I brought my new Bushy M4 upper mounted on my old lower to he range today to sight in a new Holosight I picked up. First of all, I have the Holosight mounted on an ARMS #5 as a riser. Anyway, the first shots at 25 yds were about 4" high. So, I zeroed in (took a LOT of clicks to move the POI down that far). After zero, I moved the target to 50 yds. Same thing - POI was about 3" high. Agian, lots of clicks to move it down to achieve zero. I checked my zero with a 3-rd group, and moved the target again, this time to 75 yds. Well, guess what? POI was 3" high AGAIN. Does this make any sense? I was using Federal Eagle 55-gr. .223. Federal's website only listed trajectory information for 50, 100, 200, etc. yds and it didn't look this this kind of deviation was normal. Any ideas?
You are using an out of date browser. It may not display this or other websites correctly.
You should upgrade or use an alternative browser.
You should upgrade or use an alternative browser.
What kind of bullet drop for .223?
- Thread starter rock_jock
- Start date
If you zero at 25 yards, your point of impact at 50 and 100 yards will be high, but it should be no where near that far off. If you zero at 25 yards, your line of sight and the bullet trajectory intersect at that point. To do that, because of the offset between sights and barrel, your rifle barrel is pointed slightly up, above horizontal. The bullet will continue to climb until gravity starts to pull it down. As I remember, a .223 zeroed at 25 yards will reach max altitude at about 100-125 yards, and then start dropping, re-intersecting the line of sight at about 250 yards, which is called the second or far zero. (These numbers are very rough, based on my fading memory.) At the peak of its trajectory, the bullet should be about 1.5" above line of sight. Therefore, for a .223 zeroed at 25 yards, it should never hit more than 1.5" high.
In short, if you zero at 25 yards, you will hit higher and higher as you move the target out to about 100 yards. Farther than that, and the POI will start to come back down. Beyond about 250 yards, the POI will be lower and lower relative to the line of sight. These numbers are all for a standard AR-15 using iron sights. If you are using optics mounted higher above the barrel than iron sights, your barrel is going to be pointed up even more above horizontal to get a 25 yard zero, and therefore is going to hit even higher as you move the target out to 50 or 100 yards. I'm not familiar with the optic or mount you mentioned. If the optic is very high above the barrel, you might want to look at an alternative mount to get it lower.
In short, if you zero at 25 yards, you will hit higher and higher as you move the target out to about 100 yards. Farther than that, and the POI will start to come back down. Beyond about 250 yards, the POI will be lower and lower relative to the line of sight. These numbers are all for a standard AR-15 using iron sights. If you are using optics mounted higher above the barrel than iron sights, your barrel is going to be pointed up even more above horizontal to get a 25 yard zero, and therefore is going to hit even higher as you move the target out to 50 or 100 yards. I'm not familiar with the optic or mount you mentioned. If the optic is very high above the barrel, you might want to look at an alternative mount to get it lower.
Sigmund,
Of course you are right, and the fact that I didn't recognize this, especially as an engineer, makes me realize how much I have to learn about firearms. This brings up a very good point, which I had never considered because I have not been analyzing ballistics from a physics standpoint (actually, I have not been analyzing it much at all). That point is: trajectory of a bullet is always dependent on the difference in distance between the bore and the line of sight, assuming of course that both are parallel. This is so fundamental and I have never thought about it! Arrgghh! (OK, end of frustration ).
I thought about this concept using an extreme example. Suppose you had a scope mounted a full 12 inches above the bore line. You would have to raise the barrel up so much that your trajectory would look like a steep sine wave. Conversely, the closer your line of sight is to the bore line, the flatter your trajectory will be (subject to limits). Ideally, your line of sight would be coincident with the bore line. Then you would only take into account the effects of gravity and not the line sight/bore line delta.
OK, so, Federal's website lists trajectory info for their American Eagle 55-gr. bullet. The tables mention that their data assumes a line of sight 1.5" above the bore line. The front sight on my Bushy is more like 2.5" above the bore, so here are a few questions:
1. The Federal ballistics data would not fit my AR because of the different line of sight/bore line deltas, correct?
2. I am assuming the military (i.e., Army/Marines) has ballistic data worked out for the 55-gr. bullet using the milspec sights on an AR, yes?
