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Chamber measurements

Discussion in 'Gunsmithing and Repairs' started by Bob72, Feb 10, 2013.

  1. Bob72

    Bob72 Well-Known Member

    Anyone know of a easy/quick/relatively accurate way of measuring the chamber of a .308. I read somewhere that you take a recently fired shell and add .001 to the diameter at the mouth. Anyone have a more accurate way of determining this measurement?. I want to neck turn some cases but am afraid that i will be getting way too much excess spacing.
  2. Doak

    Doak Well-Known Member

    Put a ring of Scotch Magic Tape around the neck of the case, or 1/4 way around for more clearance. Trim the excess off the front w/a razor blade. The tape is .0025'' thick.
    Do it to a fired case, an unfired round, or a resized case, & see if any of 'em will chamber, w/out tearin' up the tape.
    Then take the measurement across the neck/tape.
  3. brickeyee

    brickeyee Well-Known Member

    Buy some cerrosafe and make a casting
  4. Reloadron

    Reloadron Well-Known Member

  5. Jim K

    Jim K Well-Known Member

    What cases are you using? Neck turning is normally necessary only when you make cases out of those with a larger neck size (like making .243 out of .308) and the neck becomes thicker. The result might be that there is not enough room in the chamber neck for the case neck to expand and pressures increase.

    It is not necessary when reloading cases for the same caliber, though case trimming may be needed at some point, a different matter.

  6. Clark

    Clark Well-Known Member

    I have cerosafe, but I never seem to use it.
    I poke pin gauges into the chamber and make a mark on the gauge with a sharpie that shows how far it went in.
  7. fguffey

    fguffey Well-Known Member

    “am afraid that i will be getting way too much excess spacing” You want to turn necks, turn necks. But as Jim K. said “neck turning is not always necessary”.

    Other reasons for truing the neck, the neck could have more brass on one side than the other, having more brass on one side than the other bothers a lot of shooters. Then there is that part where it has not been decided if the bullet is pushed out of the case first, or if the neck expands first then the bullet makes the jump to the lands.

    Lag, the bullet is setting still, it is not moving then suddenly and without warning pressure builds, back to the neck, the neck expands effortlessly, the bullet is setting still. Back to the neck releasing the bullet, could be the bullet is released first when the neck expands, meaning the neck expands first.

    Excessive spacing? Between the neck of the chamber and case neck? A friend ask for help, he builds bench rest type rifles, he suggested the necks in his chambers are too large, I suggested he get off the Internet or avoid Internet reloaders. I ask him about the accuracy of his rifles and it came down to one hole groups, then I ask him what was his goal, “Half hole groups?” Not my barrels, not my cases, not my reamer, I boxed up a few tools and went for a visit.

    The diameter of his necks for loaded rounds measured .335”, neck diameter of fired cases measured .344”, with no shortage of 30/06 cases we started forming 308W cases using 30/06 cases. When finished his neck diameter for unfired/loaded rounds was .341” (before firing). We reduced the difference in diameter between his chamber and case necks. Next, we are starting on forming cases using anything as long or longer than a 338 Winchester Magnum, problem I can not find my short 300 Win mag forming die.

    Effort, time and money, check with RCBS to determine if the still have neck sizer plugs in different diameters, when sizing cases use different neck sizer plugs for different neck diameters. (OR) Learn to form cases, a 308 W formed from a 30/06 case will have a thicker neck, when sizing the formed cases start with the smallest (in diameter) expander plug and work up. The Forming/trim die is also a good tool for determining the length of the chamber from the bolt face end of the chamber at the end of the chamber neck.

    I have ball micrometers, blind end micrometers and transfers, and if a reloader can determine the diameter of a bore with a soft lead ball the reloader can determine the diameter of the neck diameter in the chamber, with a soft lead ball.

    Then there are expanders, unknown to the reloading world, meaning, there is no rule that says the lead has to be in the form if a lead ball, it can be in the form of a cylinder.

    F. Guffey
  8. Johan.r

    Johan.r New Member

    Dear All I just joined the forum and need to know if I can use 338 Win mag in my 338 Lapua Mag chamber. There is a significant difference in price but to by wrong or useless ammo will cost more . Appreciate proper info all the best Johan
  9. rcmodel

    rcmodel Member in memoriam


    Not even close to the same cartridges.

  10. Jim K

    Jim K Well-Known Member

    When the pressure builds, the case neck expands first, but the bullet is kept in place by its own inertia. The result is high pressure gas shooting through the small opening between the case neck and the bullet. That acts like a cutting torch, with heat, pressure, and burning particles of powder eating into the steel of the barrel throat. That is what causes throat erosion.

