MOA Used Incorrectly

Discussion in 'General Gun Discussions' started by giggitygiggity, Jan 13, 2021.

  1. giggitygiggity

    giggitygiggity Member

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    I often see people say that their gun will shot “1 MOA at 100 yards.” Technically, the “at 100 yards” portion is incorrect. I will explain.

    I suspect that most people who state that a gun will shoot “1 MOA at 100 yards” really mean “1 inch at 100 yards,” but want to sound sophisticated or as if they are gun savvy. “MOA” is not a synonym for “inch.” Minute of Angle (MOA) is a measure of the spread of a gun’s projectiles; it is an angular description. 1 MOA equates to approximately a 1 inch group at 100 yards. At 200 yards, 1 MOA equals about 2 inches; if you said, my gun shoots “2 MOA at 200 yards” and your gun consistently produced 2 inch groups at 200 yards, you would be incorrect because 2 MOA would equate to a little over 4 inches at 200 yards.

    Think of a triangle with a 1 inch hypotenuse and two 100 yard legs that meet at the gun’s barrel. Those legs can then be extended to 200 yards to produce a 2 inch hypotenuse and so on and so forth.

    The point is that if a gun shoots a certain MOA (ie 1, 2, 3, etc), it does so at all ranges. The actual measurement, in inches, of those shot groups will vary depending on your distance.

    I hope this helps those who might not fully understand MOA.
     
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  2. Walkalong

    Walkalong Moderator Staff Member

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    It’s been discussed here many times. Never hurts to go over it again
     
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  3. jdh

    jdh Member

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    1.047” if we are picking nits.

    Here’s one for you. What is one MOA at 100 meters?
     
    Last edited: Jan 13, 2021
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  4. Stevel

    Stevel Member

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    Yes.
     
  5. Stevel

    Stevel Member

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    If 100m = 109 yards.
    100 (yards) ÷ 109= .917
    1.047 (1 moa) ÷ .917 = 1.141 moa at 100 meters.
     
    Last edited: Jan 13, 2021
  6. JCooperfan1911

    JCooperfan1911 Member

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    One MOA is 1.047” at 100 yards. So if there rifle groups at an inch at that distance, it is more than correct to say they own a “1 MOA” rifle.

    However, I very often see people posting groups at 50, 75, or even 25 yards of 1 inch groups with whatever gun and they say the gun is shooting 1 MOA this is absolutely NOT true and demonstrates the persons lack of understanding of the angular concept of minutes of angle.
     
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  7. Milt1

    Milt1 Member

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    Helps me, thanks!
     
  8. Bwana John

    Bwana John Member

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    Real men use radians.
     
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  9. WestKentucky

    WestKentucky Member

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    Not necessarily. There are some additional influences that may kick the MOA measurements around a bit.
    1- guns like certain ammo. They shoot it well and it is a notable improvement over something else.
    2- vertical stringing is truth. You may still have similar windage, but as a barrel heats up the shot lands in a slightly different point almost universally in a near vertical line.
    3- tumbling changes things, so does the sound barrier. At range bullets lose stability and tumble. Often around the point at which they become trans-sonic. I can’t claim to understand the microphysics involved but the gyroscope effect of the spinning bullet breaks down and groups can easily open up very very quickly beyond that range.

    Perfect example is the old 30-30 carbine. They shoot ok but not great generally so let’s call it a 2 MOA gun. Loaded with premium ammunition or hand loads you can cut group size down to 1.5 MOA shooting from cold bore. Multiple shots in fairly rapid succession will heat the barrel up somewhat quickly and you begin to see POI change. That means your growing in error and as such The error measured in MOA increases. Take that 30-30 shooting 150gr JSP bullets and use it at 400 yards. You can hear the bullet start to tumble and “sing”. An unstable bullet taking input from air resistance goes from 2 MOA to 4MOA quickly and it’s generally in the trans-sonic area where bullet speed is crossing over into subsonic ranges but air flowing around the bullet is still supersonic.

    So in practice, you are generally correct, but with those caveats it’s hard to say that a rifle is a 1MOA rifle PERIOD, because it may be a 10MOA rifle at its extreme range.
     
  10. bassjam

    bassjam Member

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    Not entirely true. I have a .22 that'll shoot sub moa groups at 50 yards with high velocity rounds. By 100 yards those groups widen to 1.5 moa.

    I've also seen discussions on this forum about bullets stabilizing at longer distances, thus the MOA might decrease from say 100 yards to 300 yards.
     
  11. ericuda

    ericuda Member

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    100 yds = 300'

    Tan(1/60) x 300 x 12 ("/') = 1.047
     
  12. mcb

    mcb Member

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    Agreed. Someone that says they have a gun that shoots a 5-shot 1-MOA group at 100yds is nice but not nearly as impressive as someone shooting a 5-shot 1-MOA group at 300 yards. Bullet dispersion is not linear with range.

