Different bullet weights=different POI

[NewportNewsMike]

The following applies to handgun bullets of varying weights and their differing point-of-impacts (POI).

Like many folks on this Forum, I have observed that, from the same gun, using the same point-of-aim (POA), lighter weight bullets will have a lower POI than their heavier weight cousins. Sometimes, depending on weight difference, a large variation of POI – on the order of multiple inches at a range of 15 to 25 yards. This effect is very observable, and very repeatable (does it every time).

Because I had no contrary evidence and it sounds very plausible, I have always accepted the explanation from many different sources that this phenomenon is the result of the presumably faster light weight projectile spending less time in the barrel which is rising from recoil when fired. Less time means earlier exit from the muzzle (which would be at a lower rotational position), and thus the bullet is given less “arc” on its way to the target, resulting in a lower POI.

Until now –

We are into the fourth season of “Top Shot” which is a television show aired on the History Channel. One of “Top Shots” treats is their outstanding high-speed photography. They have amazing footage of bullets in flight, bullets hitting targets, targets exploding / shattering, and bullets leaving the muzzle – all in glorious slow motion splendor.

This high-speed photography showing bullets leaving the muzzle is what this post is all about. In all the various guns shown – semi-automatic pistols, single-action revolvers, double-action revolvers – the bullet is well away (an inch or more) from the muzzle before the gun starts its recoil rotation / movement. This shows that barrel rotation / elevation caused by gun recoil cannot be having any effect on the flight path of the bullet. The bullet is well out of the barrel before there is any barrel or shooter’s arm movement.

This seems to be in direct opposition to the well-publicized and accepted theory above on why lighter bullets have different POI than their heavier cousins.

Has anyone else observed this on the high-speed photography?

Any other thoughts or theories about what is happening here to cause differing POI’s?

 

[rcmodel]

Top Shot photography not withstanding, there is no other explanation then recoil moving the gun before the bullet is out of the barrel.

If you can't except that, we would have to alter Newtons Third Law of motion.

And then explain why hard kicking revolvers have front sights that point the barrel well below the expected POI when aligned with the rear sight.

rc

 

[highorder]

You can't believe what you think you see, especially from an edited TV show.

Physics often provides explanations contrary to what we think we see.

 

[avan47]

What rc and highorder said! When the bullet starts moving, the gun starts moving.

 

[Haxby]

Somebody should strap a pistol down, so the barrel can't rise, and fire a couple different-weight bullets.

 

[BullfrogKen]

Agreed.

The heavier bullets spend more time in the barrel. It only requires the movement of a fraction of an inch to adjust sights so the bullet impact matches the Point of Aim. Or, if you're filing a fixed sight, or adding a taller post to replace one that's too short, again we're only talking 16ths of an inch to move the strike of the round an inch or two at 25 yards.


I don't know about you, but my eyes are not good enough to determine if the recoil has moved the gun horizontal by only a few 16ths of an inch before the bullet leaves the barrel. Even watching it through extreme high-speed, slow-motion photography.

You may not see it, but you can rest assured it is occurring.

 

[rcmodel]

by only a few 16ths of an inch

It's actually measured in a few thousands of an inch in front sight height, not 1/16's.

A change of .006" on most 4" barrel handguns would change the POI an inch at 25 yards.

.006 is about the thickness of a course human hair.
Like my eyebrow & nose hair for instance.

A credit card is .030" thick, and would move the POI about 5".

1/16" (62.5) would move it 10".

rc

 

[BullfrogKen]

And there you have it . . .

 

[kestak]

I noticed when I use a led sled and when I shot prone my POI changes slightly with rifles at 100 yards. But at 300 yards, it changes A LOT! Anyone noticed that too?

 

[rcmodel]

It is very noticeable with 30-30 and .44 Mag lever-action carbines.

If you hold them down on the bench when sighting them in, they group where the sights are pointing, and where they will shoot when shooting off-hand with normal forearm pressure.

If you don't hold them down in front on the bags and let them jump, they shoot WAY high, and will do so when shooting off-hand.

rc

 

[1SOW]

rc model +10

JMO: Shooting Bullseye with a pistol, even a .22 cal pistol, you don't adjust the sights from a firm rest. You adjust the sights from one hand offhand shooting, or it's going to be way off target unless you have a LARGE RED "S" on your chest..

re video: http://www.trippresearch.com/tech/video.html. Watch 'til the slo-mo semi shows the slide movement and muzzle blast "first" then the bullet. When the slide moves, so does the bbl---up due to the leverage. The higher the bore axis, the more the leverage.