    But if the chamber neck is too tight, and the case neck can't expand, the result is high pressure. Many years ago, it was the practice for target shooters to lubricate the case necks by dipping them into grease. The grease acted to prevent case neck expansion and some rifles blew up. Lesson learned; they stopped greasing the case necks.

    The same situation exists in the case of old German rifles in 8x57J being fired with 8x57JS ammunition. The larger bullet (.323" vs .318") is no problem. But the larger bullet means a larger case neck, and if the chamber has not been reamed to expand the chamber neck, pressures will increase dramatically.

  11. fguffey

    fguffey Well-Known Member

    On a reloading forum that is more belligerent than informative the question was about gas passing the bullet, my thought was anybody involved in knowing the sequence of events from pulling the trigger to the bullet leaving the barrel knew gas passed the bullet before the projectile left the barrel. Guessing? NO, based on a rare and unseal photograph in the early teens as between 1910 and 1919. The photograph clearly shows a smoke ring ahead of the projectile, ahead of the smoke ring was a white cloud. The white cloud was compressed air with visible moisture, the ring was made up of gas that passed the projectile. As I said the forum has a belligerent nature.

    I started digging for the photo, then the though occurred to me, maybe this is more about a social event than it is a sharing of ideals. as the story goes, once the military discovered the photo in an art exhibit, it took them months if not years to determine how a still camera could record the sequence of events, before the photo they had decided “This is what and how it happens” They were forced to change their mind. The picture is in a book of ‘world’s most famous pictures/photographs’, not about guns but about art.

    Fast, so fast none of the events recorded by the camera were visible to an observer. All the observer saw was the blast behind the projectile. It is possible to see a 105 Howitzer round leave the barrel if the observer is standing behind the field piece. My wife's brother was at Fort Seal and part of the crew that shot the 280MM cannon, going for distance they were shooting over Lawton, OK, it did not make it, the round landed in a school yard.

    F. Guffey
  12. Clark

    Clark Well-Known Member

    It has taken me a lot of work to get rid of that noisy smoke ring.
    I need a bullet, cartridge, chamber combination that seals the gas, so all the gas is behind the bullet, and not going out the muzzle and making noise.
    I now have a 50 caliber powder burning gun that sounds like a BB gun, but can stop a raccoon with a body shot.

  13. Jim K

    Jim K Well-Known Member

    That gas escaping ahead of the bullet is called the precursor wave. It has three sources. The first is the initial gas forcing its way around the bullet before the bullet begins to move, as discussed above. The second is gas escaping around the bullet because the bullet jacket does not fill the grooves. That is common with the .45 pistol and is the cause of erosion in the edges of the grooves in GI pistols. The third is the air that was in the barrel being forced out by the bullet itself.

    None of that matters much except in suppressor design, where the precursor wave produces noise that in the ideal suppressor would be reduced or eliminated. Clark's idea would probably eliminate or severely reduce the gas escape around the bullet, but it would not affect that part of the precursor wave made up of the air in the barrel unless a means were devised to pump the air out of the barrel and seal it before firing.

  14. fguffey

    fguffey Well-Known Member


    It has taken me a lot of work to get rid of that noisy smoke ring”

    My effort was to give credit to a photographer that is unknown in the world in the world of reloading, in the big inning no one claimed they discovered anything, they did not dampish his contribution.

    Clark, You got rid of the noisy smoke ring with a lot of time and effort?

    The story goes on to say the photographer, by accident, picked the perfect day. they decided the white cloud created when compressed air made the moisture visible, in other words had the photograph been taken on a day with low relative humidity the cloud would not have been created. Then they went on to describe ‘behind the projectile, remember, the white cloud was in front, the smoke ring was next then came the projectile.

    The most impressive feature of the picture was the escaping gas behind the projectile, the gas escaping from the muzzle behind the project came out at a 90 degree to the muzzle creating a flat round disk. The flat dish revealed a pattern of the rifling, not my concern but I have been told there is throat erosion, there is no muzzle erosion, only the cleaning rod causes muzzle erosion,

    The escaping gas was hot, high pressure, metal cutting gas that did not follow the projectile, at the first opportunity the gas made a 90 degree turn between the projectile and and muzzle. Before the person labeled the blast in front of the projectile 'precursor’ the original examiners were impressed with the definition of the rifling in the round flat disk behind the projectile.

    And before you discovered, with your time and effort, they would have said the noise behind the projectile would have made what ever noise you claim is created with the smoke ring insignificant.

    F. Guffey
    Last edited: Feb 14, 2013

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