Where Does the Barrel Point On Firing Pin Impact?

[macgrumpy]

Barrel harmonics are a consideration when you reference the sight line in relation to the point of aim, it isn't an issue if you are only considering where the bore line is pointed.

Harmonics change the location of the muzzle in relation to the sight line not the location of the muzzle to the bullet's trajectory, that trajectory is still the same it just begins a little higher or lower according to the vertical component of the harmonics theory. Note - barrel harmonics actually cause barrel to move in all directions. The muzzle moves in a circular motion, the muzzle moves backward and forward as the barrel shortens and lengthens, and the diameter of the bore reduces and increases. But the harmonic theorists usually only consider the vertical motion since they believe that their theory will have the greatest effect on that direction of motion.

 

[Llama Bob]

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,

The reason this doesn't happen to the degree described is by that tool is that no one lets their rifle recoil freely. They're holding it, and have their shoulder against the stock. In many cases the rifle is preloaded against a bipod, rest, or the shooter's support arm.

The physics of this are really quite simple: the rifle DOES move while the bullet is in the barrel, the amount it moves is very much relevant to POI, and the extent to which the shooter counteracts that movement determines the final POI.

It's really amazing how many people who think they know rifles are completely unaware of what's actually going on when one is fired

 

[Caliper_Mi]

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...

Duh, of course.

But, why go futher and make them so their paths cross?

 

[Llama Bob]

Honestly, given the lack of accuracy of most safari doubles, I don't think you could hardly tell the difference. Then again I don't spend much time bench resting safari doubles so what do I know I think most people would consider two flight paths that were perfectly parallel to be totally acceptable. What's not acceptable is flight paths that cross too soon, or that diverge, or are one high/one low etc. Because of all the recoil-related factors, my general understanding is that you can only make things work right with one "hotness" of loads - hence why guns are regulated to one kynoch load or whatever.

Given a choice though, I'll just take my .45-90 with stopping loads. Too much wrong with doubles...

 

[MachIVshooter]

The reason this doesn't happen to the degree described is by that tool is that no one lets their rifle recoil freely.

That "tool" states "barrel vibrations simulator", not "free recoil unrestrained rifle jump simulator".

I maintain that barrels do not whip anywhere near 70 arcminutes.

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.

That assumes that your laser dot isn't several MOA @ 100 (most are), and that it's exactly true to bore (none are). When using laser boresighters, I rotate them around to find center, and that's where I put my crosshairs (with zero range elevation accounted for).

 

[Llama Bob]

That "tool" states "barrel vibrations simulator", not "free recoil unrestrained rifle jump simulator".

None the less that's what it simulates. Hence why it asks for the center of mass.

Barrel vibration/timing is a separate (and even more complicated) issue.

 

[murf]

llama bob,

benchrest shooters let their rifles "free recoil". there is not a lot of poi difference at 100 yards, but there is a difference. a lot of people guess at what is going on when a bullet travels down a barrel. fancy computer models do this all the time. but, there is not a lot of real test data around to back up these guesses.

machivshooter,

i agree with you about the six foot muzzle rise fantasy. i'm thinking more like six inches.

 

[benEzra]

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

50 yards, if I remember correctly. As I recall, the 55's grouped an inch or so higher. It's less with an AR because the bore axis is closer to the CG.

A lightweight bolt-action in a Monte Carlo type stock shooting a .243/.270/.30-06 will shoot a couple of inches or more higher at 100 yards if you hold it at the rear of the stock and let it lift, vs. hold it down with sling/strong forend grip/top-of-scope pressure. It's not just the angle of barrel rise, which is significant, it's also the vertical component of the bullet's velocity vector as it exits the barrel, which can cause the actual departure angle to be a smidgen higher than the bore line. (Think of the edge case in which the muzzle velocity is small compared to the rotation velocity, and you'll see why.) That's one reason why benchrest rifles are so heavy; it minimizes inconsistency from variances in rifle movement from shot to shot.

 

[Bart B.]

benEzra comments:

It's not just the angle of barrel rise, which is significant, it's also the vertical component of the bullet's velocity vector as it exits the barrel, which can cause the actual departure angle to be a smidgen higher than the bore line. (Think of the edge case in which the muzzle velocity is small compared to the rotation velocity, and you'll see why.)

What formula and component numbers (section of barrel length for radius and rate of angular change) did you use to calculate the rotational velocity (speed at the radius of the arc the muzzle moves at) to get a number greater than 3000 fps a bullet leaves at?

If the bullet has at least a 3000 fps bullet velocity in the vertical component and 3000 fps bullet component in the horizontal velocity, would not the bullet's velocity be at least 4000 fps and its departure angle at least 45 degrees up from horizontal?

 

[benEzra]

What formula and component numbers (section of barrel length for radius and rate of angular change) did you use to calculate the rotational velocity (speed at the radius of the arc the muzzle moves at) to get a number greater than 3000 fps a bullet leaves at?

If the bullet has at least a 3000 fps bullet velocity in the vertical component and 3000 fps bullet component in the horizontal velocity, would not the bullet's velocity be at least 4000 fps and its departure angle at least 45 degrees up from horizontal?

Not sure I follow your question. But if you think of a ridiculous edge case, a barrel that is recoiling upward at 10 ft/sec measured at the muzzle, and the bullet exits at 1 m/sec at the instant the barrel crosses the horizontal plane, the bullet will not fly horizontally, but will be traveling more upward than forward.

With more realistic numbers (lightweight hunting rifle), let's say we have a 3000 ft/sec bullet with a ballistic coefficient of ~.37 and an instantaneous vertical component of 2 ft/sec at the muzzle; this will put the point of impact 5 inches higher at 200 yards than the same round fired from an immovable barrel pointed at the same angle. A vertical component greater or less than 2 ft/sec will change the POI shift accordingly.

As long as the POI shift is consistent, it is corrected for by adjusting the sights or optic so that the POI matches POA, so the shooter never really notices. But with a lightweight rifle, inconsistencies in forend grip, forend pressure, sling tension (if any), and buttstock placement can open up groups. And remember that in a rifle, not all of the recoil rotation is in the vertical plane, though the majority is.

 

[MistWolf]

Never mind

 

[Bart B.]

Here's the best reasoning I've seen on where the barrel points on firing pin impact in the vertical plane:

http://www.varmintal.com/apres.htm

You can calculate the muzzle rise speed using "rate X time = distance" math at bullet exit points on the muzzle curves shown and a barrel length section of a few inches.