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M16A1/A2/M4 barrel diameters.

35 Whelen
What are the major differences in barrel diameter between the original M16A1 "pencil" barrel, the A2, and the M4, where the front sight base pins on?

I ask, because I picked up a front sight base for my A1-style AR build, 20" barrel, and as luck would have it, it fits a larger-diameter, apparently newer-style barrel.

Thanks.[:)]
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4 comments

Date Votes
  • sandwarrior
    35WhelenClassic,

    Standard for today is .750. But, they come in all sizes now. I carried, but never worked with, the older -A1 style. I am seeing blocks made for .625" barrels. So, that may be the one you are looking for. FWIW, I remember the barrel being lighter on the -A1 than the -A2's I see today, but not a pencil barrel like the very first M16.

    In any case you'll have to get a set of calipers and measure the journal area where the gas block/front sight is mounted and see what the diameter is. I've had to take parts back a couple times because I got the wrong size.
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  • Txs
    M16A1 was .625 at the front sight base, dropping down to .570 just forward of it.

    To illustrate just how great the barrel diameter difference is between the M16A1 and M4, consider that the M203 grenade launcher was designed to mount on the M16A1 barrel. Due to this, that 'notch' area where these mount on an M4 barrel is cut down to the M16A1's .625 diameter.

    BTW - Nice idea on the A1 build. The M16, M16A1 and civilian SP1 - especially in their carbine versions - balance SWEET compared to the later, heavier barreled models. [^]
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  • 35 Whelen
    Thanks for the help, Sandwarrior and Txs. That's the exact info I was looking for.[:)][8D] I'll measure both journals to confirm what I have.

    This build has been a lot of fun, but I was quite unaware of the difference in barrel diameters. I'll definitely look more carefully next time, that's for sure![:D]
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  • MrM1A1
    As stated, A1=.625" and A2/M4=.750".

    Whelen, if you have ANY more questions or need guidance in any way, don't hesitate to shoot me an email. I've been heavy into researching and building retro AR's for the past year. Would be happy to help.
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