Where Does the Barrel Point On Firing Pin Impact?

[lysanderxiii]

Is it significant? Is it minuscule? I guess that's a matter of semantics.

A minuscule error can be significant.

An angle of less than .1 degree is a minuscule angle.

A shot 5 inches off target is significant.

At 100 yards an angular error of .0795 degrees yields an error of 5 inches

 

[J-Bar]

I shot 4 position competition with .22 LR in high school ROTC. Not much recoil in a heavy .22 target rifle.

When I let other team members shoot my rifle, or when they let me shoot theirs, we would always have to adjust the sights. We all used the same ammo, same distance; the only thing that changed was how the rifle behaved when recoiling in someone else's grasp. Even with a minimally recoiling cartridge like the .22 LR, it was a significant difference; we would not score as well if we had not adjusted the sights to accommodate our personal characteristics.

 

[Sam1911]

we would not score as well if we had not adjusted the sights to accommodate our personal characteristics.

Of course. But specifically those due to recoil? I'm going to stay skeptical there. There's a lot more variety in how one shooter holds a rifle, sees the sight picture, manages the trigger, etc. than there would be in how much the recoil of a target velocity .22 cartridge moves an 8-10 lb position rifle.

 

[Llama Bob]

Refer to Pejsa's drop formula for horizontally-fired projectiles.

http://mathscinotes.com/2015/05/pejsa-bullet-height-versus-distance-formula-for-a-zeroed-rifle/


The key here is that we were asked about the relationship between the bore line and the point of impact, not the line of sight and the point of impact.

The height of the sight has nothing to do with how far the bullet drops from the bore line, it's all just bullet trajectory - basically the amount of drop due to gravity for the time of flight over the range.

Correct - this is a key point that many, many people are missing. Nothing about this problem would change if the sighting system were removed entirely, the rifle placed in a vice, and zero was achieved by walking the impacts into the target.

The sighting system is a method for (hopefully) achieving correct alignment of the rifle. It doesn't determine what that correct alignment is.

 

[Llama Bob]

Of course. But specifically those due to recoil?

On a centerfire rifle the recoil-during-barrel shift can be on the order of an inch or so. The effect is bigger on bigger internal case diameter cartridges, higher pressure cartridges, lighter rifles and those with stocks that have a lot of drop.

On rimfire it's going to be small because the pressure is low and the case diameter is small.

 

[Sam1911]

Can you find a slow-mo video that shows that happening? A rise of an inch of barrel -- or anything close-- before the bullet is gone?

So far, I can't. I just see videos of much lighter handguns firing 200 or 230 gr bullets hardly wiggling a tiny fraction before the bullet leaves the barrel.

 

[Bart B.]

Sam1911, rifle holding and trigger management will vary across all of us; I agree with that. But disagree with your comment about using sights.

We all look at target images through sights the same way. Light from the target goes through all sights into our eyes the same way for everyone.

Proved this to a bunch of new shooters on a military rifle team with two rifles in a fixed mount, one with aperture metallic sights and the other with a scope. Asked all six of them to adjust the sight to center on the bullseye target down range. All did so within 1/10th MOA on the metallic sight knobs and 1/30 MOA on the 20X scope; all with the sight's clicks removed to get finer increments tha1/4 MOA.

 

[Llama Bob]

Sorry, I didn't explain myself well. I meant enough to move impact by an inch or so at 100y. Or a little less than .02 degrees if I did the math right.

Where it really comes into play is on light mountain rifles in magnum calibers, where it might be 4" at 100y, or enough to cause a miss at 400y. There you have to repeatably control for it and dial the muzzle rise into your scope, or you will miss. Often times people notice bad POI shifts moving from the bench to slung with those guns and have a hard time getting a good field zero.

 

[macgrumpy]

Actually the question was worded poorly. I assumed that the poster is asking where the barrel is pointed in relation to the aiming point. Asking where the barrel is pointed when the firing pin impacts means nothing, the barrel can be pointed anywhere when the firing pin impacts; up, down, left, right, where ever, you need to ask where it's pointed in relation to something. Since the answers all include references to the aiming point I'm assuming that the original question should have been -

Where does the barrel point, in relation to the aim point, upon firing pin impact.