3. If a take the ARMS #5 riser off my flattop, the reticle on my Holosight is in line with the front post. Where do you think it should be?
Of course you are right, and the fact that I didn't recognize this, especially as an engineer, makes me realize how much I have to learn about firearms. This brings up a very good point, which I had never considered because I have not been analyzing ballistics from a physics standpoint (actually, I have not been analyzing it much at all). That point is: trajectory of a bullet is always dependent on the difference in distance between the bore and the line of sight, assuming of course that both are parallel. This is so fundamental and I have never thought about it! Arrgghh! (OK, end of frustration ).
I thought about this concept using an extreme example. Suppose you had a scope mounted a full 12 inches above the bore line. You would have to raise the barrel up so much that your trajectory would look like a steep sine wave. Conversely, the closer your line of sight is to the bore line, the flatter your trajectory will be (subject to limits). Ideally, your line of sight would be coincident with the bore line. Then you would only take into account the effects of gravity and not the line sight/bore line delta.
OK, so, Federal's website lists trajectory info for their American Eagle 55-gr. bullet. The tables mention that their data assumes a line of sight 1.5" above the bore line. The front sight on my Bushy is more like 2.5" above the bore, so here are a few questions:
1. The Federal ballistics data would not fit my AR because of the different line of sight/bore line deltas, correct?
2. I am assuming the military (i.e., Army/Marines) has ballistic data worked out for the 55-gr. bullet using the milspec sights on an AR, yes?
3. If a take the ARMS #5 riser off my flattop, the reticle on my Holosight is in line with the front post. Where do you think it should be?
rock_jock
The difference between the line of sight and the center of the bore is called the offset. All other things being equal, the greater the offset, the higher the arc of the bullet trajectory. Conversely, for a given offset, the farther out the zero, the flatter the trajectory. If you want to use your rifle as configured, you might want to zero it out about 50-75 yards or more.
Keep in mind that, no matter what the offset, you have to compensate for it when shooting very close. The difference between point of aim and point of impact is called holdover. For example, a standard AR-15 with carrying handle and iron sights has an offset of about 2.5". That means that if you shooting at something point blank, you would have a 2.5" holdover. You would have to aim 2.5" high to hit an exact spot. Even at 5 yards, you would have about a 1-2" holdover to hit a very precise spot. Once you have a good zero, you can figure out the short range holdover by experimentation. The longer shots should be okay as you flatten out the trajectory.
I have an Aimpoint red dot optic on my Bushmaster, mounted so that I can see the iron sights through it. That keeps the offset low, and also allows me to use the iron sights if Murphy kills my Die-Hard when I need it most. It's fairly easy to learn to ignore the iron sights while using the red dot.
For more discussion of this and most other topics, the AR-15 bible is: "Some of the Answer, Urban Carbine" by Jim Crews. Only place I know to get it is directly from Jim at
http://www.marksmans.com
Check out his training schedule also. I highly recommend his course. He not only teaches the mechanics and the tactics, he fine tunes you and your rifle to fit each other.
Good shooting.
[Edited by sigmund on 06-07-2001 at 02:52 AM]
The difference between the line of sight and the center of the bore is called the offset. All other things being equal, the greater the offset, the higher the arc of the bullet trajectory. Conversely, for a given offset, the farther out the zero, the flatter the trajectory. If you want to use your rifle as configured, you might want to zero it out about 50-75 yards or more.
Keep in mind that, no matter what the offset, you have to compensate for it when shooting very close. The difference between point of aim and point of impact is called holdover. For example, a standard AR-15 with carrying handle and iron sights has an offset of about 2.5". That means that if you shooting at something point blank, you would have a 2.5" holdover. You would have to aim 2.5" high to hit an exact spot. Even at 5 yards, you would have about a 1-2" holdover to hit a very precise spot. Once you have a good zero, you can figure out the short range holdover by experimentation. The longer shots should be okay as you flatten out the trajectory.
