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Thread: Techie Question

  1. #11
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    Quote Originally Posted by MistWolf View Post
    "See, Capt'n, that's why, although you fly it, it's my airplane. You ride her hard, use her, abuse her and at the end of the day, it's this lowly airman that sweats and bleeds over her until she's put back to rights. I understand her. And trying to translate what you told me is wrong into what's really wrong adds time to the troubleshooting. You'll tell me the hydraulics don't lock the landing gear (when I know the hydraulics weren't designed to lock the landing gear that they lock mechanically) when the real trouble is the the landing gear light didn't go out because the position switch is bad"

    They are locking lugs not tabs. Not everyone knows that all the cam does is rotate the bolt or somebody wouldn't have made the claim that the cam along with the locking lugs locks the bolt.

    The dwell time is measured from the time the bullet clears the gas port until it exits the muzzle. Once the bullet clears the muzzle the gas pressure drops. Self loading small arms are designed that the bolt does not extract the spent casing until the pressures in the barrel have dropped to safe levels.

    Carbine length gas systems have their ports closer to the chamber where pressures are much higher than they are further down the bore. In a 14.5" barrel, the distance from port to muzzle is shorter than it is with a 16" barrel. The BCG is exposed to these raised pressures for a longer time. Again, that's because the bolt must stay locked until pressures in the barrel drop to safe levels. If you used a carbine length gas system with an 18" or 20", dwell times are even longer.

    Sure, Go Middy, YAY! But all you have to know capt'n is how much more violent the middy is compared to the rifle length system because you can see it doesn't really matter. Except to those who have made an effort to understand how it all works so they can be kept running
    Wow crew chief. I am not quite sure how to interpret this. Thank you for the corrections. Always appreciated, honestly.

    Flying is just another hobby for me. My profession is keeping extremely complicated machines operating. I am a field service engineer. I still bust ass in the field covered in nasty crap. I understand multiple system cooperation quite well. Responding to this post was as much to reinforce my knowledge base as to respond to the OP's question. You don't know what you don't know until you explain it, step by step in detail. What knowledge I lacked, I made no attempt to disguise. I was quite open in what I don't know.

    I appreciate the effort that goes into knowing everything that is required about everything in a system to keep it running. Its what I do too. I was in no way trying to derail the knowledge train. I was just chiming in with what I know.

    I fail to see how the cam does not have a part in locking the bolt when the rotation of the bolt allows the lugs to engage. Please excuse my poor parts I'd. Using incorrect terminology does not invalidate the information stated.

    Thanks for the clarification on dwell definition. Now that we know dwell times biggest factor is the amount of barrel after the gas port I have a question. Will the bolt open in either system with any popular barrel length, before the bullet leaves the barrel? You stated that the bolt will not open until pressures drop to a safe level. How is this compensated for in production rifles of differing gas tube lengths? With the increased cycle rate of the carbine I would guess that maybe they are not.

    Btw, the monologue was great, just pointed at the wrong guy
    "Oh, its a wonderful day! My sun is shining, my birds are chirping, my humongous chicken defeated Elmo." Huxley

  2. #12
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    I must tell you I appreciate you taking it well, pilotguyo540. Thanks for being a stand up guy.

    Think of the deadbolt on your front door. You put the key in the slot and turn it one way and the door locks. Turn it the other way and it unlocks. The key and the mechanism slide the bolt into the recess. The key and the mechanism does not lock the door shut. It merely moves the bolt into it's recess. Remove the key and the bolt is still locked into it's recess. Kick at the door and it's the bolt in it's recess that locks everything place.

    In this analogy, the cam and the carrier are the key and mechanism. The bolt is- well, the bolt.

    The bolt lugs engage the recess in the barrel extension. When they do that is when the action is locked. If you were able to remove the cam and the carrier, the action would still be locked.

    The bolt must remain locked until pressures drop to safe levels or things will get very exciting for the shooter. As a technician yourself, you know what happens to folks when high pressure gasses meet mortal flesh. That's what makes case ruptures and kabooms so deadly is the gasses escaping at the action end before pressures drop to a safe level.

    Looking at the design of the AR, it seems that it's a triad of reciprocating mass, spring rate and cam timing that keeps the bolt locked until it's safe.

    We talk about the direct impingement system of the AR as though it lacked a piston & cylinder. This isn't so. Instead of placing the piston & cylinder on the barrel, Stoner moved it inside the action. The bolt is the piston and the bolt carrier is the cylinder. Because of it's placement, it's reciprocating mass has less effect on the barrel. Clever, eh?

    Cam Timing. Take a look at the slot the cam sets in. It's straight for a bit then turns. The bolt does not start unlocking until the cam rides the turned portion of the carrier. During the time the cam rides in the straight section of the slot, the bolt remains locked

    Reciprocating Mass- The larger the mass, the more energy it takes to move it. (Also the greater the energy to stop it!) The same amount of gas on a bolt carrier and buffer with little mass will accelerate them more quickly than it will a bolt carrier and buffer with greater mass. It will take more time to move them and this will delay when the bolt unlocks.

    Spring Rate- Higher spring rates will resist the motion of the bolt carrier group and delay the unlocking of the bolt.

    The trick is to balance all three of these things. Too much mass or too high of a spring rate and the rifle will short stroke. Not enough mass or spring rate and the bolt carrier group will beat your rifle to scrap. Cam slot profile is important as well. Unlock the bolt too early at your peril.

    I believe carbines use a heavier buffer and stiffer spring rates than rifles, but I am unsure.

    Now add into the mix gas pressure and volume which is controlled by gas port size and location and dwell time. Fortunately, the AR gas system seems to be fairly forgiving of changes

    "Capt'n, she's on the pad ready for ya. Try to bring her back in one piece this time!"
    Last edited by MistWolf; 08-23-10 at 05:27.

  3. #13
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    Thanks Mistwolf.
    Last edited by catargadelendaest; 08-29-10 at 13:39. Reason: misspellin'

  4. #14
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    I've done a bit of thinking on Dwell Time and realized that I was getting too focused on the dwell time in which the system is charged with high pressure gas.

    Dwell Time defines the time something sorta just sits in one spot. A good example is the piston in your car engine. It moves to top dead center and stops. It's dwell time is from the moment the crank shaft & connector rod pushes the piston to TDC and the piston stops to the moment the crank & connector yanks the piston back down. The same thing happens when the piston reaches bottom dead center. During those dwell times, the piston just sits there moving neither up or down.

    The bolt and carrier do the same thing during the operation of the action. So yes, the bolt does have dwell time as well.

    My definition of Dwell Time was confined to that which describes the time it takes the bullet to travel from gas port to muzzle. I apologize for any confusion this created
    Last edited by MistWolf; 08-29-10 at 14:00.

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