    And ultimately saying a gun will shoot a 5-shot 1-inch group at 100 yards is exactly the same as saying a gun with shoot a 5-shot .95493-MOA group at 100 yards or a 5-shot 2.7778 mrad group at 91.44m. A group size to be meaningful has to include three things IMHO, number of shots in the group, the size of the group, and the range the group was shot at. Any of the above notations does that just fine for me.

    integers, mrads, and meters being my favorite. :D
     
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  13. taliv

    taliv Moderator Staff Member

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    to flip it around, imagine you're talking to someone in the LGS and he says he can shoot 1 MOA at 1000 yards, but he's only ever shot 1 MOA at 100 yards and is just extrapolating. You'd probably come up with a dozen reasons off the top of your head that he's unlikely to be correct.
     
  14. TimRB

    TimRB Member

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    The hypotenuse is the longest leg of a right triangle. There cannot be a right triangle that has a 1 inch hypotenuse and two 100 yard sides.

    Tim
     
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  15. giggitygiggity

    giggitygiggity Member

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    All the additional caveats are valid.

    The practicality of MOA becomes invalidated when we work in extremes such as when the distance is beyond a given firearm’s/caliber’s effective range. If we were to fire a .308 at a target 10,000 yards away, we would probably have an infinite MOA rifle because the shots would not hit the target. Alternatively, holding a target at the muzzle, firing a few shots, and then saying that the gun is “0 MOA” because all shots land in the same hole (would probably blow the target apart too) is not valid.

    Therefore, the generally accepted standard by which one can state their gun’s MOA is its performance at 100 yards. Using that standard, shooters have an idea of what groups to expect at various effective ranges for their rifle/cartridge.
     
  16. giggitygiggity

    giggitygiggity Member

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    You are correct. Good point.
     
  17. jbmillard

    jbmillard Member

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    I don't know why you assume that everyone is incorrect and I disagree with your assertion. Someone saying that there gun shot 1 MOA at 100 yards means they shot a group of 1 MOA at 100 yards. It definitely has a meaning and is not incorrect. What would be incorrect would be shooting a 1 MOA group at 100 yards and then claiming your gun shoots 1 MOA at 1000 yards. Probably not true.

    No true. Bullet dispersion typically increases as an angular measure as range increases. This is mostly due to small changes in BC between bullets, effects of wind down range, small changes in muzzle velocity, etc. These changes are non-linear - you typically don't end up shooting 1 MOA 1000 yards when shooting 1 MOA at 100 yards.
     
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  18. giggitygiggity

    giggitygiggity Member

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    True. I like to think of MOA as a measure of the rifle’s capability and to use “X inch group at Y yards” to describe the raw combined performance of the shooter and rifle. Similarly, my car’s speedometer may go to 200mph, but I may not be able to drive it 200mph (speed limits and terrain aside) due to me not being a confident/competent driver.
     
  19. jbmillard

    jbmillard Member

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    Really? Angles become invalid at extreme ranges? MOA is an angle. Period. That's all it is. It's not invalid or valid depending on the way it's used. Just because some people use it incorrectly or shoot at such long ranges that dispersion overcomes any hope for accuracy does not invalidate the concept or use of MOA. It's still an angle.
     
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  20. South Prairie Jim

    South Prairie Jim Member

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    Stop talking that crazy talk real Americans use Imperial system..;)
     
    Last edited: Jan 13, 2021
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  21. JCooperfan1911

    JCooperfan1911 Member

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    It’s called when the bullet “goes to sleep” and the trajectory stabilizes out.
     
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  22. jbmillard

    jbmillard Member

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    Would you please describe the physics of "goes to sleep"?
     
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  23. Jim Watson

    Jim Watson Member

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    So we really ought to be saying IPH (Inches Per Hundred yards).

    The metric system has been legal for trade in the United States since 1866.

    But the radian is not necessarily a metric unit any more than the degree is an Imperial unit. The milliradian (mil) subtends 1/1000 of the range. So one yard at 1000 yards, .1 yard, 3.6 inches at 100 yards. It just works out neater in metric, 10 cm at 100 meters.


    A common legend. There was a benchrest shooter who set up a target at 335 yards and an Oehler acoustic target at 100 yards. He said he saw no case in which a group was larger in angle at 100 than 335. One ballistician said he would pay your travel if you could demonstrate the "sleepy bullet" on his range.

    A while back, there were graphs posted showing corkscrew helices of bullet flight damping out as they went down range. But the units were obscure and did not make it obvious that, as the NRA once reported, the maximum "air spiral" of a .30 bullet was about 0.1". And that just the nose, it was still around an axis through the center of gravity.

    But there is the phenomenon of "compensation." As Bart B. on TFL describes it, you get the lower velocity bullets to exit the muzzle at the top of its vibration pattern and the higher velocity bullets to exit at the bottom of the vibration so their trajectories converge way down range.
    The British observed compensation when .303 Enfields were shooting more accurately at 600 yards than expected.
     
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  24. jbmillard

    jbmillard Member

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    Yes, I agree. It's a legend. I get emails about it to this day ("do you programs take into account the bullets going to sleep?"). I just don't know why people keep perpetuating it.
     
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  25. Arkansas Paul

    Arkansas Paul Member

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    Y'all think too damn much.
     
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