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"Until pressure drops to a safe level"

Discussion in 'Rifle Country' started by mp5a3, Jan 1, 2010.

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  1. mp5a3

    mp5a3 Member

    Jun 12, 2009
    I see this term used a lot on the description of auto loaders. Can someone here explain this in more detail for me. I know that many rounds like like the 5.56 are very high pressure. Also what keeps the bolt locked ? The reason I ask is because a handgun has no type of rotating bolt and unlocks as soon as the powder is ignited. Wouldn't the bolt move some even before the gas hits the piston (on an AK) or carrier key (on an AR) Thanks. You guys are a ton of help on these things. I find the design and operation of firearms very cool.
  2. Ridgerunner665

    Ridgerunner665 Member

    Feb 3, 2005
    Upper East Tennessee
    AR's have a rotating bolt with locking lugs that lock into the barrel extension. (most modern semi auto rifles have a similar set up...not sure about AK's)

    Semi auto pistols work in different ways...a 1911 unlocks after the slide has moved back a short distance, most pistol use a similar set up (this is called locked breech)....but there are variations such as in most 380 pistols which are blowback operated.
  3. benzy2

    benzy2 Member

    Apr 24, 2008
    In an AR, along with most auto loading rifles, the bolt does not move until the gas has reached the operating system (be that DI or piston driven). The lugs on the AR bolt turn when the bolt is completely forward so that they overlap a matching set of lugs on the end of the barrel. When the round is fired the bolt face is pushed back into these barrel lugs and cannot move. When the gas hits the carrier key the bolt carrier starts to move rearward. As the carrier moves rearward it cams the bolt to rotate and unlock. Once the lugs on the bolt have cleared the lugs on the barrel the entire bolt assembly will then move rearward. If the gas system of most semi auto rifles is removed you end up with a single shot that won't cycle. As far as I know, all semi auto rifles firing rifle calibers have a locking bolt system that keeps the bolt locked during initial firing pressures, though I am sure some funky system is out there that I am unaware of.

    Google search for animations of how an AR15 or M16 work and you should get some good videos that show in detail the steps of how everything works. Same for the AK or M1/M1A. Sometimes a visual is better than a bunch of words.
  4. 9mmepiphany

    9mmepiphany Moderator

    Dec 27, 2002
    northern california
    this actually isn't true.

    most handguns, i take it we're takling about semi-autos here, which are locked at the time of ignition, do not unlock until the bullet has left the muzzle and pressure has dropped...i'm sure there is an exception (i'm thinking the H&K P9S)...they just don't use a rotating bolt...unless you count the AutoMag or the Wiley (the Beretta PX4 has a rotating barrel)
  5. rcmodel

    rcmodel Member in memoriam

    Sep 17, 2007
    Eastern KS
    In an AR-15 for instance, chamber pressure can reach 55,000+ PSI. But the gas port in the barrel is 12" or so further down the barrel and pressure there is less then 1/3 of what it was in the chamber where the powder burned. What reaches the gas port is not burning powder but rather expanding hot gas that was a result of the powder burn in the chamber.

    By the time the bullet passes the gas port and the gas can flow down through the gas block and back through the gas tube to the bolt carrier, the bullet is long gone and bore pressure has dropped to nearly atmospheric level.

    Only at that point is the bolt unlocked to begin extraction & ejection.

    If the bolt unlocked at full chamber pressure, the brass case would fail and the rifle would blow up.

    Recoil operated locked-breech pistols operate the same way.
    The barrel & slide are locked together at the moment of firing.
    A delay built into the system by the barrel cam or barrel link keeps them locked together until the bullet is gone and chamber pressure has dropped to a low enough safe level to allow extraction.
    Again, if the barrel unlocked from the slide at the time of ignition and max chamber pressure, the brass case would rupture and the pistol would blow up.

    A third type of action is the straight blow-back. In that case the bolt is not locked and depends on inertia to keep it closed long enough for chamber pressure to drop to a safe level.
    But those guns are only chambered is relatively low pressure calibers like .32 & .380 ACP, etc., and the brass case is able to contain the max chamber pressure, even though they do start to open at the moment of firing.

    Last edited: Jan 1, 2010
  6. briansmithwins

    briansmithwins Member

    Aug 1, 2005
  7. Uncle Mike

    Uncle Mike Member

    Nov 5, 2008
    never never land...never land here!
    Check this out--

    Scroll to the bottom of the page...instructional videos...click on the video, 'Bolt Carrier and Bolt'.
    This video explains how the AR system locks up, the pressure check and lock, so on....
    Last edited: Jan 1, 2010
  8. MachIVshooter

    MachIVshooter Member

    Aug 11, 2005
    Elbert County, CO
    Most handguns, virtually all .380 and larger, use a short recoil locking system. With this system, the barrel and bolt travel in unison a short distance before the barrel cams down (or rotates) to unlock from the side.

    Many smaller caliber guns (.22 rimfire, .25 ACP, most .32 ACP, some .380's) use blowback, in which only spring pressure keeps the breech closed. Blowback designs are less tolerant of varying ammunition power, and the stronger spring required in this design means that 1) they are more difficult to rack and 2) the design becomes excessively bulky when used with larger cartridges, such as the hi-point pistols.