ALSO powder burn rate matters. With a slower powder, The "longer" the bullet is in the bbl the "more" muzzle flip/POI will be affected, even with the exact same bullet.

 

[Walkalong]

It is very noticeable with 30-30 and .44 Mag lever-action carbines.

My son & I sighted in both a .30-30 and a .35 Remington this last fall. After getting them on paper using a rest we shot them offhand to fine tune the guns for hunting. The difference is significant.

 

[chrome_austex]

the bullet is well away (an inch or more) from the muzzle before the gun starts its recoil rotation / movement.

Nope. See this Top Shot video. At 0:56 you can see a revolver recoiling before the bullet leaves the barrel. http://www.youtube.com/watch?v=FV78dNU5IvY

 

[popper]

recoil force HAS to exist only when the bullet is in the bbl! The rest of the 'recoil' is momentum of the weapon system. Lots of factors in the POA/POI change.

 

[Owen]

the heavy bullet phenomenon is much more pronounced in revolvers and other fixed breech guns. A 1911 or a Glock won't show it as much as Revolver, for instance.

 

[RugerOldArmy]

I don't know if I can explain this well. Perhaps 1911Tuner or someone can say this more clearly:

Browning-style tilt-barrel lockups will shoot lower with 'hotter' loads, since the barrel tilts, just as the slide begins moving...and it tilts down. This happens early in the slide's movement, and barrel time for the bullet comes into play. As I understand it, you'll see this sometimes with lighter bullets. (When the sights are set for heavier bullets. Heavier bullets have more barrel time as a rule. In a browning tilt-barrel lockup, think how the barrel tilts to lock-up.) It is counter-intuitive, and the opposite of, say, a bolt action rifle.

I had the same question once...and saw it a few times. I'm not sure if I got this from Jerry Kuhnhausen's book, or from 1911Tuner.

 

[rcmodel]

A propely fitted 1911 barrel cannot tilt down once it is in battery.
It is a solid lock-up between the locking lugs on the barrel and in the slide, and the barrel cam lugs pressing down against the side stop pin.

The only direction the barrel can go is down in the back / up in the front, once the slide recoils far enough for it to start unlocking.

rc

 

[RugerOldArmy]

A propely fitted 1911 barrel cannot tilt down once it is in battery.

The key words are 'in battery'. In battery, the round fires. If it stayed locked, how would the slide overcome inertia?

I WAS specifically talking about 1911 style pistolas.

Consider how the barrel tilts, and that a lighter bullet will go down the barrel faster exiting earlier, and a heavier bullet will take longer. The breech-end of the barrel tilts in what direction?

Consider as well how heavier loads with the same bullet weight (OAL, etc.) show up at the point of impact...although you might have needed to play with a ransom rest to see this.

I won't argue the point, I'll submit it to the discussion, with relative confidence. If you conclude I'm wrong, we simply disagree. You're out nothing for considering this.

 

[rcmodel]

The slide and barrel both recoil a short distance locked together while chamber pressure drops to a point the case can be extracted.

By then, the bullet is out of the barrel and the barrel link pulls the barrel loose from the slide and allows the slide to continue to the rear.

http://www.m1911.org/loader.swf

rc

 

[Hondo 60]

the bullet is well away (an inch or more) from the muzzle before the gun starts its recoil rotation / movement.

While, yes, I have seen this on TV, it's just that, it's television.
Do they edit it the tape to look like that?
Or is it just that many frames per second?

I don't know, but rcmodel is correct in mentioning Newton's Third Law of motion.
(for every action, there's an opposite & equal reaction)


The following is stolen from (http://www.physicsclassroom.com/class/newtlaws/u2l4a.cfm)

The force on the rifle equals the force on the bullet. Yet, acceleration depends on both force and mass.
The bullet has a greater acceleration ...away from you than a rifle... due to the fact that it has a smaller mass.
Remember: acceleration and mass are inversely proportional. {end quote}

This is also why a smaller/lighter bullet will impact lower than a larger/heavier bullet (from the same gun)

 

[1911Tuner]

The gun is in recoil while the bullet is still in the barrel.

