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why not neck size in auto's

Discussion in 'Handloading and Reloading' started by remmag, Mar 18, 2011.

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  1. remmag

    remmag Member

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    hey there
    i have been reloading for about a year, so far so good. was loading up a batch of 30-06 tonight and the question got into my mind
    why can you neck size for bolt actions and you must full length re-size for auto rifle's

    thanks for all of the input in advance
     
  2. Steve in PA

    Steve in PA Member

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    Eventually, even a case neck sized for a bolt rifle will chamber. With a bolt rifle you tend to have greater force in chambering a round. In a semi-auto rifle you could end up with your rifle locked up nice and solid.

    Never neck size only for a semi-auto rifle.
     
  3. dmazur

    dmazur Member

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    I believe the best answer for "Why should I always full-length resize for semi-auto rifles?" is simply, "Because it's safer."

    As the previous post stated, a bolt-action rifle has tremendous power to chamber an over-sized case through the camming action of the lugs.

    However, a semi-auto action does not have this power available. The result may be a partially closed bolt, with the lugs not properly engaged. While most actions have safety mechanisms to prevent firing in such a dangerous condition, these can wear to a point where they no longer function.

    The result is generally termed a slamfire. It can be as benign as having the appearance of an unintentional discharge. As long as you maintainted muzzle discipline, you fired an extra round downrange and probably hit the dirt under the target. If you were lucky. It can also blow the bolt back toward your face and disassemble the receiver.

    The basic safe practice is to full-length resize for semi-autos, even at the expense of reducing brass life, so that every round you make will chamber. Absolutely and without doubt, it has to chamber.

    Other safety practices related to preventing slamfires include uniforming primer pocket depths with a cutting tool, to guarantee primers are seated below flush, and using primers which are not designed to be extra sensitive, such as mil-spec primers.

    There are quite a few posts on this if you are still curious. Search for "slamfires"...
     
  4. remmag

    remmag Member

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    that makes good sence

    thanks guys
     
  5. gamestalker

    gamestalker member

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    It's good to think you had the good common horse sense to ask that question. Some have assumed nothing is any different since the cartridge is what it is, 30-06 or what ever high powered rifle round, and then necked it as with their bolt actions. A man came into the gun shot I worked in with a really nice, well was a really nice Browning semi auto, I think it was chambered in 270 win., no matter. He said he had been necking for a while for that rifle without problems. I doubt he considered now and then chambering issues as problems. Either way, all it took was one slam fire to ruin his day and rifle, and fortunately not his life!
     
  6. remmag

    remmag Member

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    i figure its worth the time to ask, if you are going to do something if figure gain all the knowledge you can

    sean
     
  7. Slamfire

    Slamfire Member

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  8. Brad5192

    Brad5192 Member

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    What problems are their with full length sizing for a bolt action.
     
  9. GLShooter

    GLShooter Member

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    No problems unless you start shoving the shoulders back to far and cause the cases to stretch. Obviously for NS only cases they need to go back in the same bolt action or you may have chambering issues. Lots of guys will resize the case to SAAMI 000 when FL sizing and that way it will run in all their rifles. I have a 223 700 that is about 0.003 longer than my tightest 223. I can't use that ammo in the other rifle as I just barley move the shoulders on it if I FL.

    Greg
     
  10. ranger335v

    ranger335v Member

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    There is seldom any real benefit to neck sizing for bolt rifles, none at all for autos.
     
  11. jerkface11

    jerkface11 Member

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    Really? Based on what?
     
  12. ArchAngelCD

    ArchAngelCD Member

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    I think some of the discussion above about slamfires is a bit off on definition. A slamfire occurs in a semi-auto when the bolt closes and causes the round in the chamber to fire before the trigger is pulled. The above uses of the word slamfire don't apply. (unless I read something wrong)
     
  13. 918v

    918v Member

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    Automatics are designed to work with a certain amount of clearance between the chamber, the round, and the bolt face. The bolt needs to rotate easily behind the round because the gun cycles in a fraction of a second.

    Neck sizing maintains zero clearance between the bolt face and the round. The bolt has to wedge itself inbetween th back of the carteige and the lug recess. This is OK in bolt action rifles because they have alot more leverage and larger bolt lugs than their automatic counterparts. Neck sizing for automatics will cause premature wear and/or breakage of the bolt.
     
  14. W.E.G.

    W.E.G. Member

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    Try sticking a spent bottleneck case that was fired in a semi-auto in a case gage.

    It won't go until it has been resized.
     
