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Thread: BCG/Buffer/Spring system - Engineering perspective

  1. #21
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    Quote Originally Posted by sinister View Post
    excellent videos that really show how the rifle exhausts the gas

  2. #22
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    Quote Originally Posted by MistWolf View Post


    The only variable between a carbine length gas system with a 14.5" or with a 16" barrel is dwell time. Gas pressure is the same for both. The only variable is what is known as "dwell time". If the BCG were to start moving while the bullet was in the barrel, both start moving when the bullet is at the same place in the barrel. Yet, the 16" barrel accelerates the BCG faster. Where is the extra energy coming from? It can only come from delaying venting.

    From the increase in kinetic energy, due to the extra mass of propellant going into the gas port before the bullet uncorks the barrel. This is why the carbine length in a 16’’ barrel is harder on the BCG.

    Also, I would imagine that dwell times in the AR system are more important for FA rather than semi-fire alone, though it would make firing a semi-only AR much easier if the numbers are within set parameters. But think about the ammo most people use.


    StainlessSlide, curious to how you approximated some of the values, and the math for integrating pdv.

  3. #23
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    Quote Originally Posted by MistWolf View Post
    Also keep in mind that the extra .7" of retraction also offers the bolt a running start before it engages the rim of the next cartridge to overcome any initial resistance to being stripped from the mag.
    This is an excellent point, and a large part of the problem with the types of egg-headed analysis in the OP. Simply being an engineer (or worse, and engineering student) is not enough. Rather than analyze the system with what you think you know, you need to understand the system as it was designed to work and why.

    I have encountered more than one AR that will not load from a locked bolt when the paddle is depressed but will chamber every time when the shooter retracts the bolt fully to the rear with the charging handle and releases the bolt.

    Additionally, there are tolerances to account for wear as well as fouling. What works perfectly in a lab somewhere in a sterile environment is not necessarily germane to what works outdoors and over the long haul.

    I had a rifle, ammo, and magazines from a manufacturer awhile back that I could easily induce double-feeds on. When I asked my contact about this I got "our engineers have 10s of thousands of rounds on this system." Awesome. Engineers. They never left the ****ing bench. All they would have needed to do was to run from point A to point B with the magazines in pouches and they would have induced the exact same malfunctions I did. Instead, it took said ammo being released to the public and other actual shooters and users spending time with it outside of a laboratory for the manufacturer to finally understand the problem and change the ammo.

    I'm way more interested in effect than cause. I guess if you think you're going to design the new hotness then testing like this is of interest, but I'd rather see someone spend some time quantifying the alleged improvements of things like smaller gas ports, redesigned buffer systems, various brakes, etc. and since how far the buffer and bolt move is wholly inconsequential to me as a shooter (unless you can explain how it enables me to shoot more accurately, faster, or for longer between malfunctions or parts failure), I'm not sure I understand the point of the OP.

  4. #24
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    Quote Originally Posted by Gun View Post
    From the increase in kinetic energy, due to the extra mass of propellant going into the gas port before the bullet uncorks the barrel. This is why the carbine length in a 16’’ barrel is harder on the BCG.

    Also, I would imagine that dwell times in the AR system are more important for FA rather than semi-fire alone, though it would make firing a semi-only AR much easier if the numbers are within set parameters. But think about the ammo most people use.


    StainlessSlide, curious to how you approximated some of the values, and the math for integrating pdv.
    Ok, one more time- If the AR used a closed system (bullet still in barrel when BCG gets pressurized and begins moving) the dwell time of an extra 1.5 inches will make no practical difference. Pressure will be the same, when the system dumps excess pressure overboard will be the same, energy to the BCG will be the same.

    The only way the difference of 1.5 inches of bore could change the energy of the gases that pressurizes the BCG is if the BCG pressurizes after the bullet exits the barrel completely (uncorks the muzzle)
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

    http://i115.photobucket.com/albums/n289/SgtSongDog/AR%20Carbine/DSC_0114.jpg
    I am American

  5. #25
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    Quote Originally Posted by MistWolf View Post
    That's what I said.

    The AR is a self regulating system. When it gets enough pressure to operate, the movement of the BCG separates the gas key from the gas tube and the rest of the gases are vented overboard.

    Therefore-

    As it would be a closed gas system until the bullet uncorks the barrel, if the gases were to begin moving the BCG at this time, pressure and time would be the same whether the barrel was 14.5" or 16".

    But with an open system, that is, the BCG does not pressurize until after the bullet uncorks, the extra distance does affect how much energy the gas carries into the BCG as the longer bore increases the amount of time it takes for the gases to vent enough to be a significant event.

