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Gunsmith notes on chamber length safety, additional related details

Discussion in 'Shotguns' started by kirbythegunsmith, Sep 7, 2008.

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  1. kirbythegunsmith

    kirbythegunsmith Member

    Nov 12, 2006
    St. Louis area
    I had written this information as response to some mistaken notions available at this thread

    (plus postings alluding to my previous post are here)


    and thought that the majority of shotgun shooters would find the material provided here to be an informative explanation that clarifies an unfounded supposition about chamber length and the safety or lack thereof.
    Where to start?

    By the way, any comment response should not be considered in too harsh of a light, since my main thrust has always been towards educating shooters, even if it means puncturing inflated opinions that contain no substantial substance, since I prefer to keep the rest of the readers better informed rather than worry about someone's sensitive ego.

    I should have noted that the damascus vs. fluid steel test (well over a dozen sample barrels, mixed) was with new production barrels, first examples as blanks, then as standard "struck" contour of 2 lb. 12 oz.
    Therefore, that would not necessarily make a corollary to old barrels on well-used guns.
    However, concerning a damascus barrel of pristine condition, no visible pitting inside or out, I would not have a heart attack hearing it had been shot. I would be more worried by ANY barrel full of deep pits in and/or out being shot, especially if the pits were of unknown depth and in a barrel that started out life as a thin lightweight example, such as a European bird gun.

    Pits inside a barrel would be harder to examine, but any tiny pits in damascus are unlikely to do the same as a tooth cavity, where a small hole on the outside masks a large rot inside. If the metal was corroding, what would stop the interior edge of the hole from also rusting? There is no harder outer layer such as the calcium of a tooth to resist pit edge corrosion.

    Now I think I'll start with Fred's comments.
    First, I do appreciate the notation that you
    a: don't claim to be an expert
    b: don't have a ballistics lab

    I also do like hearing about any particulars such as the frame cracks noted.

    One must understand that what you have seen and reported in no way alters the aspects that I have explained, and would be called "anecdotal evidence" in any research project.
    First, we do not have any history of those guns with cracked frames, especially serial # range to denote era of production. I would suggest that in future, you collect any such evidence that you can, and I would also consider performing inspection of any such damaged guns prior to disassembly and scrapping.

    Do you consider the fact that a gun had not happened to be rechambered was a material cause of cracking, when you do admit to not a large number of examples seen, but a huge number of unaltered guns had to have been at these matches, and with no obvious cracking?
    In other words, is lack of cracking a sign that they would survive very little extended use IF they did not have chamber work,
    or that those that had chamber work (surely not a huge percentage of guns present, or of manufacture total) were now immune to cracking,
    or just had created a minor chance that they were less likely to crack?

    By the way, were any Chinese guns of particular note in wear or other fault?

    Those notes about keeping barrels snug on takedown 97's are definitely noteworthy, but since my last posting was rather lengthy, I had decided on not touching on that point, then.

    I had an 1100 here with a frame crack and barrel crack, but it was a 40 year old well-used TRAP gun. The barrel may have had less retention due to wear of the mag. cap detents that prevented normal snugging of the forearm/barrel assembly; in other words, some operator mistakes in lack of due diligence. (That is similar to closing your car door only halfway and driving fast over ruts)
    THAT was a barrel of obvious full 2-3/4 length, so should I make any allusions about a full length chamber being a part of the culprit that caused the problem? Maybe one thing has absolutely nothing to do with the other, since we are looking at such an extremely small sample size.

    OK, now it's your turn, 1858.

    First, I need you to redo your math.

    For one thing, there has never been a forcing cone in any 97 of 1/8" length. PERIOD.
    A very short typical cone is at least 3/8" (measured in a Win. 37A) with a larger than normal bore size, which skews the length measure vs. ANGLE measure (normal chamber reamer is about 5-1/2 degree at the forcing cone section)

    You also state you had measured your chamber at "about" 2-9/16".
    I'm not grieving you over an estimate, but you are evidently not an expert on measurement of chambers or have sufficiently precise measurement tools to go into the chamber measuring business.

    See a small sample of the tools that I have posted at the bottom.

    I have absolutely no idea how you could have been so mistaken about bore reduction in your second response post.
    Reduce a bore by 2.25 times?
    Pardon me, but HAR de HAR.
    You try to equate an 1/8" section of shell length as being of a 1/8" thickness. Have you ever seen a hull with that thick of a wall? I thought not. Maybe you think the edge of the hull is standing up at a right angle, acting as a bulwark against shot passage.
    Where are your calculations now? Total non-starter with that one and all of the specious conclusions that follow. As said before, no such a forcing cone of 1/8" in a 97 or 2-9/16 chamber, either, but closer to 2-5/8. As it is, how do you know that is REALLY at 2-3/4 now, with your lack of measurement precision? It may be at a 2-5/8 blend line and you wouldn't know for sure.

