9mm trajectory funk

[DoubleA]

I've read and heard in more than one instance that the heavier bullets fired out of a CZ have a higher POI in relation to the lighter bullets(130 grn and less). I'm having a hard time wrapping my brain around how that is possible when the heavier bullet is traveling slower and has more time to drop. Anyone have thoughts or possibly and answer?

[Shadow]

With the heavier bullet/lower velocity, the gun has more time to rise in recoil before the bullet exits the barrel.

[DoubleA]

So theoretically if you pushed the bullet to the same speed as the lighter bullet then it would solve the problem.

Is it the barrel starting the cam action before the bullet exits? That was my original reasoning, but I thought that the bullet has exited the barrel before the front sight lifts. If that were not true then calling the shot would be a matter of seeing the sight picture mid recoil or is it just knowing where it will impact in relation to where you saw the sights upon ignition?

Is this just a thing with CZ's, 9mm's, or all pistols or guns for that matter?

Edited February 16, 2012 by DoubleA

[kevin c]

My thoughts:

POI is where the bullet meets the paper, but what exactly are you trying to relate that to?

If it's the bore axis, you are right. All bullets drop from the moment they leave the barrel, and the slower slug will drop more over the same distance because it takes longer to get there. To compensate for the drop, all barrels have to be tilted upwards to compensate for the drop, with the bullet travinging up over the plane between eye and POA and back down into the target. The slower slug will need more tilt, allowing the bullet more loop up and back down.

But with the barrel tilted, your sighting plane can't be parallel to the bore axis, it has to be adjusted downwards onto the target to match POA with POI. In this set up, the bullet starts below the line of sight (because the bore is below the sights), eventually crosses the line of sight (about 15 yards, I think), and then travels over the line of sight, up and down, until it hits the target, at about 25 yds.

So, if you are between 15 and 25 yards, your shot will be a bit over the line of sight (POI over POA) for just about any pistol round, but more so with a big slow slug.

And we haven't even gotten to the issue of how the gun may shift/unlock during ignition and whether that has any effect.

Edited February 16, 2012 by kevin c

[Sarge]

With the heavier bullet/lower velocity, the gun has more time to rise in recoil before the bullet exits the barrel.

That would have to be a mighty slow bullet in my opinion. Slow motion video always shows the bullet being clear of the barrel before anything happens as far as the gun unlocking etc.

[Shadow]

1. Too many forces/variables for me to want to attempt an explanation. In my experience, its true in all pistols to include revolvers. A 200 grain 38 special shot from a J frame will hit higher on a target than a 125 grain if you use the same point of aim at normal shooting distances.

[Shadow]

With the heavier bullet/lower velocity, the gun has more time to rise in recoil before the bullet exits the barrel.

That would have to be a mighty slow bullet in my opinion. Slow motion video always shows the bullet being clear of the barrel before anything happens as far as the gun unlocking etc.

________________________________________________________________________________________

Didn't say anything about the gun unlocking. When the bullet starts moving forward, the gun starts moving backward.

Edited February 16, 2012 by Shadow

[MarkCO]

With the heavier bullet/lower velocity, the gun has more time to rise in recoil before the bullet exits the barrel.

Nope, dwell time in the barrel has no effect with pistol length barrels and velocities.

Kevin's explanation is pretty close. I sat here for 30 minutes and wrote it out, but when I read it, I could not visualize what I wrote, maybe tomorrow. Unless you know, it is better illustrated graphically. Two straight lines (sight line and bore axis) that cross downrange must be compared to a variable parabolic (actual path of bullet) that is initially tangent to the bore axis and may touch or not cross, or crosses the line of sight twice make up the geometry.

[DoubleA]

I guess you get the same effect when limp wristing?

I don't know, it just sounds strange to me that the recoil is pushing the barrel up before the bullet leaves it after seeing so many slo-mo videos showing it dead still until the bullet leaves. That would mean that the slightest change in grip pressure would dramatically change POI wouldn't it?

Are we talking .5" at 15 yards or 3 or 4"?

[DoubleA]

With the heavier bullet/lower velocity, the gun has more time to rise in recoil before the bullet exits the barrel.

Nope, dwell time in the barrel has no effect with pistol length barrels and velocities.

Kevin's explanation is pretty close. I sat here for 30 minutes and wrote it out, but when I read it, I could not visualize what I wrote, maybe tomorrow. Unless you know, it is better illustrated graphically. Two straight lines (sight line and bore axis) that cross downrange must be compared to a variable parabolic (actual path of bullet) that is initially tangent to the bore axis and may touch or not cross, or crosses the line of sight twice make up the geometry.

I understand the bore axis and sight line relationship and how the bullet in most cases will cross the sight line twice. What I don't get is that if the sights were not in the equation and there was nothing but a barrel, how could a heavier bullet fly higher than a light bullet at medium range. All this of course at similar power factors.

Shadow has a sound explanation, it is just alien to me until now. I guess it does make the rule of followthrough even more golden in that you can still screw the shot up even after the trigger is pulled and the hammer drops.

