Howdy all.

As a newbie to the AR world, I'm trying to read up on all the AR stuff that I can.  I purchased a Rock River AR15A2M4 version that I can't wait to shoot.  However.....

Sighting-in instructions on both this website and others state that mechanical zero is established using the elevation at the 300 meter mark with a target downrange at 25 meters.  

Uhhhh, forgive my stupidity, folks, but this doesn't compute in my feeble brain.  Shouldn't I be sighting in at a different target range (say, 300 meters)?  What if I never intend to shoot anything other than 100 yards (the length of the longest range around here)?

It's the same, only different:

Once I establish the mechanical zero, when I go to shoot different distances do I adjust the front sight post elevation to move point of impact up/down or do I turn the knurled knob to move point of impact up/down?

Be gentle with me; as I said, I'm a FNG here and I need y'all to make me smart.

Thanks

Mike Sigman
[email protected]

No Mike, When the horizontal thumbwheel is at the 3/800 mark the rear sight elevation is actually at Zero, look at the sight base and you will see that the sight is almost flush with the receiver. You use the Front sight to sight in at the 25 yds range (elevation) and the Vertical thumbwheel for windage you only use the the Horizontal thumbwheel after you have sighted in to Dial in your elevation beyond 250 to 300 yds. The .223 is such a flat shooting round that you really dont need any elevation adjustments between 25 and 300 yards.
Beyond 300 yards Dial in 1 click and you should be good out to 400 yards and so on.

Of course after you get a good group at 25 yards its up to you to fine tune your sights depending on ammo used etc.

With 55 grn. 193 at 25 yards and 1.5 inches high you should be dead on at 100 yds.

leave that Thumbwheel at the 3/8 mark and use the front sight to sight in at 25 yds. Then fine tune it later at longer ranges.

Mike, many here on ARFcom, myself included, have had great luck with the Santose method of zeroing...

[url]groups.msn.com/TheMarylandAR15ShootersSite/improvedbattlesightzero.msnw[/url]

Believe it or not, the bullet path actually arches after it leaves the barrel.If zeroing at 25 yds, the bullet will be point of aim at 250 yds as well.In between 25 and 250 yards, the bullet will be a bit higher than point of aim.It's also going to be a bit lower than point of aim at distances less than 25 yds.This is with Federal 55g.To get technical, at 250 yds POI will actually be -0.61".

Do bullets arc because of imperceptible muzzle rise that occurs before the bullet exits the barrel?

If not, physics dictates an immediate projectile drop (assuming a level barrel) as soon as the bullet exits the barrel.

If the barrel were truly level, yes, it would begin to drop... after a certain amount of time.  Let me explain.  Actually, when you are shooting, the barrel is aimed upwards, although ever so slightly, as to be nearly inperceptable to the human eye.  Afterall, what you are doing when you manipulate the sights is make the barrel move while you are retaining a level sight alignment and sight picture.

So, if you were to see a battleship shoot a load (hahaha) towards a target 23 miles away, the barrel of it's 16" gun would be pointed rather high.  Does the projectile begin to drop immediately?  Not yet; it's LOADED with kinetic energy, and it's not done expending it yet.  Bullets from our AR's are the same way.  When we shoot, they are expending quite a bit of energy as they leave the barrel, and are going to travel flat due to energy and aerodynamics before the energy bleeds off, and then it starts to drop.

I used to be a traffic accident investigator when I was a Marine MP, and we had to know physics.  This was actually brought up in class once because of flip/vault formulas for motorvehicles.  I'm not saying I remember it 100% correct, but it's something I've seen explained before, so I thought I'd pass it on.

Thanks for the info, folks - I really appreciate it.

Edge767 - who do you fly for?  Bet I fly for the same outfit with 23% less morale than last week!

Thanks again

Mike

Uhh NO!  If you zero at 25 yards your far zero will be near 400M.  A zero at 25M (27.3y) will result in a far zero of 375M.

