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runway1
12-05-2011, 11:38 AM
All the trajectory tables I see have the bullet rising above the line of sight then dropping. Why? Is the barrel a few tick up from the LOS? Thanks!

Reductio
12-05-2011, 11:40 AM
I'm assuming this is for an AR-15? Look at your gun, and you might notice that your sights are higher than your barrel. They're like that on most guns. The bullet starts dropping the second it leaves the barrel, so to get the best trajectory, you actually have to 'lob' it out there a bit.... so it will come from under your LOS and zero once, go above, then re-zero with your LOS at some point past.

runway1
12-05-2011, 11:58 AM
So, you're saying the barrel pointing slightly up?

I have a .308 Remington 700P but I'm talking any rifle. I was just looking at tables comparing the .17HMR with .22WMR (yes, I know they're rim fires) and the tables show ~+1.5" at 50 yds and then back down.

Nathan Krynn
12-05-2011, 12:00 PM
Technically yes.


The zero is where your sights and the trajectory meet. Before your zero it will be higher, after lower.

Reductio
12-05-2011, 12:56 PM
So, you're saying the barrel pointing slightly up?

I have a .308 Remington 700P but I'm talking any rifle. I was just looking at tables comparing the .17HMR with .22WMR (yes, I know they're rim fires) and the tables show ~+1.5" at 50 yds and then back down.

Yep, since your sights are above the bore, the bullet will *always* have to rise to meet your line of sight, and the only way to do that is angle it up slightly. This is why people talk about .223's having a 25/300 yard or 35/200 yard zero... it zeros at both distances, is low before and after, and high in the middle.

runway1
12-05-2011, 1:40 PM
Yeah, ok. It zeros just in front of the barrel and then later at your 100/200 whatever point. My 700P is zero'd at 100 yds. Ok, thanks all!

So the barrel's axis and the scope's axis are NOT parallel. That would then make sense.

brando
12-05-2011, 2:21 PM
Ballistic objects move in parabolic arcs in the presence of gravity. It's not a straight line to the target.

IntoForever
12-05-2011, 2:28 PM
The only exception would be if your sights zero was at the peak of the trajectory.

NytWolf
12-05-2011, 2:37 PM
All the trajectory tables I see have the bullet rising above the line of sight then dropping. Why? Is the barrel a few tick up from the LOS? Thanks!

Simple answer: Because your sights are always above the barrel.

The line of sight, whether scope or iron sights, is always a straight line. The bullet's trajectory is always parabolic due to gravity. In order for the bullet to hit its target at the intended sight-in distance, the bullet has to rise to point of impact. In other words, the bullet has to rise from its starting point (in the cartridge) to the line of sight.

Most people sight in their rifles at 100 yards, which is well before the peak of the parabola. Because of the sight in, the bullet's path crosses the line of sight exactly two times.

five.five-six
12-05-2011, 2:44 PM
The only exception would be if your sights zero was at the peak of the trajectory.

that is only useful if you are only target shooting at a fixed distance inside of point blank



in a 25/200 or 35/300 the bullet is always within about 2" of zero from the muzzle to ~450 or so

shy 7th
12-05-2011, 3:38 PM
Ballistic objects move in parabolic arcs in the presence of gravity. It's not a straight line to the target.

UNLESS you are aiming directly at or away from the gravitational source.



...but most people will never find themselves aiming straight down or straight up.

five.five-six
12-05-2011, 3:54 PM
UNLESS you are aiming directly at or away from the gravitational source.



...but most people will never find themselves aiming straight down or straight up.


it happens

c4GYxIwKA6Y

Fjold
12-05-2011, 4:02 PM
Here's an exaggerated picture to show how the barrel and line of sight relate.

http://img.photobucket.com/albums/v214/Fjold/Guns/5-bullet-trajectory.gif

G-forceJunkie
12-05-2011, 4:27 PM
Its already been covered pretty well, but think of it this way. Gravity. Your line of sight is a straight line, typically level with the earth (unless you are shooting up or down a hill.) Gravity makes the bullet start dropping the instant it leaves the barrel. Unless you tip the barrel up a little bit, it would never cross your line of site.

Army
12-06-2011, 6:52 AM
Think football. You and the receiver are on level ground. To throw it far, you throw it high.

Gravity affects the bullet the instant it exits from the barrel. To compensate this immediate drop, the barrel is angled upward to "football" the bullet to the target.

Coyote Brown
12-06-2011, 7:12 AM
Fjolds diagram is awesome.

The only thing to add is , place the barrel at level. The scope will point downward. Where they intersect is zero. Say 100 yards is where they x. Now when the diagramed rifle is shouldered and the sight leveled at the target , the barrel is a angled up for the shot to cross at 100 yards. This is where each individual caliber ,bullet , load and conditions change the parabola . For theory a 100 yard zero .223 55gn 3000fps round would drop back down into the level sight line at 300 yards. While only ever reaching 4 inches above sight level line at 175 yards.

Hope this helps.

Nathan Krynn
12-06-2011, 7:14 AM
Pretty sure its 36 yards and 300 yards.

or 30 meters and 300 meters.

Coyote Brown
12-06-2011, 7:17 AM
Fjolds diagram is awesome.

The only thing to add is , place the barrel at level. The scope will point downward. Where they intersect is zero. Say 100 yards is where they x. Now when the diagramed rifle is shouldered and the sight leveled at the target , the barrel is a angled up for the shot to cross at 100 yards. This is where each individual caliber ,bullet , load and conditions change the parabola . For theory a 100 yard zero .223 55gn 3000fps round would drop back down into the level sight line at 300 yards. While only ever reaching 4 inches above sight level line at 175 yards. Starting at the barrel 1.75" low , this is sight height over bore on our theory rifle. At 50 yards 7/8" low.

Hope this helps. CB

Coyote Brown
12-06-2011, 7:22 AM
Well yes NK my math is erroneous . Just trying to get the theory across. May I point out again there are many variables in the equation that are unknown from the OP. Given more precise data I could run several ballistic calculators.
A troll writing a term paper doesnt need that .

donking
12-06-2011, 7:32 PM
There is a great explanation and diagrams here
http://www.ar15.com/forums/t_3_18/328143_.html

Cokebottle
12-06-2011, 7:40 PM
All the trajectory tables I see have the bullet rising above the line of sight then dropping. Why? Is the barrel a few tick up from the LOS? Thanks!
It doesn't have to, but if it doesn't, then you are always going to be shooting low.

The bullet begins "falling" at roughly 32ft/sec/sec the moment is leaves the barrel. That means that for the first second of flight time, the bullet will "fall" roughly 32ft (a little less due to aerodynamics).

To be able to hit "zero" at anything beyond 25 yards, the bullet must actually cross "zero" twice... once at some close distance (25-75 yards), and again at the ultimate "zero" distance... 200-300 yards.

You could fire the bullet "straight", but doing that would cause it to impact as much as 18" low at 200-300 yards.

By adjusting for the ballistic trajectory and compensating for the arc, it is possible to have the bullet impact within a 6" vertical range (still a good center-mass kill shot) at any distance from zero to 300 yards. From 0 to 50 yards, you'll be a hair low. From 50 to 200 yards, you'll be no more than 2" high. From 200-300 yards, you'll again be low.
Beyond 300, the impact point will drop off rapidly (for the 5.56).