walmart next time see what they have for ammo
If that's the case. A 3.5" gun needs a 4" chamber to ope ???
A 3.5" she'll is 3" unfired NOT 4" opened
If it was 4 " you'd have a 4" magnum gun?
I'dove to see one lol
 
Problem with a lot of the older USSR Baikals is that chamber size is not marked on them. I had 2 over/unders and an IJ58 sxs and neither were marked. They were all 2 3/4" chambers if I remember correctly.
 
I have two 7's stamped on the barrels. They aren't side by side but close so its not really 77. What is the best way to measure the chamber, I roughly measured 3" but I would like a more accurate way then having my wife stick her finger in the chamber till she feels the forcing cone then measuring her finger LOL
 
Easy way (taken from the LC Smith Collectors Forum)

"Take a 3" x 5" index card, roll it lengthwise, slide it into the chamber (it will partially unroll and expand to the chamber diameter), gently slide it until it stops at the end of the chamber, use a pencil to mark the index card at the breech end of the barrel, and repeat the steps to see if it always comes to same spot. Then use a tape or ruler to measure length marked on the card. The chamber must be clean, and it works best with a new 3” X 5” card."
 
You can also wrap tape around a dowel rod until it fits the chamber snugly then gently insert it until it stops and then measure. You can also take an old credit card and cut a strip that is the width of the chamber. Insert it until it stops and measure.
 
If it''s not too expensive I would open the full choke to modified. Where I am, there's a guy who opens the chokes for $40.00/barrel. Mind you, you will have chrome lined bores and that takes a special reamer. If you did open it up, as a thought, you should also have the modified barrel measured or patterned. Even though it is marked modified, it coud be a very tight modified bordering on full choke.
 
gunsaholic said:
If it''s not too expensive I would open the full choke to modified. Where I am, there's a guy who opens the chokes for $40.00/barrel. Mind you, you will have chrome lined bores and that takes a special reamer. If you did open it up, as a thought, you should also have the modified barrel measured or patterned. Even though it is marked modified, it coud be a very tight modified bordering on full choke.
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When I used my calipers to measure the barrels (roughly as my digital calipers were out of battery) i came up with .700 bottom barrel and .690 top barrel. From what I understand this is roughly the sizes of standard MOD and FULL chokes.
 
That would be improved modified (on edge of full) and full ( bordering on extra full). Of course this is assuming the bores started out at .729 This is where the patten board will help. When actually testing, you might find it is throwing full and extra full patterns.
 
It shoots a really tight pattern and really reaches out. I've dusted some clays really far out with the top barrel. If any bird jumps up too close the bottom barrel does a number on them too.

Ill have to look into opening up the barrel. Could be a really good all around gun.
 
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