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Thread: Clinic-3: BFC

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  1. #11
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    Hood

    Quote Originally Posted by John
    I have a question, that I die to ask:
    So, now as the slide/barrel combo starts to recoil, ins't the barrel hood participating in sharing the load of the thrust? Until the barrel starts unlocking, I think it is.

    Come on Johnny, enlighten me.
    Nope. Think about the way the pistol operates, bossman. Remember that the front faces of the barrel lugs engage the rear faces of the slide's lugs.
    When the bullet hits the rifling under pressure, it holds the barrel hard forward. That same pressure is slamming the slide backward. That would tend to try to force the breechface away from the hood. If it's any consolation, I once had a gunsmith give me a lecture about "The Forgotten Fourth Lug." He was talking about the hood and breechface junction. Took me half an hour to get the light to turn on for him. There are a lot of practicing smiths whose specialty is the 1911 pistol...who don't completely understand how it functions.

    That's why excessive headspace due to lug setback is dangerous. The slide moves backward and away from the barrel by the same amount of the deformation, partially opening the breech. The case backs out as it follows the slide, and loses chamber support. Ka-boom!

  2. #12
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    So, in reality, the barrel is pushed forward? I am afraid I do not understand. Especially when you say that the same pressure is slamming the slide backward.

    In my (non-gunsmithing) mind, while the bullet is still in the barrel, pressure pushes at the base of the bullet and at the inside area of the case head. Due to the mass difference, the bullet moves forward much faster than the slide/barrel (which are engaged) move backward, so the bullet exits the barrel and now the slide/barrel combo is moving rearwards. The reason they are moving rearwards, is of course because the lugs are engaged. If as you said the front surface of the barrel lugs are engaging the rear surface of the slide lugs, and the hood is touching the face, and since it is the barrel that initiated the movement to the rear, not the slide, the area that puts pressure on the slide to move back, is the hood, not the lugs.

    I am confused, even more so now, than before asking.
    John Caradimas SV1CEC
    The M1911 Pistols Organization
    http://www.m1911.org

  3. #13
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    Recoil

    Almost there, boss. The barrel doesn't initiate the slide's movement. The thrust vector between the slide and bullet does that. The slide pulls the barrel backward. If the barrel drove the slide, the lug contact areas would be reversed. Rear of the barrel lugs to front of the slide lugs.

    The pressure of firing drives the barrel and slide in opposite directions. Due to the slide's greater mass, it conserves its momentum more efficiently than the bullet does. The greater mass overcomes the lesser mass...and the unit moves backward. In reality, the recoil-operated, locked breech 1911 operates exactly the same way as a straight blowback. It just uses a different mechanism to delay the opening of the breech until the bullet exits and pressures fall.

  4. #14
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    The thrust vector between the slide and bullet does that. The slide pulls the barrel backward.
    I think I got this. Physics 101 - for every action there is an equal and opposite reaction. The force of the exploding powder/gases is pushing the bullet out of the barrel and exerting force onto the barrel pushing it forward. At the same time the gases are pushing rearward against the spent cartridge and breechface.

    If it weren't for the lugs locking the two together, the barrel would slide forward to the extent the link will allow and then stop. The slide would go rearward until the dustcover hit the frame bed. The gun would not come back into battery and the barrel would have to be "reset" by hand after each shot.
    "The 1911 was the design, given by God to us through John M. Browning, that represents the epitome of what a killing tool needs to be. It was true in 1911 and it's true now." - Col. Robert Coates commanding, U.S. Marine Corp Special Operations Command Detachment 1 (DET 1)

  5. #15
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    Physics 101

    Bingo garrett...except for the part about:
    >The gun would not come back into battery and the barrel would have to be "reset" by hand after each shot.<
    ************************

    You forgot about the recoil spring...Springs work in both directions.

    Your post basically described the straight blowback design, which uses added slide mass and a heavy recoil spring to delay the breech opening. The short recoil system of the 1911 does it exactly the same way, except it "adds" to the slide's mass temproarily by locking the barrel to the slide, which negates the need for having to use a heavy slide. When the bullet exits, the pressure drops and the barrel starts to unlock. The slide, now free of its added burden, can move at full speed toward the frame's impact surface.

    If the 1911 in its present form were to be converted to straight blowback by removing the locking lugs from the barrel, the recoil spring rating would likely have to be doubled...or greater... in order to keep the breech closed until the pressure dropped to a safe level.

  6. #16
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    Wow, 1911Tuner - you have my head spinning in amazement that I can actually follow most of the physics you're talking about!

    However, I'm coming at this from a very different approach - just trying to keep a "shooter" shooting, as opposed to precise match-fitting of the barrel and I'd like a little straightforward advice if you don't mind, Sir.

    I have two Argentine pistols that I just acquired (see posts in the "foreign contracts" forum) - both were purchased from guys who had shot them some, but not much, after they had bought them used. Both of the barrels are pretty well worn, but they are the original barrels. So it has been suggested to me that I should maybe shoot them a little as-is to check functionality (they pass the non-firing function tests), but to install new barrels for general shooting and save the originals for collector's sake.