For every action, there must be an equal and opposite reaction. Once the bullet is gone, the force that accelerates it and the gun is gone with it. If the bullet is gone, the action side of the equation is over. Without action, there is no reaction, and any gun movement after the accelerating force is removed comes from momentum that was generated and conserved during acceleration.

The actual recoil lasts only a brief instant. By the time our brains can recognize that something has happened, it's over...and what we feel as recoil is momentum. Think about it. How long does it take a bullet to get to the muzzle?

Revolvers show more POI change with ammunition than autopistols because the reaction side is immediately transferred to the recoil shield and frame. With autos, it's directed at the slide...essentially a breechblock...and the slide moves straight back on rails with a spring as the only connection between the gun and the gun mount...the frame.

In a .45 caliber locked breech/short recoil operated pistol, the bullet exits at nominally 1/10th inch of rearward slide/barrel assembly movement with the only factor pushing on the frame being the spring. Actually the mainspring also has an effect on the frame, but I'm simplifying for demonstrative purposes. 1/10th inch. You need stop-action photography to see it.

About 90% of what you recognize as recoil with an autopistol...muzzle flip...comes from the slide impacting the frame. By the time that happens, a 230-grain bullet with a mv of 830 fps, is about 20 yards downrange and its point of impact is already decided.

Here is a very old fluroscope photo of a very old 1911 pistol caught during the firing phase. The bullet base is approximately one inch from the muzzle. The position of the link and the alignment of the rear of the slide with the frame indicates that the barrel and slide have moved roughly .75 inch..which is just about right for the bullet to exit at 1/10th inch of slide travel. Top shot's cameras just can't catch that.

If you look closely, you can see the bones in the shooter's hand...and that the pistol has moved very little, if at all.

Interesting, no?

Note also the upper barrel lugs and their mating slide lugs. The slide is driven rearward by recoil forces...grabs the barrel by the lugs...and drags it backward with it against the frictional drag the bullet is exerting on the barrel. This is a point not often considered by many people. This picture provides strong clues as to how the pistol operates...for those who can see it.

 

[RugerOldArmy]

Thank you 1911Tuner.

 

[PapaG]

Learned this years ago with a customer's Remington replica in 45 Colt. Shot a foot high with factory 255's. I worked my way down through 230, 220, 200, 185, and finally got it point on with the old Lyman 450229, a hollow base semiwadcutter originally designed for the C&B Remington. Each step lighter brought poi down lower. All were kept at similar velocities. Found out the hard way that if you don't control the variables, the results aren't valid.

 

[1911Tuner]

Y'welcome.

Let's go back to this:

Browning-style tilt-barrel lockups will shoot lower with 'hotter' loads, since the barrel tilts, just as the slide begins moving...and it tilts down.

The slide drags the barrel backward. At 1/10th inch, the bullet exits. At or just after that point...maybe .105-.110 inch...the link reaches the point that it just starts to tug on the barrel. At .200 inch, the upper lugs are completely disengaged, and at .250 inch, the barrel is down as far as the link can take it. Ideally, about .003 inch off the frame bed.

Hand-cycling the gun, the barrel starts to drop almost immediately due to the lower barrel lug backing up on the slidestop pin. During live firing, that doesn't happen because the bullet is dragging forward on the barrel while the slide is dragging the barrel backward, and the barrel and slide are horizontally locked in opposition, and can't move vertically. The barrel remains at its in-battery position until the bullet exits and breaks the frictional lock and the link starts to pull on it...at just about the same time.

The link not only pulls the barrel down and disengages it from the slide...it times the barrel's linkdown. This is why it's not a good idea to play with different link on-center lengths without understanding what it can do to barrel drop timing.

 

[kestak]

Roughly 800 feet/second with 45acp
Barrel is roughly 5 inches
That means 9600 inches/second
Considering the acceleration is done over 5 inches and it is too late to calculate it precisely, let's say the bullet goes about 8500 inches/seconds in the barrel. Someone here can try to be more precise.
That means the bullet stays in the barrel roughly 0.00058 second in the barrel after ignition.

Does it make sense or I just flunked my math?