Mag Length Dasher Loads

Discussion in '6BR, 6BR Improved & Wildcats' started by REELDOC, Dec 24, 2017.



    Sep 18, 2006
    I come to the well again............ Would like some info on AR mag length dasher loads. I think I need to drop to a 95-100 grain bullet but if you guys have any info I'd rather not reinvent the wheel.
  2. tonysnoo

    tonysnoo Silver $$ Contributor

    Mar 30, 2012
    I was going to build a brx with ar mag bottom metal(ptg). It was going to be doable with my Berger 108s, but if memory serves I was going to have to jump them about .020. with a .104 free bore. That would be using my fatrat grendel mags that I have stretched to accommodate a 2.3 oal.

    Some of the vlds would need to jump over .050-.080.

    The 87 class bullets were looking really good in un mod mags, but I ended up going P-mag "bottom plastic" with easily modifiable plastic mags and fillers. It runs any bullet I want to try, even though I came full circle back to my beloved Berger 108s
  3. reubenski


    Aug 17, 2008
    This almost more of a freebore question. At least it starts with FB. For most bolt guns, folks who spec their own reamers in the FB realm do so to seat a particular bullet's body/ boat tail junction in a certain place in relation to the case's neck/ shoulder junction bc they almost always have enough mag length. In the case of AR's like you're asking it is a delicate act of balancing trying not to shove the bullet too far down in the case which will eat up capacity, and still allow the tip of the bullet to clear the mag and frankly with about .030 - .050" to spare so the shooter can chase the lands a little and not run up against the mag COAL limit. The 6mm Fat Rat and 6mm AR's are a great example in this exercise. The 6 Fat Rat has a reamer for single-load to maximize case capacity and a "varmint" reamer to fit mag length and not have to jump .120" With this cartridge you cannot have both max case capacity and feed from a mag, regardless of the bullet. Mine has the longer FB and here is what I measured last night trying to do the same thing you are.

    I cut the front out of one of my mags to allow a max COAL of 2.365". My goal is to get a COAL that corresponds to at least a .020" jump to the throat that fits inside of 2.365". The CBTO measurements below are what the loaded cartridge and bullet's measure at the throat of my rifle so the corresponding COAL's below don't take into account the extra .020" needed for jump. So realistically the number I'm trying to beat is 2.345", not 2.365".

    105 HYB - Bullet Length: 1.285", Bullet BTO: .558", Case BTO: 1.658", COAL: 2.385"

    103 ELD-X - Bullet Length: 1.260", Bullet BTO: .575", Case BTO: 1.659", COAL: 2.344"

    108 ELD-M - Bullet Length: 1.274", Bullet BTO: .593", Case BTO: 1.680", COAL: 2.361"

    95gr TMK - Bullet Length: 1.181", Bullet BTO: .478", Case BTO: 1.695", COAL: 2.398"

    I give bullet length and bullet's base to ogive measurements so you can subtract and get the length of the bullet's nose. The length of the nose is the difference between meeting the lands and everything that is left over that must get inside the mag length. I don't bother identifying or writing down the length of the nose bc it will shake out in the COAL measurement if the bullet will fit, but long nosed bullets like the new SMK's are very problematic for this purpose. Choose more traditional shaped bullets like the older SMK's or Nosler CC's to minimize the nose and get the bearing surface to the lands with the shortest nose possible to "limbo" under the mag COAL limit. Going to a lighter bullet really just helps with case capacity but doesn't necessarily address excessive bullet jumping to the lands.
  4. 4xforfun

    4xforfun Gold $$ Contributor

    Apr 25, 2010
    Dumb question..since I am not a big gas gun guy.....wouldn't a dasher need to be built on an AR10 type platform? If so, why the concern with mag length?

    If it can be built on the ar15 type platform...well....never mind....I see the concern.

  5. IdahoSharpshooter

    IdahoSharpshooter Gold $$ Contributor

    Dec 7, 2007
    A Dasher in an AR-10 would be a really neat project. Magpul SR-25 would allow a 2.800" oal.
    Mag conversion might be a lot easier using a metal magazine, though.

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