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Thread: Clinic-3: BFC

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  1. #21
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    I think we are getting somewhere. What you are saying is that the case is sealing against the chamber wall, and it is the case which is pushing the slide backwards, which then drugs the barrel along?

    In other words, if there were no lugs to mesh on the top of the barrel, the slide would move back alone? Of course, that means disaster, since the pressure in the barrel is still high, but is that how it works?
    John Caradimas SV1CEC
    The M1911 Pistols Organization
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  2. #22
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    Eureka!

    Quote Originally Posted by John
    I think we are getting somewhere. What you are saying is that the case is sealing against the chamber wall, and it is the case which is pushing the slide backwards, which then drugs the barrel along?

    In other words, if there were no lugs to mesh on the top of the barrel, the slide would move back alone? Of course, that means disaster, since the pressure in the barrel is still high, but is that how it works?
    Bingo!

    I've had people argue that, once the case is sealed under pressure, that it can't move backward...but that's absurd. If that were true, excessive headspace wouldn't be a safety concern. Excessive headspace allows the case to back out of the chamber when the force drives the bolt away from the barrel/breech area. The case backs out and becomes unsupported in the thin area forward of the webbing...and ruptures. Essentially, it causes the breech to partially open...and that ain't good.

    Think a little deeper. If the barrel moved backward and drove the slide, the locking lug contact would have to be reversed, and the rear faces of the barrel lugs would bear against the front faces of the slide's lugs.

  3. #23
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    Exactly, that's what draw my attention to that. Also, my initial thinking is flawed because the barrel is in equilibrium, while the bullet is inside it. Pressure is directed at both ends, so it doesn't move itself. but of course, the case CAN move, as the bullet can. And the case would move, if it was not for the breechface, which is right behind it. So the case pushes on the breechface, which means it pushes on the slide, which in turn pulls back the barrel.

    Thank you Johnny, now it makes sense.
    John Caradimas SV1CEC
    The M1911 Pistols Organization
    http://www.m1911.org

  4. #24
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    Function

    Quote Originally Posted by John
    Exactly, that's what draw my attention to that. Also, my initial thinking is flawed because the barrel is in equilibrium, while the bullet is inside it. Pressure is directed at both ends, so it doesn't move itself. but of course, the case CAN move, as the bullet can. And the case would move, if it was not for the breechface, which is right behind it. So the case pushes on the breechface, which means it pushes on the slide, which in turn pulls back the barrel.

    Thank you Johnny, now it makes sense.
    YES! YES! YES! Another light goes on!

    John, you'd really be shocked at the number of experienced people who don't understand how the gun functions...including some experienced pistolsmiths.
    One in particular who claims to build the finest 1911 that the world has ever seen...but I ain't namin' names. Wichaka knows... Hard to soar like an eagle(<---CLEW! CLEW!) when the turkeys keep clippin' your wings...and he's clipped a lotta wings.

  5. #25
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    LoL, I guess it is not that easy, isn't it?

    Now that we have established how the whole thing works, another issue. Please correct me if I am wrong somewhere.

    As we established, the slide is pushed back by the case. So in reality, it is a good thing if the case touches the slide when it is in the chamber, I guess you call that headspacing, in other words the case should not be allowed to move to the rear, at least in an ideal situation. Correct so far?

    Now, since it is the slide which pulls the barrel to the rear, why on earth do people get so crazy, about the hood touching the breechface, when the pistol is in lock? I've heard that if it is not touching the breechface, the felt recoil increases. According to the discussion we just had, this is **.

    Another thing. How are the two touching surfaces at the left and the right of the chamber supposed to be matted? I mean obviously the first thing is to make sure that the lugs are matted properly, but that might require some trimming of these areas, in addition to the hood. Am I right? How much can you trim there? And do you trim the slide or the barrel.
    John Caradimas SV1CEC
    The M1911 Pistols Organization
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  6. #26
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    Quote Originally Posted by John
    LoL, I guess it is not that easy, isn't it?

    Now that we have established how the whole thing works, another issue. Please correct me if I am wrong somewhere.

    As we established, the slide is pushed back by the case. So in reality, it is a good thing if the case touches the slide when it is in the chamber, I guess you call that headspacing, in other words the case should not be allowed to move to the rear, at least in an ideal situation. Correct so far?

    Now, since it is the slide which pulls the barrel to the rear, why on earth do people get so crazy, about the hood touching the breechface, when the pistol is in lock? I've heard that if it is not touching the breechface, the felt recoil increases. According to the discussion we just had, this is **.

    Another thing. How are the two touching surfaces at the left and the right of the chamber supposed to be matted? I mean obviously the first thing is to make sure that the lugs are matted properly, but that might require some trimming of these areas, in addition to the hood. Am I right? How much can you trim there? And do you trim the slide or the barrel.
    Correct. When the case moves to the rear, it's moving toward losing chamber support. Re-read the differences between static headspace and working, or live headspace. Headspace can be excessive in two different ways. One is dangerous, and one is not...assuming all the exxcess is in one direction or the other. Most guns that do have loose headspace more or less divide the static dimension somewhere in the middle unless it's due to lug setback. Then it's all in the dangerous direction.
    *****************************

    Hood touching breechface:

    When the hood is closely fitted to the breechface, the slide pushes the barrel forward to its limit within the slide, and engages whichever lug or lugs that will horizontally engage...eliminating the fore and aft slack between barrel and slide...which eliminates slap-seating...which prolongs the life of the lugs.
    Imagine pushing two hammer heads together with 500 foot pounds of force.
    Now, imagine slamming them together from a foot apart with the same force.
    in which case are the two heads going to deform faster?

