.223 ballistic trajectory/setting up a scope

[gerritm]

I am having a hard time setting up my scope on my DPMS .223 with a flat top and 20" bull barrel. I think I know basically what the trajectory of a .223 55gram bullet is. When you sight it in at 25 yards to start at the bull it is flat and low. Going out to 50 yards the scope is now raising and high. At 100 yds it is still rising. I set the 3x9 Acog Clone NC Star scope to shoot slightly low at 50 yds and then it will be slightly high at 100 yds. When you come back to 25 you have to raise the scope up to compensate for the flat trajectory. Does this sound right? I have checked this with a second scope I have and it seems to be the way. Am I missing something?

 

[codygjohnson]

With an "standard" loading of .223 w/a 55 grain FMJ, at 25 yards, your point of impact is typically 1.5" lower than at 50 yards (depending on zero distance).

 

[codygjohnson]

If you don’t wish to deal with dope or holdover, I would suggest zeroing the rifle at 50 yards. Your point of impact will be zero at 50 yards and 215 yards (if I remember correctly…) It will be 2” low at the muzzle and at 250 yards, and 2” high at 100 yards. These are all approximate figures and depend greatly on your particular ammunition, scope height, rifle and environmental conditions, but it’s a good starting point. Keep in mind that the things you read on the internet are all hearsay, the best info is found on the target at the range.

 

[navyguy]

I've set mine about 1/2" low at 25 yards, which puts a 55 gr about 1 1/2 high at 100 and should be about dead on at 200, although I've not shot it at that distance yet.

Mine is a carbine length and I don't see myself shooting much past 200 yards unless just for grins. Your 20" bull barrel will put a little more zip on that 55 grainer, so if you're wanting long range POI you probably want to be 1" low at 25, 2" high at 100 and dead on at 250. If you're shooting targets for pin point bulls eye groups, than you will need to compensate for the various POI as it goes. If you shooting critters like coyote, and such, you should be able to pretty much hold straight on out to 250. I worry more about cross winds when you start getting out past 100 yards with those light bullets.

 

[thopkins22]

Takes a while to load...but worth it.
http://www.combatshootingandtactics.com/published/thebattlefieldzerofullpictures.pdf

 

[gerritm]

I think I need some more range time with this scope and rifle. I like the idea of zero @ 50 and a couple of inches high @ 100. It seems like if I zero @ 50, I am about 8 inches high @ 100. I need to spend more time and write down what is exactly going on. I just know that I was surprised at zero at 25 and 10 inches high @ 50. I think I will start @ 50, then 100, and see where I am @ 25.

 

[Dawico]

The biggest problem with a sight like that is the height. The sight line is about 2"+ above the bore I am guessing. It makes a big difference when you are trying to figure all of your dopes at those distances. I have had better luck in that situation just getting a zero at a known distance and them memorizing where to hold at shorter or longer ranges.

 

[codygjohnson]

IF you are having a problem getting the rifle to zero, spend the money on some ammo and fire 10 shot groups to see where your point of impact actually is. You’d be surprised at how easy it is to be off with firing only 3 shot groups. A 10 shot group will really let you know where your POI is.

 

[gerritm]

IF you are having a problem getting the rifle to zero, spend the money on some ammo and fire 10 shot groups to see where your point of impact actually is. You’d be surprised at how easy it is to be off with firing only 3 shot groups. A 10 shot group will really let you know where your POI is.

No problems getting it to zero. It fired a quarter sized group at 50 yards. I shot 100 rounds yesterday. The problem is the inconsistency of elevation between 25 and 100 yards. Windage is dead on. At dead on @ 25, when you go to 50 you are 8-10" high. Re zero it at 50 and you are 3-4 inches high at 100 yards. Go back to 25 and you have to bring the scope up 2 dots on the mil dot reticle. I think the trajectory must be an arc that reaches it's peak somewhere beyond 100 yards before it starts to drop. So at 25 it is flat, at 50 it is rising, and at 100 it is starting to level out.

 

[codygjohnson]

Man, it's hard to imagine that kind of trajectory unless that ammo is dropping like a rock. Is the scope mounted to a flat top receiver or is it mounted on a carry handle?

 

[Texas1911]

No problems getting it to zero. It fired a quarter sized group at 50 yards. I shot 100 rounds yesterday. The problem is the inconsistency of elevation between 25 and 100 yards. Windage is dead on. At dead on @ 25, when you go to 50 you are 8-10" high. Re zero it at 50 and you are 3-4 inches high at 100 yards. Go back to 25 and you have to bring the scope up 2 dots on the mil dot reticle. I think the trajectory must be an arc that reaches it's peak somewhere beyond 100 yards before it starts to drop. So at 25 it is flat, at 50 it is rising, and at 100 it is starting to level out.

It's not flat, you are just so close that the bullet is beneath the sight height. The difference between a 25Y and 50Y zero at 100Y is huge, but the difference in a 100Y zero and 200Y zero at 100Y isn't. That has to do with geometry and how the sight over bore axis dictates your barrel trajectory. At close range, the taller the sight axis the more upwards elevation you have on the barrel.

Here's a picture to illustrate that at a 25Y zero.

The black line is the sight axis. The red line shows a short "sight over bore" height like you would get on a bolt rifle, and the green line shows an AR15's height. Notice how the down range trajectory places the AR15s rounds significantly higher, and how the disparity between the two is magnified down range. When you start to shoot at longer ranges the amount of difference between the two becomes less, again due to geometric principle.

 

[gerritm]

It's on a flat top. I just changed the upper from a carry handle. The results are basically the same but not quite as extreme due to the lower mount.

 

[gerritm]

That graph makes perfect sense. I have shot larger caliber rifles and it is not as pronounced as with the .223 round. Thanks, that is what I thought. I think I will attach a small red dot for shorter range and use the scope for 50 plus yards. I feel better after seeing this.