Question about come ups

[MIGHTY MOUSE]

This may be an idiotic question and I apologize for my ignorance, but how is the number of clicks of elevation calculated for each increase in distance of 100 yds?  I have memorized them for my use in the field but I always had a problem with just memorizing without understanding.  I am a conceptually minded  person and if I can see the whole picture and how it fits together then I will understand it for life and be able to explain to someone else much easier.  How is this related to IMC? I can see the pattern in the elavation increase except at 700 to 800yards and beyond. Is that repetition of clicks just a coincidence? I assume this is a trigonometry question but I was hoping that a more gifted marksmen could help me wrap my mind around this concept.  Thanks in advance.

                                 

[AR-180]

I don't know of a formula, but it deals with drop, BC, muzzle velocity, bullet weight, and sight height above the bore.  That is why the elevation drums are different for the AR and the M1A.

[kool aid]

I had a chance to ask the head honcho of the US Army Rifle team  about AR come ups. According to my notes:

The BDC on the sights of an AR are set up for the SS109 round. They are set for the full length,1-7, set up. The same parts are used on the m4. So even they have to cook up their own dope.

The short answer is..... You have to develope your own dope for your own particular rifle/ammo combo. Sorry. It is a real pain but the worst day at the range beats the best day fishing, right? 

[wcmartin1]

The short answer is..... You have to develope your own dope for your own particular rifle/ammo combo. Sorry. It is a real pain but the worst day at the range beats the best day fishing, right? 

I agree.  That's the fun part - learning how your rifle responds to different loads at various ranges.

[Bravo]

Please note though that these are approximations only.

Let's say you sit down and develop your dope with LC M80, from 100 to 1000 yards. At that point you can not say "this British FMJ ammo will follow that dope". It *MAY* happen that way, but probably won't. The dials are built on theoretic velocities, and as anyone with three different rifles in the same caliber can tell you, the velocities you get with the same ammo out of all three WILL NOT be the same.

Environmental factors apply as well - sometimes multiple MOA clicks.

There is only one -ONE- way to know what your come-ups should be, and that is to shoot and log. This is either fortunate or unfortunate, depending on your outlook.

One time I was offered the opportunity to verify zero and dope before a big shoot. I replied that I knew my rifle, ammo, and myself so I'd be fine. The incoming comment was that if I hadn't verified zero and dope THERE, then I didn't have dope at all. He was right, and I got much less cocky.

In the same vein, I once mentioned that I was using H4895, as it was about as temperature indifferent as Varget, but a touch faster. The incoming comment I got was "NOTHING is as temperature indifferent as Varget" - and again I learned the hard way  

[Son of Martha]

Go here and play around with different input data:

http://www.handloads.com/calc/index.html

The comeups obviously have to increase with range, and the "3,3,4,4,5,5,6,8,8" list is an approximation which is close enough for military rounds.  Even the military rounds and sights are not EXACTLY on, but are so close that a rifleman using irons can't pick the differences out of the noise.

How are they calculated?

"For the accurate computation of a trajectory, the forces on the projectile as a function of yaw angle must be known, and the projectile treated as a rotating rigid body. Then, with the drag known, the trajectory can be found by numerical integration, beginning with the initial velocity and spin. Not only is the problem a difficult and involved one, but the calculations are exceedingly tedious. In fact, they were so tedious that they stimulated the creation of digital computers in the 1940's, leading to the general-purpose computer, which has now revolutionized technology and life."

http://mysite.du.edu/~jcalvert/phys/ballist.htm

Note that the above is abridged...there is no mention of how the drag is found, nor is there mention of the fact that a projo fired at 3000 fps that slows to 2000 fps will behave differently at 2000 fps than the same projo fired at 2000 fps to start with...

(Sometimes I start to suspect that, yes I might be a geek...)

Good stuff here:
http://www.frfrogspad.com/extbal.htm

http://en.wikipedia.org/wiki/Trajectory

SoM

[wcmartin1]

(Sometimes I start to suspect that, yes I might be a geek...)

LOL.  Yes, but knowledge, experience, and truth are usually the best ammunition.  Thanks.

[MIGHTY MOUSE]

Thanks for the info. Let me chew on that for alittle while I am sure I will have more questions.

[funfaler]

The short, easy answer for me is that a "click" = 1 MOA (M1a sight).  So we only need to know how many inches of bullet drop for each of the ranges.

Bullet Drop with Rifle Zeroed at 300yds
                    Point of Aim (POA) is the same as Point of Impact (POI) at 300yds
      308 (147gr)               5.56 (62gr)
100yds      +5"   +5 moa                    +3.5"   +3.5 moa
200yds      +6"   +3 moa               +5"   +2.5 moa
300yds      0    0 moa                0       0 moa
400yds      -14"     -3.5 moa            -12"   -3 moa
500yds      -37"   -7.5 moa            -34"    -7 moa
600yds      -71"   -12 moa               -70"   -12 moa
700yds           -120"   -17 moa                     -123"   -17.5 moa

These are approximate numbers, but you can see that the bullet drops @14 inches from 300-400 yards, so 14 inches at 400 yards = 3.5 MOA, we chose 4 clicks

[TreadCarefully]

A simple answer might be this: Rear sight elevation usually moves linearly i.e. each 'click' in the rear sight moves the aperture up the same, fixed amount. Gravity, on the other hand, accelerates a bullet towards the earth proportionally to the square of the time it is in the air i.e. non-linearly.

So, the farther out you are shooting, the more non-linear your changes have to be with the rear sight because the bullet will be in the air longer, and gravity will have a longer time to affect its flight.

Obviously, this doesn't address how to calculate rear sight settings, but, hopefully does address the non-linearity conceptually.


Hope this helps.

T

[Old Dog]

here's something else.  hope it helps