Middle Georgia 3 Gun Match Saturday Feb 23

[Quiet Riot Firearms]

Thats not always true! Think of gravity as always enacting force on the bullet. The drop at 50m is way less than 300m, that much is clear Im sure. Gravity has much more time to cause the bullet to drop over 300m than it does over 50m.

In the graph you posted, the shooter has zeroed to 200m, not 50.

http://www.loadammo.com/Topics/February02.htm

"For example, let’s look at the trajectory for the .223 Remington, shooting a 55-gr. bullet with a muzzle velocity of 3,600 f.p.s. The bullet coefficient is 0.237. The scope-mounted rifle is zeroed for 200 yds. The end range is set at 400 yd. and the trajectory range increment is 20 yds. If we were trying to dispatch a varmint that had a kill zone of 4 inches and we were using the .223 Remington as sighted in Fig. 2, the bullet would be within the 4 inch span (+/- 2 inches) from the muzzle to about 250 yds."

if the shooter had really zero'd their rifle to 50m, the bullet wouldnt have nearly that much arc on it, and it would be in the dirt at 200m.

This is exactly the misconception I am talking about.

You're losing me here man. With a 1.5-2" height gap from scope to bore, there is no physical way to make an arch touch only once on a 50m zero with 223/556 bullet velocities. The bullet is still travelling up relative to the aiming line, then drops back down, crossing the aiming line at 2 different points. BA's graph is spot on.

ETA - I'm seriously considering going to this. Never shot a 3 gun, only carbine matches.

 

[jsquared]

I should have rephrased that: yes, the bullet will cross the horizontal plane twice for any zero distance.

The thing is, with a short distance zero, the first time may be 5 inches from the muzzle, and the second may be at 50 yards. There is nothing that says a rifle zero'd to 50 yards HAS to hit the target on the up arc, it very well could hit it on the down arc, after which it continues downward. The bullet arc could be so miniscule that the bullet only ever rises a fraction of an inch before starting to descend, at which time it hits POA at 50 yards.

In the Army I taught soldiers to zero their rifles using a standard set of steps proven to work for setting a battlefield zero on a 20" M16A2 using 55gr M193 ammo. These steps only worked if you followed them exactly, using an A2-height front and rear sight on a 20" barrel, with M193 ammo. With this combo, the bullet would hit POA at 25m (sight-in distance) and 300m (zero distance).

My point is, most people these days aren't using a a rear sight that can be adjusted for elevation, they are using a fixed rear sight with an adjustable front sight or a red dot/optic. In all of these cases, it is very difficult (impossible?) for the average shooter to know if their bullet is traveling on the up arc or the down arc when they zero at 25 or 50 yards, UNLESS they have access to a longer distance range and can see where the bullet hits at, say, 100 or 200 yards.

IF you are following a set of steps proven to work for your rifle, and ammo proven to be effective for those steps, it is possible. Otherwise you have to shoot your rifle at 50, 100, 200, 300, etc to see what it does and adjust accordingly.

Edit: now that I think about it we were using M855 by this time but thats not really the issue.

 

[Quiet Riot Firearms]

Physics man, physics. There is no physical way, with the velocity a .224 bullet is travelling out of a 10-20" barrel, with a scope height of 1.5-2", for a bullet to ever be going down at 50m. If it crossed 5" in front of the muzzle, it will lead to a much longer second crossing point. Its a simple parabola. Velocity stays the same, starting point stays the same, scope height stays the same. The only thing that changes is the initial angle of launch. Hence why the 25m zero gives a 300m second point, a 50m gives a roughly 210~m second cross, etc. Bottom line is in an AR platform, if you are zeroed at 50, you will be in the ballpark at 200. period. There are not 2 different 50m zeroes. However you do need to back out to 200 to check and refine.

ETA read here:
http://m4carbine.net/showthread.php?t=65679

 

[jsquared]

Ahh, I see what youre saying. Yes that makes sense for any two-sight system (front and rear) you use. My 5" example was wrong but it still holds true that the second crossing of the horizontal plane could be at 50m, whereas the bullet is traveling downward. Youre right though the closer the first crossing is the further away the second crossing is until the angle reaches 45 degrees.