    A third system is the roller delayed blowback, employed by HK in older handguns and in the Czech VZ-52 pistol. Similar to the conventional tilting barrel design, the slide and barrel travel together a short distance until the rollers go into a recess, allowing the barrel to unlock

    Another system is the locking block, best example being the beretta 92. it works in basically the same fashion as a tilting barrel, except that the barrel remains level. Instead, a block attached to the barrel drops to unlock.

    What all of these sytems have in common is that the case does not come out of the chamber until the bullet has left the barrel, hence the "safe pressure level" denotation.

    With a gas operated system, like the AR, AK and all others, the gas port is located so that by the time the bolt unlocks and begins to travel rearward, the bullet is gone from the bore.

    The effects of the pressure being too high can be witnessed in any case of out-of-battery firing, where the system is not locked, or unlocks prematurely. Blows the case head off and often damages the firearm.
  9. Slamfire

    Slamfire Member

    Dec 29, 2006
    The pages below are from AMCP 706-260.

    Automatic firearms mechanisms are designed to unlock at pressures that won't rupture the cartridge. All gas operated mechanisms that I am aware of unlock when there still is residual pressure left in the barrel. This residual pressure is energy that is still available for work. Typically the action will unlock when the residual pressure is less than 650 psi.

    In the first chart, the residual pressure drop curve is there for a reason. The pressure drop rate is a critical issue for a designer.

    Timing is critical to the proper function of these firearms. Once the mechanism is desinged, a powder that is slow burning and thus has a high residual pressure, will cause function problems.

    The second chart below shows that during the design phase, the timing of the mechanism is carefully calculated.



    I have been involved in a number of discussions on my use of lubricated cases in semiauto firearms. To have an informed opinion, I needed to increase my understanding on the design of firearms. I have found that firearms design books are very rare. There are hundreds of “eye candy” books, with pretty pictures of firearms, but very few books that provide the information to understand operation and design principles. I have found a few and hope this list is useful for those looking to expand their understanding of the fundamentals of firearms design.

    1. “Technical Notes, Small Arms Design”, Author: John G. Rocha , available from Armalite (800) 336-0184, stock number NA1085 $12.50 . This soft cover pamphlet is a copy of the course material handout to a firearms class taught at Rock Island in the 60’s. Very interesting, a good number of formulas, but still very readable.

    2. Brassey’s Essential Guide to Military Small Arms, Design Principles and Operating Method, Author Allsp and Popelinsky, Brassey’s Inc, 1997. IDSA Books , (937) 773-4203 price was lower than Amazon.com’s. This starts off simple and ends up very technical. This is a serious book and the final sections really require education in math, science, or engineering.

    The above two books are the only ones that I have found that are currently in print.

    3. The Machine Gun, Volume IV, parts X & XI, LTC George Chinn, pub 1955. Out of print. This book should be the absolute first book to buy for someone who is interested in the principles of automatic weapon design. Really an excellent statement of principles. It is a comment on the general state of technical ignorance in our society that this book is extremely rare, but the general public is not that much interested in it. A Gun Show book dealer told me more people wanted the volumes with the pretty pictures of old guns!

    4. The Bolt Action by Stuart Ottenson. First edition by Winchester Press 1976. Ottesnon later added a Volume II which came out in a two volume edition by Wolfe Publishing in 1985. These books are very non mathematical for a general audience, but the principles, particularly expounded in Vol 1 on the Mauser 98 are, in my opinion, fundamental to the understanding of bolt action design.

    5. AMCP 706-260. Engineering Design Handbook: Automatic Weapons. Out of print. The most technical and mathematical design book I have found to date. Assumes a high level of knowledge in firearms design, mathematics, and Mechanical Engineering. A technical degree, preferably in Mechanical Engineering is really needed to attempt to understand the presented material. This was created in the early 60’s, and reflects the designs, and the design knowledge of the day.

    6. AMCP 706-252 Engineering Design Handbook: Gun Tubes. Out of print. Very interesting, not limited to small arms.

    AMCP stands for Army Material Command Pamphlet. There were about one hundred AMCP Pamphlets covering information ranging from Statistics to Automotive Design. A copy of AMCP 706-260 and other out of print AMCP pamphlets can be ordered from NTIS at 703-605-6000. These are not cheap, they want $45.00 to $150.00 for some of them.

    7. Small Arms & Cannon, Smith and Haslam, RMC of Science, Shrivenham UK, 1st Edition, 1982, Brassey's Publishers LTD. No idea for a source really excellent read.

    Firearms design books are so rare because the market is so limited. I would suggest to anyone who has a technical background, and is interested in firearms design, to acquire those books in print. Because once they are out of print, you may never find one. Or you will have to pay, like I did, about $100.00 for Chinn’s Volume IV. And be happy to get it, because the price is now $250.00! SlamFire.
  10. briansmithwins

    briansmithwins Member

    Aug 1, 2005
    I was able to find copies of most of the books mentioned above by using the 'Worldcat' search at my library's website.

    Interlibrary loan is great!

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