The Army forgot this when they came up with that 8/3+1 method - as a result your round will be very high if you use that method.

Follow the Santose Improved Battlesight Zero listed above.  The trajectory is much more useable, and it [b]will[/b] calibrate your elevation dial.

Thanks for the link.

Alex

Actually, it begins to drop immediately at 32fps per second, the speed of any falling object. The bullet just can't drop much because of the speed/time involved. As already stated, the barrel is tilted upward to counteract the force of gravity.

Uhh NO!  If you zero at 25 yards your far zero will be near 400M.  A zero at 25M (27.3y) will result in a far zero of 375M.

The Army forgot this when they came up with that 8/3+1 method - as a result your round will be very high if you use that method.

Follow the Santose Improved Battlesight Zero listed above.  The trajectory is much more useable, and it [b]will[/b] calibrate your elevation dial.[/quote]

Well that is from the Sierra software my instructor had.According to the Siera software at 0 yds, the bullet path is -1.5" and went up to 300yds with a bullet path of -5.42".At 400yds, I would have a very,very low shot.I have an ARMS #40 so the improved battlesight zero won't work for me.We were trained for using the AR at distances less than 100yds and for now, this way works best for me.I personally don't see myself using it in a real-life scenario in anything farther than that.

Mike: It took me a while, but you figured out I fly by the "767" in my name.  Well, I do fly, but just the little birds: Cessna 150, 152, and 172.  My ex-wife didn't want me to fly (she was afraid I'd run off with a flight attendant!) so I didn't take up my grandparents' offer to pay for flight school up to 737 type-rating and went to work for Compaq instead.  (Yeah, great career move!)  This from the same guy that turned down WOFT in the Army for being an MP in the Marines!  (I don't regret joining the Marines, but I often wonder about how cool it would have been to fly for a living, even if it was shithooks!).  As it turned out, I became a software engineer, made huge cash, and then, like about 30,000 other Compaq/HP employees, I got the pink slip (with a good severance, though).  I'm now a college student finishing my undergrad and then going off to (of all places) law school.  Having been an MP for 12 years, and then working on the design of software for servers at Compaq/HP, I should be able to get into criminal or intellectual property law.  Time and economic conditions will tell.

I have a good friend that flies for the airline based in Houston: boy are THOSE boys happy!  [ROFL2]  NOT!  He's being made to fly ERJ-145's around the nation all day, and they call it commuter flights.  Even though he leaves IAB for DFW, then ORD, then JFK, then ATL, then MIA, then some weird places in Mexico... it's an interesting way to keep pay down.

Oh... the 767 comes from my birth month and year: 07-67.

Redbone,
The sights on the AR are about 2.6" above the bore, not 1.5" like most hunting rifles.  That'll affect your trajectory as it takes a lot more up-angle to get the bullet up to the zero that quickly.

I don't see why you can't use the IBZ with your ARMS#40.  I believe most people here do.  Well, I guess you'd only be on with the small aperture.  Large aperture would shoot lower, but within 100yds probably not enough to matter.  Might even actually be closer at 100.

I use the GG&G A2 BUIS and the improved zero procedure works for me.  If you go to the Maryland Shooters page referenced above, and read the zeroing instructions all the way to the bottom, then it should work for you.

-Zmeja

Let me see if I can clear this up.

First, all objects will be acted upon by gravity, and will fall towards the earth's core (down). This can be prevented by another object supplying an opposite force. Gravity is measured in terms of accelleration (32 feet per second per second). This means in the first second an object will fall 32 ft and 64 feet in the next second and so on.

Now, lets apply this to bullets. If a gun is perfectly level when the round is fired, gravity will immediately cause the bullet to start dropping at 32 ft/sec2. If the bullet travels for 1 second then it will drop 32 feet. Simple enough.