Edited February 16, 2012 by DoubleA

[reptoid]

Actually, this has nothing to do with line of sight or sights at all for that matter. It has everything to do with bullet weight and barrel time. Don't believe me? mount your pistol or revolver in a ransom rest and shoot groups with 3 differant bullet weights with the same (appropriate)powder charge without changing anything else at a range of 15-25yds.

You will get three separate groups with the heaviest bullet striking highest, middleweight in the middle and lightest and fastest on the bottom. Doesn't matter if it's a single shot contender, a revolver, or a semi-auto. Heavier, longer barrel guns will show a smaller change.

An extreme example would be a 2" fixed sight airweight revolver going from a 110gr. +P JHP load to a 158gr SWC. I've seen a change in POI of 3"-4" in elevation @ 7yds and usually 1" or more to the left in windage for a RH shooter.

At longer range external ballistics will become a factor and to figure POI you have to chronograph the load,zero at one especific distance, then enter: Sight hieght above bore, zero distance, bullet data and chrono results into a ballistic program to predict POI for other ranges.

Edited February 16, 2012 by reptoid

[mcracco]

And to hurt your head even more:

You are on a perfectly flat range with your gun in a machine rest with the bore parallel to the earth's surface. You fire a round and at the same time drop a bullet to the ground from the same height as the chamber of the gun. Which hits the ground first, the fired round or the dropped bullet?

[MarkCO]

Both hit the ground at the same time. That is the simple one.

[DoubleA]

So I guess if the barrel is fixed in way that it cannot move at all, then the results would be inverse.

Edited February 16, 2012 by DoubleA

[Steve RA]

If the barrel were fixed, the heavier bullet would impact lower. How much lower would depend on the distance from the firing point to the target.

As pointed out above, the reason the heavier bullet will impact (POI) higher on the target - hand held - is because the gun has more time to recoil upward before the bullet leaves the barrel.

[mpeltier]

I guess you get the same effect when limp wristing?

I don't know, it just sounds strange to me that the recoil is pushing the barrel up before the bullet leaves it after seeing so many slo-mo videos showing it dead still until the bullet leaves. That would mean that the slightest change in grip pressure would dramatically change POI wouldn't it?

Are we talking .5" at 15 yards or 3 or 4"?

Limp writing is another issue altogether and may or may not effectively change poi signifacantly. Don't believe the slo motion videos either. I have seen many, and some show no movement and some do. There's a lot we as the viewer do not see regarding how the auther set up many of these shots. Dispite what many will claim rearward motion does start at the moment of bullet movement, well before the bullet leaves the bore. Combine this with a bore axis above the center line of the wrist/forearm and soft tissue compression of the wrist and hand and you get your answer. It is very very little movement we are talking about and the movement due to initiating recoil forces is very consistant, as is the make up of our anatomy.

[MarkCO]

I understand the bore axis and sight line relationship and how the bullet in most cases will cross the sight line twice. What I don't get is that if the sights were not in the equation and there was nothing but a barrel, how could a heavier bullet fly higher than a light bullet at medium range. All this of course at similar power factors.

Shadow has a sound explanation, it is just alien to me until now. I guess it does make the rule of followthrough even more golden in that you can still screw the shot up even after the trigger is pulled and the hammer drops.

I guess I was too tired... If the bore axis is level, and fixed, a slower (initial mv) bullet's parabolic flight path will always be below that of the of a faster bullet's flight path.

What messes with peoples heads is that the ballistics calculations all assume a level line of sight and an upward tilted bore axis. For that reason, people look at the path graphs and see that a heavier bullet lands after a lighter bullet fired from the same gun. But they have not taken into account the tilt of the bore axis.

[MarkCO]

Limp writing is another issue altogether and may or may not effectively change poi signifacantly. Don't believe the slo motion videos either. I have seen many, and some show no movement and some do. There's a lot we as the viewer do not see regarding how the auther set up many of these shots. Dispite what many will claim rearward motion does start at the moment of bullet movement, well before the bullet leaves the bore. Combine this with a bore axis above the center line of the wrist/forearm and soft tissue compression of the wrist and hand and you get your answer. It is very very little movement we are talking about and the movement due to initiating recoil forces is very consistant, as is the make up of our anatomy.

Yes. That is a momentum evaluation and the first correct explanation. I was just too tired to think straight last night.

[kevin c]

If the barrel were fixed, the heavier bullet would impact lower. How much lower would depend on the distance from the firing point to the target.

It will hit lower if it travels slower, taking more time to get to the target and therefore having more time to fall. So, if it is at the same PF as the lighter bullet, it will hit lower because of its lower velocity. If it is travelling at the same speed as the lighter bullet (HIGHER PF) it will hit in the same spot as the lighter bullet (making no allowance for differing ballistic coefficients). If faster than the lighter bullet (+P .45 ACP vs subsonic .22 short ) it should hit higher.

Ain't touching the unlocking vs recoil issue - way too many variables.

Edited February 16, 2012 by kevin c