    Also note that a solid chamber gauge is made a bit smaller than the expected diameter of the mouth of a chamber and would likely give a reading equating to a slightly longer chamber than was present IF the forcing cone lengthening cut away the chamber edge blend line, since gauges normally have chamfered edges or are rounded at the end.

    I would also bring to your attention that Brownells is a tooling seller, not a research department. Where anybody can pull out a misleading figure of a 10 to 25% increase in chamber pressure, with no evidence, is quoting "old wive's tale" evidence. No such pressure increase, no such battering, no such fatigue (increase beyond what is expected as normal). BUNK.
    Brownells sell many tools that are good, has a tech department that can be very helpful, has outstanding warranty and other service second to none, but they do sell tools and more that I do not even consider using due to defective design and/or manufacture.
    For instance, the choke opening tools commonly called Critchley reamers are not proper for precise choke opening, but they will sell you a set at any time, regardless of lack of suitability.
    Don't believe I will use them for non-lab research answers, either.

    Remember in the original posting in the 97 chamber thread I made mention of the Remington research lab tests that bears out this conclusion.

    By the way, if you haven't searched out my previous postings, you wouldn't have known that I am a proponent of forcing cone alteration, but I do believe that my method and geometry of forcing cone work (as well as including a fine finish as part of the work, and for less than $85 AND a whole lot longer and more effective than what you show) has better pellet deformity reduction since it is much longer than most factory typical forcing cones. That is why I call my modification a Superlong forcing cone.
    Maybe you can do some comprehensive reading so you won't waste time trying to preach to the choir, when you are talking to the music professor.

    Everyone may get a kick out of the graphic example of forcing cone performance I had made and have posted below.

    Here are some measurements of interest:
    97 chamber forcing cone diameter change in the first 1/8" .018" of .070" total reduction
    Typical 2-3/4 shell (remember that those are expected MAXIMUMS) 2.72" with a .020" stretch when fired
    Many of the 3-1/2 shells that I have measured are 3-1/4 to 3-5/16 fired length

    Plastic or any hull lying in the first portion in the forcing cone will flatten under pressure of the shot column speeding past. The shot will actually be MORE cushioned in that section, since it has a plastic layer of the shot cup AND the deformability of the hull to add more cushioning. Plastic is not as hard as steel or lead, so guess which flattens out first.
    I do not recommend using a hull for pellet deformity reduction, just stating the facts.

    Now let's do some shell investigation.
    A shot column is maybe 7/8" long, so 1/8 or 12% is being affected by that bit of plastic (end of crimp lying in the beginning section of the forcing cone) at any one time. The shot is no more than 2.2" from the bolt face in a loaded shell in this example. The shot travels over 3/8" while pushing open the crimp and that has to be inducing some load against the shot and it's acceleration curve and is considered a normal included load factor.
    (Shot is moving about 300 fps at the 1/2" mark)
    However, the inside diameter of the shell is hardly larger than bore size, with the shot growing in diameter slightly when the hull expands at firing , and the shot progresses forward into the crimp section where the hull wall is sometimes markedly thinner than the other sections. See the tapered hull in some cutaway examples of shells.
    I just measured one hull about .034 thick behind the crimp, and tapering through the crimp to about .014" thick at the edge.

    You see, the shot sits at near bore size in the shell, grows slightly with the hull expansion at firing, expands a small amount through the thinning wall section of the crimp, and then gets squeezed back to not much smaller (if at all) than when it was in the hull.
    Where is the bottleneck? Proof shell- NOT!
    That minor expansion cross-section is just going back to the approximate original area cross section with hardly a reduction in diameter at the end of the transition cycle. Remember, during that passage of the shot over the 1/8" section referenced (the thinnest bit of the hull, too) that only that tiny bit is any way shape or form different than a non-short-chamber example. The other 88% is still either the same before or after.
    Think about it this way: if the shot is halfway past that bit of hull laying in the forcing cone, the 44% behind is at the size it would be in the alternate example, and the 44% AFTER section is being squeezed through the forcing cone exactly like it would be otherwise, just an eighth inch sooner but at an identical angle.
    The front of the shot has moved a good 1/2" before it could have even that minor input difference.

    Chamber exam:
    diameter about .798 at edge with cross section of .5 square inches
    bore diameter of barrel @ .730 is .42 square inches
    Bore reduction is a total of 16% during the entire distance of the forcing cone taper

    bore diameter of .780 at 1/8" into the forcing cone of a 97 barrel, .477 square inches
    Bore reduction is about 4.6% at that point without taking into consideration the cushion effect of the plastic of the hull

    bore in a tight 12 ga. of .715 is .4 square inches.
    Total bore area reduction is now 20%

    See that a tight bore can add as much reduction to bore area as a bit of chamber length change (~4%). It does happen at the end of the transition but will still affect the entire shot column. If you downgrade that effect, imagine if it were squeezing down into a 16 or 20 bore barrel and say it would not have any input.
    The tight bore example was an Ithaca 37 MAGNUM that passed proof, so realize that the input I am showing is not a massive change vs. a normal barrel size, just as a backbored and larger bore size is not a huge pressure reduction, either.