Think about this though: an optic only has one reference point, not two. There is no guarantee with an optic that the bore is angled upward to begin with, it could be angled down or be completely flat. So, depending on the position of the shooter's eye and the height of the reticle or dot in the optic the bullet could start out at any trajectory. A shooter could start with a completely flat bore, shoot a group and see that they are 2" low at 50m (just from bullet drop), then move their reticle down to impart a slight arc on the bullet, and be hitting POA at 50m. At this point, depending on how much they moved the reticle, they might be hitting POA on the Upward (reticle adjusted a small amount) or the Downward (reticle adjusted lower) arc.

 

[HelluvaEngineer]

Ahh, I see what youre saying. Yes that makes sense for any two-sight system (front and rear) you use. My 5" example was wrong but it still holds true that the second crossing of the horizontal plane could be at 50m, whereas the bullet is traveling downward. Youre right though the closer the first crossing is the further away the second crossing is until the angle reaches 45 degrees.

Think about this though: an optic only has one reference point, not two. There is no guarantee with an optic that the bore is angled upward to begin with, it could be angled down or be completely flat. So, depending on the position of the shooter's eye and the height of the reticle or dot in the optic the bullet could start out at any trajectory. A shooter could start with a completely flat bore, shoot a group and see that they are 2" low at 50m (just from bullet drop), then move their reticle down to impart a slight arc on the bullet, and be hitting POA at 50m. At this point, depending on how much they moved the reticle, they might be hitting POA on the Upward (reticle adjusted a small amount) or the Downward (reticle adjusted lower) arc.

It would have to be on the upward trajectory of the arc since you'd have to angle the muzzle to the sky and essentially motor the round to hit at 50 yards. Hold on, I'll draw a picture.

 

[jsquared]

Im loving this ballistics talk, I think about this junk all the time and no one ever wants to talk about it with me haha.

For the OP, sorry for the thread hijack, I will ask a mod to delete all this if you want and we can start a new thread. To everyone else, got to this match! 3 gun is a blast and even if you have never shot at a match before you will love it.

 

[HelluvaEngineer]

Im loving this ballistics talk, I think about this junk all the time and no one ever wants to talk about it with me haha.

Don't worry, I'll set you straight!lol If you start a new thread let me know and I'll jump in.

As you can see below you would have to angle the muzzle at roughly 147.5 degrees above the horizontal to get that kind of trajectory...obviously even if a reticle would allow that range of adjustment the muzzle would then block your view through the optic. And your shot would travel over 8 miles into the air under ideal conditions.
Please excuse the poor drawing, I was on the phone.



I only used a velocity of 3000 feet per second. Didn't see where you said 3600. For reference 3600 ft/s would require an even larger angle.

 

[dial1911]

I have no problem with the ballistics talk... I've done a ton of reading on exterior ballistics too. Engineering school + gun nut = a serious need for data.

Besides that, it's keeping the 3 gun match info from sinking to the bottom of the forum with me having to bump it.

I'm staying out of the ballistics just because I started the thread. That's probably not a good reason.

 

[HelluvaEngineer]

I have no problem with the ballistics talk... I've done a ton of reading on exterior ballistics too. Engineering school + gun nut = a serious need for data.

Besides that, it's keeping the 3 gun match info from sinking to the bottom of the forum with me having to bump it.

I'm staying out of the ballistics just because I started the thread. That's probably not a good reason.

Engineering school does kind of do that doesn't it. For reference I used the simple equations that don't care about air resistance and therefore aren't quite accurate...but it would still require an angle somewhere around the 147 degrees

 

[HelluvaEngineer]

With a scope you still have more then one reference to line up...only one reticle with a cross hairs but you adjust the POA and POI by adjusting the center line of the lenses.

http://2.bp.blogspot.com/_wbkZg1wE4C8/TJy8uIszSqI/AAAAAAAAABg/VZWjpRmrNDk/s1600/scope+cutaway.jpg