    If I go with a Wilson "drop-in" barrel/bushing kit, and the stock-equivalent barrel link, am I likely to find myself wishing that I had just gone ahead and paid someone to do it right, or is the amount of fitting expected to be relatively minor and confidence-building for my 1911 skills? I understand that I will need to check the lug and hood enagement, as well as bushing-to-slide fit, but is this relatively straightforward? (Kuhnhausen's book is on the way)

    Also, I know this is probably a dumb question, but should I plan on having the headspace checked after installing the "drop-in" barrel?

    I appreciate your tolerance with me for these basic questions, but the advice on these forums has never failed me yet, and I'm just looking to decide whether my time is worth more than the aggravation I'm facing, and if I'd be better off just paying a 'smith to do it. Thanks.

    flcracker

  7. #17
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    I am by no means in the same league as 1911Turner.

    I have installed a few Wilson "drop ins". All that I have used have been pretty much drop in, you may need to take some of the hood both in length and width. If you can work a file and take your time, file a little and check you should not have any problems. Check the lug engagement both top and bottom, check that the link is not trapping by putting the slide stop through the link (with the slide off the frame) push it back against the back of the lugs and there should be a little side to side movement of the link. And when ever you put a new barrel in check the headspace.

    Shoe

  8. #18
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    Headspace

    Howdy FLcracker,

    Actually...on any pistol of unknown history and as old as the Argentine Colt...
    I'd do a headspace check before firing it with the OEM barrel. You'd be shocked at the new in box factory guns that almost fail the NO-GO test...
    and some actually do.

    Always check the headspace on a replacement barrel. The fingers that you save might be yours.

    You can make a poor man's gauge by using a resized or unfired empty case
    and a set of ingition feeler gauges...the little ones. Measure the case and use the largest gauge that will slip between the case and the breechface
    with just a little drag. Add the thickness of the gauge to the case length, and add another .001 inch just to be on the safe side. It won't be as precise as a real gauge, but it'll be close. If the total adds up to more than .920 inch...proceed with all due caution.

  9. #19
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    Quote Originally Posted by 1911Tuner
    Almost there, boss. The barrel doesn't initiate the slide's movement. The thrust vector between the slide and bullet does that. The slide pulls the barrel backward. If the barrel drove the slide, the lug contact areas would be reversed. Rear of the barrel lugs to front of the slide lugs.
    Johnny, this is the part that puzzles me. Since the bullet is inside the barrel, I was expecting the barrel to start moving backwards, pulling the slide along with it. How can the bullet affect the slide? I mean, what is actually pulling/pushing the slide back? I was under the impression that the force which pushes the bullet out, (or rather the equal and opposite force to the one pushing the bullet out) was pushing the barrel back, which in turn pushed the slide back. If that is not what happens, what initiates the slide's movement, so that it pulls the barrel along?
    John Caradimas SV1CEC
    The M1911 Pistols Organization
    http://www.m1911.org

  10. #20
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    re:

    Quote Originally Posted by John
    Johnny, this is the part that puzzles me. Since the bullet is inside the barrel, I was expecting the barrel to start moving backwards, pulling the slide along with it. How can the bullet affect the slide? I mean, what is actually pulling/pushing the slide back? I was under the impression that the force which pushes the bullet out, (or rather the equal and opposite force to the one pushing the bullet out) was pushing the barrel back, which in turn pushed the slide back. If that is not what happens, what initiates the slide's movement, so that it pulls the barrel along?
    The same pressure in the chamber that drives the bullet forward drives the slide backward. If this weren't the case, the straight blowback action couldn't work at all. The slide is nothing more than a breechblock...a bolt. The breechface is often referred to as the thrust face, because the force vector thrusts against it with the same power that it drives the bullet.

    >>>The Colt/Browning short recoil system works exactly the same way as a straight blowback design. It just uses a different system to delay the breech opening.<<<


    Hmmmm...Lemme try this from a little different angle. Let's build an imaginary gun.

    The slide is welded to the frame, and it has a channel in it that will allow the barrel to slip in and out of the slide freely. A slip-fit barrel within the slide.
    The barrel has no locking lugs or link, and isn't mechanically tied to anything.
    Aside from that, it's the same as any other .45 caliber barrel on the inside.

    Imagine that you slid the barrel out of the slide, chambered a round, slipped it back in, and fired the round. Would the barrel slam backward into the breechface...or would it be launched along with the bullet? Remember how tight the bullet is when it's in the barrel...

    When the 1911 goes bang, the lugs engage, and the slide drags the barrel backward as the bullet is in transit. Just shy of .10 inch (nominally) of rearward slide travel, the bullet exits, the barrel starts to unlock, and the slide continues via the momentum that it conserved during that first .10 inch of travel...while the bullet was still in the barrel.

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