Next, lets consider the speed of the bullet, which does not alter the effect of gravity. Most 5.56 bullets travel at approx 3300 fps out of a 20" barrel. That means it takes about .1 seconds to reach 100 yds (300ft). Gravity only acts on the bullet for .1 seconds causing the bullet to drop about 1.5 feet if my calculator is right. This is based on the above.

To counteract this gravity pull, the bullet must have an upward force applied, the easiest being aiming high. A quick trigonometry calculation shows the angle upwards must be around 1 degree elevation. Because the gravity and up angle cancel themselves out at the half way point (50 yds), the bullet has only gone up a max of 9 inches high.

The numbers I used are rounded off and very, very general. All bullets are aerodynamically designed and as such generate lifting forces to reduce the affect of gravity even more.

The reason faster bullets are flatter shooting is because gravity has less time to affect them.

As point out above that is your problem.  You need to start the bullet path at [b]-2.6"[/b].

Do that then run the trajectory (M193 = .243 BC, 3250 FPS @ 15', standard conditions for sea level) (M855 = .308 BC, 3025 FPS @78', standard conditions for sea level)

I think you'll see the difference then.


Sure it does.  I use the 50 yard IBSZ with my 20" with the ARMS#40 and it works great.  I also use with my C7 uppered carbine - works great there as well.


Which is perfect for the Santose IBSZ - the bullet is never outside 2.5" from the line of sight within  your specified range - the same CANNOT be said for zeroing at 25Y (or meters)

As point out above that is your problem.  You need to start the bullet path at [b]-2.6"[/b].

Do that then run the trajectory (M193 = .243 BC, 3250 FPS @ 15', standard conditions for sea level) (M855 = .308 BC, 3025 FPS @78', standard conditions for sea level)

I think you'll see the difference then.


Sure it does.  I use the 50 yard IBSZ with my 20" with the ARMS#40 and it works great.  I also use with my C7 uppered carbine - works great there as well.


Which is perfect for the Santose IBSZ - the bullet is never outside 2.5" from the line of sight within  your specified range - the same CANNOT be said for zeroing at 25Y (or meters)

Forrest, thanks.I was confused thinking it was for A2 sights only.

duhhhh...if I shoot straight ahead, will I shoot myself in the back of the head??  Relatively & theoretically speaking? 'Specially if that's where I am sitting/ or is it s*itting??

Hey guys, thanks for some good responses ...who said we wuz unthinking, illiterate rednecks??  Not me!!
Mike

The physics part sound good, but I disagree.

For my arguement I will use a baseball.  Theoretically, the instant a baseball leaves the pitcher's hand, it will start to drop, and would theoretically hit the ground before it reaches the plat.  Wouldn't the pitcher need to throw the ball at an incredible arc to keep it going?  However, the forward force from the throw keeps the ball moving in a relatively straight path.  

Yes, a FALLING object falls at an acceleration rate of 32 fps per second, but a bullet isn't falling, it's moving forward.  after a while, of course, the force of gravity and friction from the air will slow the projectile and it will start to descend, but even then the rate of 32fps per second doesn't apply until the object is only falling, and not moving in any other direction.  

Also, an object only falls at the above stated acceleration rate untill it reaches a speed determined by its mass and volume.  It then maintains that speed until it is acted apon by some other force.  This is why a popsickle stick dropped off the sears tower wouldn't be traveling so fast that it would go straight through a human being.

Edge, for a moment there I thought you were a physics professor, then I remembered you are something much better than that...you are a former Marine.  [:D]

Semper Fi, the Marines have taught you well.

We're confusing falling with being acted upon by gravity. If you throw your baseball straight up, it's not falling, but it's being acted upon by gravity nonetheless...i.e. being slowed at 32fps squared.(air resistance disregarded.)
A bullet isn't "falling" immediately, because as stated above, it's launched at an upward angle. But it's still being acted upon by gravity. If shot perfectly level, it will fall at 32 fps squared, whether moving forward or not. The forward momentum is immaterial, and the only thing that limits the fall is speed in that it hits the target before it can fall much at all.