I have an Aimpoint red dot optic on my Bushmaster, mounted so that I can see the iron sights through it. That keeps the offset low, and also allows me to use the iron sights if Murphy kills my Die-Hard when I need it most. It's fairly easy to learn to ignore the iron sights while using the red dot.
For more discussion of this and most other topics, the AR-15 bible is: "Some of the Answer, Urban Carbine" by Jim Crews. Only place I know to get it is directly from Jim at
http://www.marksmans.com
Check out his training schedule also. I highly recommend his course. He not only teaches the mechanics and the tactics, he fine tunes you and your rifle to fit each other.
Good shooting.
[Edited by sigmund on 06-07-2001 at 02:52 AM]
Sorry, past my bed time and my eyes are getting blurry. Site corrected in previous post.
P.S. For a lot more on the AR-15, go to
http://www.ar15.com
[Edited by sigmund on 06-07-2001 at 03:01 AM]
P.S. For a lot more on the AR-15, go to
http://www.ar15.com
[Edited by sigmund on 06-07-2001 at 03:01 AM]
chickrepelent
New member
rock~jock try zeroing at 50 yards. That should make it shoot pretty flat.
Try this ballistic link, very complete. It should answer almost any hypothetical situations. An interesting site.
http://internet.cybermesa.com/~jbm/ballistics/ballistics.html
http://internet.cybermesa.com/~jbm/ballistics/ballistics.html
Here is a table for a 52gr .223 bullet at 3000fps with a 25 yard zero and a 2.5" sight hight.
Grns Brand Style Dia B/C Sght
52 SIERRA BTHP .224 .234 2.50
Range Drop Velocity Energy Flight Time
0 -2.50 3000 1039 0.0000000
25 0.00 2894 967 0.0255875
50 2.24 2791 900 0.0518819
75 4.21 2689 835 0.0794466
100 5.89 2590 775 0.1078010
125 7.24 2493 718 0.1373987
150 8.27 2399 665 0.1680107
175 8.91 2307 615 0.2000126
200 9.18 2218 568 0.2331056
225 9.04 2132 525 0.2673009
250 8.39 2047 484 0.3034837
275 7.25 1965 446 0.3408954
300 5.61 1886 411 0.3795376
325 3.30 1808 378 0.4204888
350 0.34 1733 347 0.4630500
375 -3.25 1662 319 0.5068813
400 -7.71 1593 293 0.5532216
425 -13.02 1527 269 0.6014658
450 -19.23 1464 248 0.6515749
475 -26.39 1404 228 0.7034787
500 -34.80 1346 209 0.7583455
As you can see the bullet is still above line of sight out to 350 yards.
I would say that 25 yards is not a good rage to sight your setup at.
Grns Brand Style Dia B/C Sght
52 SIERRA BTHP .224 .234 2.50
Range Drop Velocity Energy Flight Time
0 -2.50 3000 1039 0.0000000
25 0.00 2894 967 0.0255875
50 2.24 2791 900 0.0518819
75 4.21 2689 835 0.0794466
100 5.89 2590 775 0.1078010
125 7.24 2493 718 0.1373987
150 8.27 2399 665 0.1680107
175 8.91 2307 615 0.2000126
200 9.18 2218 568 0.2331056
225 9.04 2132 525 0.2673009
250 8.39 2047 484 0.3034837
275 7.25 1965 446 0.3408954
300 5.61 1886 411 0.3795376
325 3.30 1808 378 0.4204888
350 0.34 1733 347 0.4630500
375 -3.25 1662 319 0.5068813
400 -7.71 1593 293 0.5532216
425 -13.02 1527 269 0.6014658
450 -19.23 1464 248 0.6515749
475 -26.39 1404 228 0.7034787
500 -34.80 1346 209 0.7583455
As you can see the bullet is still above line of sight out to 350 yards.
I would say that 25 yards is not a good rage to sight your setup at.