# Accuracy is not linear

Discussion in 'General Rifle Discussion' started by Yunus, Apr 9, 2010.

1. ### YunusActive Member

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Example, if a rifle shoots 1 MOA @ 100 yards, it does not mean it will shoot 10 MOA @1000 yards.

I know this(or think i know it). But I can't explain fully why. The biggest variable I am assuming is wind, but what other variables come into play? Exclude the shooter from this discussion, i guess that actually is the biggest variable.

Wind
bullet
powder amount
powder burn consistency
scope
velocity

What am I missing from the list?

2. ### ThorsHammerNew Member

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How about "distance" as well, but that was kind of implicitly included in your post.

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4. ### mrm14Active Member

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Although not as large a factor as wind deflection, gyroscopic drift, or spin drift as its commonly referred to as is an issue for long range shooting, and something worth considering.

5. ### DillingerNew Member

• Wind on multiple axis ( Head, tail, cross, angled )
• Effects gravity will play on the round
• Reduced velocity as it goes further, compounding gravity's effect
• Super sonic vs. sub sonic speed of the round
• By-product is FPS as round leaves barrel
• Overspin / balance of the rounds center - leading to:
• Spin Drift
• Mirage
• Humidity from zero location to location of shot taken from
• Elevation from zero location to location of shot taken from
You also have to factor in Ballistic Coefficient of the round and realize that some rounds will REACH 1000 yards, but will not be truly spinning on their true centralized axis.

This condition is why some shooters in benchrest mic and test their reload bullets in a sectional density solution to check for consistency.

You have realize that when you are shooting 1,000 Yards, it will truly be one of the hardest things you will ever do. It really is THAT hard even with good equipment....

6. ### cpttango30New Member

1moa at 100 yards is aprox 1" 1 moa at 500 yards is 5" 1 moa at 1000 yards is 10" or there abouts.

If a rifle shoots 1moa at 100 yards that does not mean it will shoot 1moa at 500 yards.

There are umpteen hundreds of things that affect bullets accuracy.
1. Distance
2. spin
3. wind
4. friction coefficient of barrel
5. friction coefficient of bullet
6. air density
7. humidity
8. elevation
9. gravity
10. Rotation of the earth
11. consistency of barrel (Variations in diameter).
12. consistency of powder burn rate
13. consistency of primer burn
14. consistency of brass
15. consistency of seating depth of bullet
16. consistency of neck tension on bullet
17. consistency of bullet diameter
18. consistency of bullet jacket thickness
19. barrel lock up
20. lug engagement.
and others that I am not thinking about.

There is a lot of factors that go into accuracy of a rifle. Also a rifle that shoots 2moa at 100 yards will never ever shoot below that at longer distance. I don't care what anyone says it just can't happen.

7. ### YunusActive Member

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I'm no where near being able to shoot at 1000 yards, I would like to try to work up to 500 someday but not anytime soon, biggest reason, is lack of a range that goes that far.

Thanks for the responses they were what I was looking for. This is more of a mental exercise than a real world thing for us. I was just trying to figure out what goes into the shot, assuming the shooter is out of the equation, and you guys delivered .

8. ### JpyleNew Member

Gravity
Elevation - air density, etc

As Tango states the list is long...

9. ### kansas45Member

That "Rotation of the Earth" part is interesting. It does come into play at 1000 yds. I'm suprised that a bullet traveling that fast would be affected by this, but it is.

10. ### mrm14Active Member

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It's called The Coriolis Effect

The Coriolis Effect occurs as a consequence of the fact that you're shooting from one point to another on a rotating planet (the Earth) The Coriolis Effect is very minor, and can be disregarded for most practical shooting applications.

Heres some basic facts about the Coriolis Effect

1. The Coriolis Effect has both vertical and horizontal componets which are independent.
2. The horizontal component of the Coriolis Effect is entirely determined by your latitude on Earth, and is unaffected by the azimuth of fire.
3. For latitudes north of the equator, the horizontal component of the Coriolis Effect is always to the right. For latitudes south of the equator, the deflection is to the left.
4. The horizontal Coriolos deflection is minimal at the equator, and grows greater as you move toward the poles.
5. The vertical component of Coriolis deflection depends on your latitude and azimuth of fire.
6. Fireing east will cause you to hit high, and fireing west will cause you to hit low.
7. When shooting north or south, the vertical component of /coriolis deflection is zero regardless of latitude.

Reference: Applied Ballistics For Long Range Shooting
By Bryan Litz

11. ### TunerNew Member

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Reading the posts on the subject I could not help but wander if you were speaking regarding a controlled vs uncontrolled environment, and I am not sure I have stated it properly. For example are you talking about firing a rifle at 100 yards and getting a one inch group and then firing it at 1000 yards eliminating all possible external forces (except gravity) and getting a 10 inch group? That would be a measure of equipment accuracy. Allowing external forces to act upon the group size, ie wind, adds "Operator Ability" and is what wins 1000 gard matches.

Anyway, one item that can impact accuracy that I have not seen mentioned is that a bullet will "precess" kind of a wabble when it exits the barrel which will affect accuracy. This tends to stabilize early in flight and is one reason why groups at 200 yards are sometimes darn near as small as groups at 100 yards. So in this case because of a "glitch" with bullet stability accuracy would not appear to be linear.

Passing through the soundbarrier. going from supersonci to subsonci will cause a "shudder" to the bullet and can impact accuracy after that point. A lot has been written about high velocity pellet rifles that break the sound barrier and how when they go subsonic it can really screw up the accuracy, I am sure this will impact to a lesser degree a more streamlined bullet.

A lot has also been written about Standard Deviation in velocity of loads. At 100 yards variations in velocity doesn't mean much and for all intent probably not measurable. Now, stretch that out to 1000 yards where your trajectory is about 37 feet (I believe that is what I read just recently) then you will probably get very measurable verticle stringing (not evident in the 100 yard groups) with a load that has a greater variation in high to low velocities.

Interesting question.

13. ### jpattersonnhActive Member

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I have one rifle that is not the norm. A Romak3 in 7.62x54r. It shoots 1.5..2" groups at 100, but will hold 4" at 400. It really does not open up past that till you pass 600. The 147..152gr bullets have a low BC. I can't explain it! The barrel is very thin, so some noodling may be occurring and the bullet takes some distance to settle down.

14. ### mrm14Active Member

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Epicyclic Swerve

http://www.appliedballisticsllc.com/index_files/EpicyclicSwerve.htm

Last edited: Apr 10, 2010
15. ### jpattersonnhActive Member

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It happens w/ mil-surp rifles more often than any other. I have read that article in years past, but it is an anomaly like no other.

16. ### mrm14Active Member

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I believe in the older larger caliber military rifles, with their "thin" barrels, that barrel whip is part, but not all, of the contributing factor for the pitch and yaw of the projectile and this Epicyclic Swerve effect.

Some of the guys that have shot the 300 Win. Mag. caliber in my club, with stock thin hunting rifle barrels, have experienced, what might be described, as the same effect of Epicyclic Swerve. Of course this is my opinion. With a heavy, longer barrel, it seems to mostly but not totally go away. So that enters the question of: How much barrel whip is causing this effect? Especially when the barrel gets hot. I just dont know for sure at this time.