Neck size for AR

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  • Jaybeez

    Ultimate Member
    Industry Partner
    Patriot Picket
    May 30, 2006
    6,392
    Darlington MD
    Because the brass is still expanding when it's being extracted.

    IN a bolt action the brass remains in the chamber long enough to transfer heat to the chamber walls and then contract slightly.
     

    Sticky

    Beware of Dog
    MDS Supporter
    Mar 16, 2013
    4,501
    AA Co
    An autoloader is not a bolt action rifle and needs a little more sizing to ensure reliable extraction, as Jaybeez said, you are often trying to extract a semi auto case before it has had a chance to fully 'relax' after firing. You will likely have extraction/sticking issues if you neck size only. Set the shoulder back a few thou and you eliminate this potential problem and still really don't work your brass much.
     

    Pinecone

    Ultimate Member
    MDS Supporter
    Feb 4, 2013
    28,175
    No.

    The reason is, the bolt action has more camming force to CHAMBER the round. So it does not have to slip into the chamber.

    Semi autos do not have the force to chamber a round that is not small enough to pretty much drop in.
     

    outrider58

    Eats Bacon Raw
    MDS Supporter
    Jul 29, 2014
    49,815
    I'm going to stick my 'neck' out and say this; If you are only shooting this ammo from one gun, after neck sizing, the brass passes the 'thunk' test in your case gauge, I say give it a try. If you run into problems feeding or ejecting, then I would abandon the idea. Remember, in a gas operated semi, ejection is solely reliant on that gas. Bolt guns work on arm power.
     

    Magnumite

    Ultimate Member
    Dec 17, 2007
    6,561
    Harford County, Maryland
    You can adjust the sizing die so the neck is sized and the case shoulder is just ‘bumped’. The case should just drop into the chamber and full battery obtained. That adjustment would be for that rifle/pistol only. Feasible for shooting needing finite accuracy, but for other purposes just full length resize. I crank out sub moa on my Dillon without much effort.
     

    Biggfoot44

    Ultimate Member
    Aug 2, 2009
    32,881
    I'm going more with Pinecone than Sticky on this one for the reasoning, but the bottom line is the same .
     

    Major03

    Ultimate Member
    Could it not be both reasons?

    OP...what's the concern about just full length sizing? Extending case life or trying to maximize accuracy?

    I've found quality dies produce ammo just as concentric and accurate as neck sizing only dies. Forster and Redding Competition dies are fantastic.

    If you're looking to work the brass less, you could always use a body die to bump the shoulder back and then neck size. Or anneal your brass to reduce the work hardening.
     

    Pinecone

    Ultimate Member
    MDS Supporter
    Feb 4, 2013
    28,175
    For a single rifle. Set your die to bump the shoulder back 3 - 5 thousandths. That is enough to easily chamber.

    If you have more than one, you can set to size 3 - 5 thousandths shorter than the small chamber rifle.

    Actually, semi auto brass have so many stresses, that the difference in case life is not huge.
     

    Uncle Duke

    Ultimate Member
    MDS Supporter
    Feb 2, 2013
    11,666
    Not Far Enough from the City
    Could it not be both reasons?

    OP...what's the concern about just full length sizing? Extending case life or trying to maximize accuracy?

    I've found quality dies produce ammo just as concentric and accurate as neck sizing only dies. Forster and Redding Competition dies are fantastic.

    If you're looking to work the brass less, you could always use a body die to bump the shoulder back and then neck size. Or anneal your brass to reduce the work hardening.

    I also think it is both reasons.

    Me, I'd be paying to lot more attention to having .223 brass that works, than I would to working .223 brass. But that's I suppose another discussion.
     

    John from MD

    American Patriot
    MDS Supporter
    May 12, 2005
    22,733
    Socialist State of Maryland
    Neck sizing is mostly used as a additional accuracy tool in bolt guns or to keep cases from splitting in guns that have oversized chambers. I see no advantage at all for neck sizing in an AR. My biggest issue with the 300BO in an AR is primer pockets opening after only a few reloads but that is because I am shooting 150 gn at supersonic speeds and using any make of brass.
     

    Pinecone

    Ultimate Member
    MDS Supporter
    Feb 4, 2013
    28,175
    It is, but we cannot push the brass back into the bottom of the case.

    And this why, oversize chambers are an issue. Every time you fire, the case gets longer, mainly near the case head. Then we resized the case, moving the shoulder back.

    Two things happen. Over time, the neck become thicker. And more problematic is that the brass near the case head ends up getting thin and caused a case head separation.

    If you shoulder bump, yes, you are sizing the whole case. But not to minimum size. The cases have some taper, so if we do not run the case fully into the die, we are not reducing the diameter completely.

    Neck sizing only assumes that your chamber is perfect and the bore is completely centered. If the chamber has a curve, or some oval shape, neck sizing only can increase issues.
     

    atblis

    Ultimate Member
    May 23, 2010
    2,011
    One could also get a case gauge that indicates SAAMI max and min for ammo and bump everything back to inside that range. Often getting to SAAMI max is a 0.001" to 0.003" bump (obviously varies gun to gun). Something like a Dillon case gauge.

    One advantage to neck sizing only is that when using a Lee collet neck die (great die btw), is that you do not need to use any lube. It's a great way to churn out very accurate ammo quickly with minimal fuss. Generally not appropriate for an AR15 though.
     

    Doco Overboard

    Ultimate Member
    I understand a few things about semi auto rifles regarding cartridge fit and chamber dimensions that a chamber with a slightly generous fit to cartridges a good thing.
    The first one is primer firing pin clearance. When head-space is on the tight side like a minimum head-space chamber and a + head-space cartridge are combined make sure primers are below flush. Sometimes cases that have been crimped have a concave head. This can happen during the manufacturing process when the primers are crimped. It also can be seen with varying primer heights when reloading. Bolt speeds alone are enough to detonate an overly sensitive primer before lugs are fully engaged and things like this can make trouble by contributing to a slam fire, OOB or even if hammer follow through happens.
    Another thing is the additional radial clearance can help minimize heat transfer. In rapid fire weapons minimizing cook off whether it be from a primer right up against the bolt face tightly or heat transfer to the cartridge through the chamber walls.
    Chamber necks that have become to thick too long or bullets that are seated to the throat can exacerbate problems with dirty, mis-reamed chambers and parts wear aggregate. Or a long firing pin for that matter.
    The act of a rapidly closing bolt usually makes the the remaining case size needed to make it fit well in an auto loading rifle and that's what can cause problems if the case is a close fit with the chamber. Full length resizing can promote better performance by making sure head-space is not to long by a shell holder that's thin or a press that's not completely cammed over center.
    Auto loading rifles are more susceptible to problems associated with cartridge fit just due to the nature of the design their purpose and lack the power to extract or seat a poorly fit cartridge just like pumps and lever actions so full length resizing could be considered a best practice in my view.

    Some auto loading rifles I own I intentionally finished reamed over Saami gauge length and allows for below a no-go gauge tolerance. Just because I cant accurately account for dimensional clearances with worn parts. Receivers bolts etc.
     

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