    Now...same two hammer heads slammed together on center, and slaming them together on a 50% offset. This illustrates the importance of full vertical depth of engagement. See it?

    Sides of the hood closely fitted:

    Accuracy doesn't come from any one fit or dimension. Many things stack up to create it by working together..and consistency is the key. When the sides of the hood have a lot of clearance, the barrel can shift a little from side to side as it cams up...which alters the angle of the chambering cartridge. A close fit eliminates a little inconsistency. Doesn't mean a lot, but it does mean something...and many little "somethings" add up to paint the larger picture.

    The other aspect of the close fit in that area is that it helps to limit the amount of torque that the barrel undergoes when the bullet hits the rifling...which imposes high rotational forces on the barrel. Again...it doesn't control it completely, but rather eliminates a lot of it..which in turn adds to the accuracy of the gun.

    The sides of the hood shouldn't be making actual contact. No such thing as a free lunch. Contact would impose added drag as the barrel cams up, and a little dirt or even heat could contribute to causing a failure to go to battery.
    For maximum accuracy, about .001 inch per side is as close as it should be. For reliability, .005 inch. .003 inch strikes a workable medium...but only for a game gun. Carry guns should be set at .005 per side.

    Likewise, a tightly fit hood to breechface is a good thing for accuracy and durability, but the drawbacks are the same. Too tight, and the gun fails to go to battery unless kept clean. I've seen bullseye shooters field-strip and clean the areas as early as every 35-40 rounds because of malfunctions.
    For the really serious ones who had their static headspace set below minimum and trimmed their brass to fit within .001 inch...they also use a dry brush in the chamber to get rid of the carbon build-up at the chamber shoulder...but these guys are chasing one-inch groups at 50 yards...and malfunction alibis are allowed.

    So, one has to determine what...and under what conditions...the gun is being built or tuned for. To strike the best compromise between accuracy and reliability, the clearances need to be a little looser. Not "rattletrap loose", but enough to allow for a reasonable amount of dirt and fouling. Also, the less able you are to maintain the gun, the looser it needs to be. If you can strip and clean it every day...even if it's just a quick wipe-down and light lubrication, you can get away with tighter clearances. If you can only manage once a week, more clearanceis needed. If you're under fire in the trenches at Belleau Wood...best go with an ordnance-spec gun.

    I've found that...under non-wartime conditions....003 inch of clearance at the hood and slide...and .005 inch on the sides works well as long as I take reasonable pains to clean and oil the gun. Stip and wipe once a month. A drop of oil in the rails with the slide locked back, and allowing the oil to run
    down for a minute or so...and detail-stripping to get the dust and lint out every 3 months is a good schedule. If I had to grab one pistol and run..I'd pick up the 1919 Colt, the 1945 Rand or one of the Norincos...but probably the '43 GI Colt that I rebuilt this year and carry a lot. It has a well-fitted Kart QUick'n'Easy fit...with .003 at the breechface and .006 at the sides and two lugs bearing the load. Static headspace is .905 inch. The only play between the slide and frame is when the pistol is dead dry. A little oil, and there isn't any. It's more accurate than I can prove without a sandbag rest, and it never malfunctions. Not even with my funky reloads...dry or lubed...cold or hot. Yeah. I wouldn't hesitate to bet my life on that one.

  7. #27
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    Hmmm, I imagine I didn't clarify myself enough. I was not talking about the sides of the hood of the barrel, but at the two areas at 3 and 9 o'clock on the rear of the chamber, which touch ledges inside the slide.
    John Caradimas SV1CEC
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  8. #28
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    9 and 3

    Quote Originally Posted by John
    Hmmm, I imagine I didn't clarify myself enough. I was not talking about the sides of the hood of the barrel, but at the two areas at 3 and 9 o'clock on the rear of the chamber, which touch ledges inside the slide.

    Ah! Sorry John.

    Those areas should have a little clearance with the slide and the only contact should be with the hood face. Otherwise the chamber face will get peened by the slide. The clearance is correctly done on the slightest forward angle top to bottom. About 2 degrees or a little less, starting with a couple thousandths clearance in the corners of the hood, and increasing toward the bottom. The angle isn't really critical as long as there's enough clearance.

  9. #29
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    Johnny,

    These are the areas I am talking about:



    Not at the sides of the hood, at the sides of the barrel.
    John Caradimas SV1CEC
    The M1911 Pistols Organization
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  10. #30
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    Pix

    Yep...that area should have just a tiny bit of clearance with the slide. I'm talkin' maybe .003 inch at the widest points.

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