  15. buttrap

    buttrap Member

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    It is all depending on the gun as some self loaders will slam a round in the chamber with a lot more hammer than a lot of bolt guns will and some wont.
     
  16. 918v

    918v Member

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    So which autos will do that?
     
  17. steve4102

    steve4102 Member

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    The link to Zediker posted by Slamfire1 is a good read. I highly recommend it along with this from Sierra.
    http://www.exteriorballistics.com/reloadbasics/gasgunreload.cfm

    When a round is fired in a bolt gun the brass expands to fit the chamber then it springs back ever so slightly which allows for easy extraction (Fireformed). Things are a bit different in a semi-auto. When a round is fired in a semi-auto the brass case expands to fit the chamber same as a bolt action. The difference is that in a semi the brass is still under a slight bit pressure when the round is extracted. This slight bit of pressure causes the fired case to be a bit larger than the chamber it was fired in. A round fired from a semi is not a fireformed replica of the chamber, therefor the case must be Full Length sized to bring it back to specs.
     
  18. dmazur

    dmazur Member

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    Guilty as charged.

    The term I was looking for was "out of battery fire". This isn't a slamfire, as the rifle didn't fire until the shooter pulled the trigger. However, I've read about this condition generally lumped together with slamfires and I thought that would be a good direction to point someone.

    Firing pin protrusion or high/sensitive primers are generally blamed for slamfires, I believe. And, not full-length resizing and using a case gauge is generally blamed for OOB and head separation incidents with semi-autos.

    With the Garand, there is a firing pin tail & receiver bridge that is supposed to keep the floating firing pin from hitting the primer until the bolt rotates into the correct position. From what I've read, this doesn't always work, especially in 60 year old rifles with mixed parts.

    There is also a camming surface on the front of the hammer that is supposed to force the bolt to rotate to "locked" if it didn't do so on the op-rod spring. This is also subject to failure if parts are worn.

    Reviewing, if the case doesn't fully chamber and you pull the trigger and fire with the bolt unlocked, it is out of battery fire (OOB) not a slamfire.

    If the rifle fires when the bolt hits a high primer, before you pull the trigger, it is a slamfire. If you are lucky, the bolt was rotated at least partially closed. So slamfires are not necessarily OOB, but they can be.

    And, while I certainly don't understand this completely, I believe that improper case resizing can contribute to slamfires. Perhaps in the last fractions of an inch of chambering with a bolt moving forward at high speed, the last thing you want to do is test all those safety mechanisms by stopping the case early, just to see if the floating firing pin can hit the primer before it's supposed to?

    Anyway, sorry for contributing to the general confusion...
     
  19. ranger335v

    ranger335v Member

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    "There is seldom any real benefit to neck sizing for bolt rifles --- Really? Based on what? "

    Facts; not "web wisdom."
     
  20. Slamfire

    Slamfire Member

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    I believe the idea that the receiver bridge was a safety device was an idea promoted by the ex Ordnance Officers who worked for the American Riflemen in the 60’s. These guys were total apologists for the Army, expecting post employment free bees and of course the Army was providing services running the National Matches and the DCM. The articles they wrote were to assure the shooting community that all the slamfires happening were due to poor reloads and monkeyed rifles, not due to any flaws in the design.

    It was a coverup.

    I do believe the receiver bridge pulls the firing pin off the primer of a fired case, something that would prevent exjection issues, but I really question if it did anything else, besides holding the back of the bolt up.

    The Army had slamfires from the very beginning. They had them so far back that the memory was totally forgotten by the time the NRA suck ups were writing their articles in the 60’s. Proof of this is the rare round firing pin. Like the AR15, the M1 Garand firing pin had it weight reduced. Obviously they were having the same problems with the early Garands as they had with the early AR’s: slamfires! Reducing the weight of the firing pin reduced the impact energy as the thing rebounded off the primer.
    [​IMG]
    [​IMG]
    This is where the firing pin is stopped by the receiver bridge. At this point that firing pin is all the way forward in the bolt face and yet the bolt is maybe .006” from closure. That receiver bridge does not do much if anything about preventing slamfires. This is a very nice receiver with all matching HRA parts.
    [​IMG]

    The NRA is not nearly as supportive or unquestioning of the military now as it was back in the 60's. Around 1968 the Army stopped running the National Matches and the NRA had to make up the difference in funding, now days the Ohio National Guard provides manpower, the Regular Army provides teams to shoot, but darn little else. No more Leg Ammo, or issue rifles.

    In fact, the Regular Army is now very hostile to shooters. Try to gain access to one of their ranges. They don't want you, and they don't consider shooting skills as important. Especially civilian shooting skills.
     
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