    If bore size on the tight side is not considered a danger that raises shell pressure by the gun manufacturers, a 1/8" chamber length difference can be considered the same, since the examples in my previous posting had also been part and parcel of many years of manufacture by Winchester and others. They were proofed, they were tested in the Winchester lab, the design spec. was approved and used and no admonishment from SAAMI was forthcoming or we'd have heard about it before this.





  2. Dave McCracken

    Dave McCracken Moderator In Memoriam

    Dec 20, 2002
    Thanks for posting all that,Kirby. I may have to go over the stuff a couple times, lots of info.

    870 Number One got a 1 3/8" forcing cone around 1980. With the ammo of choice,Winchester 00, pattern size at 25 yards reduced about 4".
  3. ReloaderFred

    ReloaderFred Member

    Apr 28, 2005
    Oregon Coast
    Well, Kirby, it's interesting how different people have different opinions. I've talked to several knowledgable gunsmiths about this issue and they're all diametrically opposed to your opinion. I won't tell you what they said about your other post, as it wasn't very flattering.

    Just for clarification, I've never seen a Chinese Model 97 with a cracked frame. I've seen them with other problems, but never a cracked frame.

    And the majority of the Model 97's in our area have had their chambers relieved, and the forcing cones lengthened. It's only the unmodified that we've seen cracked.

    Hope this helps.

  4. MCMXI

    MCMXI Member

    Jul 1, 2008
    NW Montana
    kirby, I've read through your post three times already and have found it to be very interesting. I was particularly interested in the chamber, forcing cone and barrel measurements that you listed. It did leave me wondering though, if the chamber length isn't important, why do you need such precise tools to measure the chamber? Wouldn't a tool with 1/8" accuracy be sufficient since you don't seem to think that it makes a difference?

    Also, let me clear something up right away ... I've never claimed to be an expert or even mentioned the notion of going into the "chamber measuring business". Good grief, I simply reported (with photos) that I bought a chamber gauge from Brownell's, measured the chamber, sent the barrel to a gunsmith to have the chamber and forcing cone lengthened (he measured it too and found it to be even shorter than I described), polished the forcing cone, measured the chamber again and found that it was longer ... that's it!! There hasn't been any attempt at obfuscation, simply a statement of what was done in good faith. If I buy another '97 I'm most likely going to do the same thing again despite your informative post, however, next time I'd probably send it to you since you're cheaper and you polish the forcing cone too!! (So how much would you charge to cut the chamber to 2-3/4", cut a longer forcing cone and polish it?)

    When I bought my '97 a few months ago (made in 1921) I knew absolutely nothing about the shotgun other than it was designed by John M. Browning and it was a popular choice in CAS events. I made numerous posts to the effect that I wanted an original, made in the USA, Winchester. I also decided that I wanted an original "riot" solid frame version. Once the shotgun arrived and I posted photos of the shotgun, a number of members mentioned that they had personally seen cracked frames on '97 shotguns at CAS events. Not just one or two but a bunch. I didn't see a single post advising against lengthening the chamber ... and even after using the search engine I couldn't find a single post to the effect that this was unnecessary. Now couple this with the "Chamber and Forcing Cone Length" document from Brownell's and you've got a fairly compelling reason to make a change.

    After all is said and done, it's clear that you feel that the chambers on the '97s are fine as is but as Fred mentioned, a number of gunsmiths believe the opposite to be true. Personally, I think you've made a good argument and anyone in the position that I was in will now have more information at their disposal. That's what this board is all about right?! If we're lucky, someone will come along with photos of a cracked '97 and then we'll really have something to talk about.

    P.S. Those are some nice looking (and expensive I bet) tools.
  5. MCMXI

    MCMXI Member

    Jul 1, 2008
    NW Montana
    kirby, based on the chamber and forcing cone dimensions for the '97, why wouldn't it be safe to shoot a 3" shell in a '97 shotgun with a 2-3/4" chamber and stock forcing cone? I measured some Winchester 3" shells that I have and they're 2-9/16" long unfired. If the hull "grows" by 3/8" at the most (approximately half the diameter of the shell), the hull is now protruding 3/16" into the forcing cone with 3/16" in the chamber. The diameter of the forcing cone at 3/16" is 0.764" based on diameters of 0.798" at the start and 0.730" at the end of the cone. The hull that you measured was 0.014 inches thick at the end so the forcing cone is reduced to 0.736" diameter which is still larger than the bore that you measured at 0.730. Given that as you said, a tight bore is 0.715" would this, in your opinion, result in an overpressure situation? What if the forcing cone was lengthened?
    Last edited: Sep 8, 2008
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