300 Blackout C.O.L / load data question

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  • John from MD

    American Patriot
    MDS Supporter
    May 12, 2005
    22,726
    Socialist State of Maryland
    I managed to get ONE Magpul .300 BO mag. I would like more, but guess that one 30 round mag of suppressed subsonic is good for HD. :)

    Are the regular Lancers good for .300? I picked up a few at Green Top.

    I have .223 Lancers and they work fine with my 150gn loads in 300 BO.
     

    cotman68

    Active Member
    Feb 21, 2010
    796
    Stewartstown, Pa
    I managed to get ONE Magpul .300 BO mag. I would like more, but guess that one 30 round mag of suppressed subsonic is good for HD. :)

    Are the regular Lancers good for .300? I picked up a few at Green Top.


    I use the .223 Lancers in the .300 blk out. Only ones I use so I don’t accidentally mix them up


    Sent from my iPhone using Tapatalk
     

    ironhead7544

    Active Member
    Oct 27, 2018
    188
    Call the bullet makers and ask them.

    You can determine the max length for your rifle with a given bullet this way:

    Find a fired cartridge case that will easily rechamber in your rifle. Pinch the case mouth slightly, I can generally do it with my fingers, so that the bullet will slip fit with a bit of pressure. Seat the bullet long and carefully, gently chamber the case and close the bolt. Carefully remove the case. Measure the OAL. this will give you the length that the bullet will touch the rifling. I would back off about .020 to start. With this measurement, make up a few dummies and see if they will feed. The 300 Blackout can be fussy about COL. And the magazine max length must be considered.

    With the Lancer mags, the factory says use the 5.56 mags for the lighter supersonic loads and the specific 300 Blackout mags for the subsonics.

    Join loaddata.com.

    I use the Magpul 300 mags with both types.
     

    Park ranger

    Ultimate Member
    Dec 6, 2015
    2,315
    Easier way. Seat a bullet long with no primer. From muzzle slide cleaning rod in barrel. Push the dummy round into the chamber and try pushing it out with rod. If get any resistance, seat dealer and repeat until no resistance is felt.
     

    RetiredArmyGuy

    Active Member
    Jan 16, 2018
    171
    Pasadena, MD
    ^this. They're also good 458 SOCOM mags.

    I agree. They work well in both 300BO and 458SOCOM. I have just decided to reload all my 300 Blackout cartridges to run smoothly through any standard AR magazine. I have been successful so far in loading both subs and supers which run smoothly in standard magazines.

    I stencil 300, 458 and 6x45 on magazines I use for those calibers as well as have the caliber engraved on each dust cover.

    A few years ago I made a huge error and grabbed the wrong upper. I will hopefully never make that mistake again. A 6x45 upper will completely load a 300 blackout. Fortunately I didn’t get a complete KABOOM but it did make the upper unusable and blew the magazine apart. Interestingly enough the 125gr 300 Blackout bullet exited the 6x45 barrel just with extreme pressure.
     

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