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Thread: difference between the bcm 16" mid lenght upper and the 16" mid lenght light weight

  1. #31
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    Quote Originally Posted by TehLlama View Post
    Check the wiki on thermal mass and specific heat. Assuming the same material used (4150 CMV) then the narrower barrel will reach a higher temperature, but still cool down faster when immersed in any fluid (air).


    The profiles between LW and Gov are identical out to the journal that the gas block or FSB is mounted too. From there forward is the difference, where the LW gets machined to a smaller OD for the next 5.7" or so.
    Actually, a larger diameter barrel will exhibit an increased rate of thermobuoyant convective cooling. The Biot number of a AR-15 barrel is only 3.4e-5, so the temperature of the barrel will not vary internally. I computed the heat transfer rate of a 16" barrel with a 10 and 20mm overall diameter as an example at 200 degrees C with 25 degree ambient air. The 10mm barrel has an average heat transfer of half a watt while the 20mm OD barrel transfers at 7.4966W. That presumes they are the same temperature, and the thinner one would require less shots to get up to that temp. So, a thicker OD definitely leads to faster cooling, at least until the thing is so thick that the Biot number approaches 1, which would require something several feet in diameter. I'll attach my calcs.

  2. #32
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    Whether one profile barrel heats up faster or cools down faster than an other barrel profile, I'm just glad they come with hand guards!

  3. #33
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    They will heat/cool at the same rate since they are identical where it counts. The only difference is going to be harmonics and how the weight up front effects the hot barrel.

    Edit: We have seen from members like trident that a chf LW CL barrel can achieve near moa groups. How much more accuracy do you expect from the same barrel but in govt profile? Im gonna say the accuracy difference, if any, will be negligable. And you would need a large test population to determine the small difference (if one exists that is inherent to the profile). If you need sub moa groups stainless may be the way to go.

    That said, as stated before the difference is gonna be weight up front. For offhand shooting, I find a little more weight helps for stability, but I dont compete for offhand groups so I chose a LW profile on my gun. YMMV. Either way, I dont think its going to be a huge difference, so I would just try and figure out which fits your shooting style and carry on. Keep in mind I'm not a professional shooter, I just shoot a bit as a hobby right now.

    [QUOTE=usdmsw21;1423459]...[QUOTE]

    you forgot to account for the increased heat energy contained in the larger barrel, since you assumed they have the same temperature and taking only the different parts of the barrel into account, one has 65-70% of the volume of the other. You also forgot to account for radiant heat, which will be substatial - hence handguard covers.
    Last edited by MegademiC; 10-22-12 at 21:24.

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