    If you study slow motion videos of a firearm discharging, you can see that the gases vent rather forcefully for some time after the bullet uncorks the muzzle. That's the energy that's operating your rifle.

    I saw a writeup where a guy cut the barrel of his rifle length AR right at the front of the FSB. The rifle functions fine although there is virtually no dwell time.

    SS, yes the system does not pressurize all at once. I'm not sure if reflecting pressure waves play much of a role in all of this like they do in automotive internal combustion engines
    When you say they're vented overboard, do you mean outside of the ejection port?

  6. #26
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    Quote Originally Posted by Boss Hogg View Post
    When you say they're vented overboard, do you mean outside of the ejection port?
    Gases are vented overboard through the exhaust ports in the BCG and when the gas key separates from the gas tube. Of course gases are also vented when the bullet uncorks the barrel
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

    http://i115.photobucket.com/albums/n289/SgtSongDog/AR%20Carbine/DSC_0114.jpg
    I am American

  7. #27
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    Quote Originally Posted by Boss Hogg View Post
    When you say they're vented overboard, do you mean outside of the ejection port?
    Vented inside the upper receiver and left to escape via the path of least resistance.

  8. #28
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    Quote Originally Posted by rob_s View Post
    I have encountered more than one AR that will not load from a locked bolt when the paddle is depressed but will chamber every time when the shooter retracts the bolt fully to the rear with the charging handle and releases the bolt.
    This is food for thought - thank you. Was there a difference with a more full or more empty magazine?

    Quote Originally Posted by rob_s View Post

    Additionally, there are tolerances to account for wear as well as fouling. What works perfectly in a lab somewhere in a sterile environment is not necessarily germane to what works outdoors and over the long haul.
    Which is why I'm trying to understand the BCG energy reserve - the thing that keeps the carbine feeding under marginal conditions

    Quote Originally Posted by rob_s View Post

    I'm way more interested in effect than cause.
    So you don't care much if you understand the system - some do.

    Quote Originally Posted by rob_s View Post
    ...since how far the buffer and bolt move is wholly inconsequential to me as a shooter
    Until you want to feed the next round.

    You're right that attempts to analyse a system more quantitatively often overlook important aspects, even when done by the best engineers. Thus the post to a forum full of heavy AR users, which has borne fruit already.

  9. #29
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    Quote Originally Posted by AR15barrels View Post
    Can you further explain your measurement process?
    Pictures may help.
    Pictures may follow, but consider this for now.

    Say you removed the stock from a carbine, and held a 2" piece of aluminum rod tightly between thumb and forefinger. Suppose this rod was small enough to enter the vent hole in the rear wall of the receiver extension (RE). You insert the rod into the receiver extension coaxially through this hole, keeping the finger grip. The rod now sticks straight forward into the RE through the rear vent hole, with your fingers against the outside of the rear RE wall. The rod doesn't touch anything if the BCG is full forward.

    Now fire the weapon. As the buffer comes back, it will push backwards on the rod inside the RE. The rod will slide through thumb and forefinger, only stopping when the buffer reaches maximum retraction. The rod will not slide back appreciably farther than this due to inertia, because of its lightness and the friction of the finger grip.

    Pull the rod back out of the vent hole, then measure the distance the rod extends forward from your fingers, and subtract the RE rear wall thickness. The result will be is how close the plastic end of the buffer came to the rear wall of the RE.

    Do a similar measurement with the BCG full forward (using the depth rod of a caliper through the vent hole, perhaps, if you can find one that fits). Subtract these two and you'll get the maximum buffer retraction.

    In my case the "thumb and forefinger" is a cylindrical plastic cap which clamps to the back of the RE. It has a center hole through which the aluminum rod slides. An adjustable spring and ball bearing provides the drag (like the fingers squeezing).

    I am assuming that the buffer and BCG are in contact at maximum retraction.
    Last edited by StainlessSlide; 06-19-12 at 11:48.

  10. #30
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    Quote Originally Posted by sinister View Post
    The gas port pressure plot given by Armalite is very interesting, thank you. But I don't see any pressure drop which would show that the bullet has exited. The graph covers 4 milliseconds, by which time the bullet is long gone.

    Where is the pressure cliff that must occur after bullet exit, somewhere around 1 to 1.5 ms of the plot? It would have been nice if they gave a source for that plot, so we could get the context.
    Last edited by StainlessSlide; 06-18-12 at 13:09.

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