Then all of the possible answers would make sense. Especially since the poster was obviously trying to distract people in to assuming that the sights were part of the offset (as many posters have done). The sights have nothing to do with the problem, the question asked where the center axis of the bore is pointed, not where the sighting line is pointed.

If the poster asked where the sight line was pointed in relation to the point of aim, at the moment of firing pin impact, then the answer would be pretty simple (assuming that the shooter had a good firing position) - it would be directly at the point of aim, there would no complex math formula to interpret, the offset would be zero.

 

[Bart B.]

I though people could think where the barrel pointed without stating relative to the aiming point. I was asking where the barrel is pointed in relation to the aiming point mentally, but didn't put it in words.

Macgrumpy's question

Where does the barrel point, in relation to the aim point, upon firing pin impact?

is better worded than mine. I apologize for my imperfect assumptions.

I was not trying to distract anyone by mentioning anything about the sights. 'Twas intended to see if anyone considered it important to their reasoning.

 

[MachIVshooter]

Guys, muzzle rise from recoil really doesn't factor in significantly. A fast recoiling rifle, usually a lightweight gun with a high velocity sledgehammer cartridge, such as my 7-1/2 lb .375 RUM, will be accelerated to speeds of 20-25 FPS in a few milliseconds, but the bullet will accelerate to ~3,000 FPS where it exits the barrel in under 1 millisecond; the bullet has left the muzzle long before the weapon makes any appreciable movement.



This is also true of tilting barrel being a factor in recoil operated handgun designs; the slide doesn't begin it's rearward motion until after bullet exits bore:





If you watch some high speed videos of barrel whip, you'll also see that very little movement occurs before the bullet leaves the muzzle.

I can vouch for the fact that a 40gr .223 shoots significantly lower than 55gr does out of a Mini-14. Even though 40gr has less drop, the rifle rotates more during initial recoil shooting 55gr. Lightweight deer rifles often shoot high with heavier/hotter loads also.

At what range? Lighter weight bullets have lower B.C.s, shed velocity quicker. Depending on the bullet and load, your initially faster 40 grain pill may well be traveling slower than the 55 gr at 200, 300 yards. Barrel whip may factor in here, too, the faster 40 gr. leaving when the muzzle is at a higher angle than with the 55 gr. Watch the videos above; again, recoil isn't really a factor.

 

[J-Bar]

Those are great videos, thanks for posting.

If the muzzle does not move before the bullet leaves the barrel, how does one explain the fact that, in handguns at least, heavy bullets hit higher on the target (at short ranges) than lighter bullets? I have always been told it was because the heavier bullet stayed in the barrel longer than the lighter (faster) bullet, so the heavier bullet is more affected by muzzle flip.

Is this not correct?

The phenomenon is real...158 grain bullets impact higher than 125 grain bullets out of my .38 Specials, and 255 grain bullets hit higher than 200 grainers out of my .45.

 

[Casefull]

I have a 5lb kimber 308 and if you do not grip the forearm firmly the rifle will not group very well. It is fine in hunting situations but from the bench you need to hold on or it jumps around when shooting for consistency. If you think about it the recoil has to affect the POI to some degree. Pistols teach a lot more about how rifles react than rifles do...trigger pull for example.

 

[Llama Bob]

You only have to look at the relative heights of the front and rear sights on a magnum revolver to realize just how much rotation is possible. It's rather a lot, actually.

 

[Bull Nutria]

i give up what is the correct answer Bart?

Bull

 

[Walkalong]

Guys, muzzle rise from recoil really doesn't factor in significantly

But it does factor in. Shoot a group, vary your shoulder pressure, and it will show on target.

 

[30Cal]

Refer to Pejsa's drop formula for horizontally-fired projectiles.

http://mathscinotes.com/2015/05/pejsa-bullet-height-versus-distance-formula-for-a-zeroed-rifle/


The key here is that we were asked about the relationship between the bore line and the point of impact, not the line of sight and the point of impact.

The height of the sight has nothing to do with how far the bullet drops from the bore line, it's all just bullet trajectory - basically the amount of drop due to gravity for the time of flight over the range.

That's what I'm thinking. I can draw a front sight of zero height or I can draw one of ridiculous proportions; but the barrel remains unchanged with respect to the target.

 

[Laphroaig]

I can draw a front sight of zero height or I can draw one of ridiculous proportions; but the barrel remains unchanged with respect to the target.

I disagree. First, if the sight height was actually zero, it would be impossible for the projectile to ever hit the line of sight and aiming point as it would start dropping as soon as it left the bore. There has to be some angular difference between the line of sight and the bore to elevate the projectile upward to compensate for gravity and friction as the projectile travels to the target.

If you compare different sight heights, say 1.5 inches and 10 feet, the inclination of the barrel would have to be substantially greater for the latter case to elevate the projectile to intersect the line of sight at the target. A rifles sighting system is an integral part of the interaction between the rifle and an aimed projectile, and thinking that you can somehow disregard the sight reduces the exercise to the effects of gravity and friction on a projectile that will strike wherever those factors lead it.

I still voted "D" because, IMO, a lot of stuff happens (barrel vibrations and movement due to recoil) in the few milliseconds between the primer strike and the projectile leaving the barrel.

Laphroaig

 

[Caliper_Mi]

d - bullet drop minus muzzle rise is the answer.

murf

This.

Sight height only affects how your sight is zeroed. The barrel and especially the bullet don't know what the height is.

 

[Caliper_Mi]

I believe that double rifles need to be regulated. That term almost always has to do with making two barrels on the same gun shoot to the same point of impact at a certain distance. (Really, the paths of the two projectiles CROSS at the intended distance.)

I've always wondered: why bother with this? Why not just set the two barrels parallel to each other. The left will hit an inch to left of POA and the right will hit an inch to the right. Will it matter so much for hunting big game?

 

[Llama Bob]

That's not the issue. The issue is that if you put the two barrels exactly parallel, they will not shoot parallel because their position relative to the center of mass of the rifle and the point of rotation at the end of the stock is not the same. If only it were that easy...

 

[Bart B.]

Here's a software program from Border Barrels showing how a barrel in a complete rifle whips vertically when shot. Rifle weighs 9 pounds. Note the bullet leaves at an angle of about 76 minutes up from the horizontal with a 1.2 millisecond barrel time (green line in picture); typical for a lot of .308 Win loads.



Here's the web site to plug in your barrel numbers then see how it whips before the bullet leaves. You'll need to know (estimate?) the approximate barrel time you bullet has.

http://www.geoffrey-kolbe.com/articles/rimfire_accuracy/barrel_vibrations.htm

 

[Llama Bob]

No doubt, if you don't pin that thing down it will move A LOT.

 

[MachIVshooter]

Here's a software program from Border Barrels showing how a barrel in a complete rifle whips vertically when shot.

Ridiculous. If that were true, your POI would end up 6 FEET above of your boresighted zero @ 100 yards. Boresighting, done right, will pretty much always have you within 8" at 100, often closer.

And barrel whip is a sine wave form (bi-directional) that can also exist on the horizontal axis, creating an oscillation.

 

[murf]

a great way to get the barrel rise amount from a rifle, machivshooter! stick a laser bore sighter in the barrel, set the scope crosshairs on the laser at 100 yards, fire a shot at the center of a target, measure the bullet hole distance above center, add 2.3" (approximate bullet drop @ 100 yards for a 308 win, a 168 gn vmax @ 2800 fps). that is the muzzle rise distance above the bore axis for that gun shooting that load at 100 yards. convert the inches to moa, or millirads, and you can calculate muzzle rise at any distance.

cool!

murf