Has the bullet left the building?

[CHLDave]

Question for the gun scientists (or maybe it's just a dumb question and I don't know). Does a 9mm round exit a 3 inch barrel before any movement occurs with the slide/barrel? As I understand a closed breach system, locks the barrel to the slide during the initial stages of the bullet being fired, at which time the bullet is simultaneously traveling down the the barrel. The reason I ask this question is because I notice on my new XD 9 compact (3 inch barrel), that even the slightest (backward) pressure on the slide relative to the pistol frame causes the barrel to begin cycling downward to it's feed position.

Now, the XD pistol is extremely accurate. I shoot better with it than my Bersa .380 which is a fixed barrel, blow back system (and smaller caliber). So my guess is that the bullet leaves the barrel before any 'action' occurs with the slide. However, that does sound kind of interesting and a bit different than I thought the locked/closed breach system worked. Maybe on longer barrels, the bullet is still in the barrel during the slide motion, and therefore more reliant on the barrel staying attached/locked to the slide for accuracy.

My apologies for not searching the forum for the answer to this, as I'm not sure what to even search for lol.

[HighVelocity]

Here's some animation that might help:

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

http://www.youtube.com/watch?v=1H3IFJXxyEs

http://www.youtube.com/watch?v=iPPgmvcs ... re=related

http://i11.photobucket.com/albums/a161/ ... kworks.gif

[pbwalker]

http://i11.photobucket.com/albums/a161/ ... kworks.gif

haha! Beautiful!

[Liberty]

The problem is with all of these animations is that they show the slide starting to go back only after the bullet is well clear of the barrel.This isn't how it really works. The slide starts going back at the same time as the bullet starts seperating from the brass. The slide of course is going to accelerate much more slowly than the bullet. Both the slide and the the bullet achieve max velocity while there is pressure in the barrel. The secret in a good gun design is to not allow the barrel to tilt until the bullet has cleared the barrel. Some designs do this better than others

[jimlongley]

I don't have the high speed video web site in my bookmarks, but there are some great ones out of a German company that show that the gun starts moving well before the bullet exits the barrel.

Newton's Third Law holds true no matter what. As the finger starts to depress the trigger some of the motion will be imparted to the gun instead of the trigger, as the hammer or striker starts its motion, some of that motion will be transformed into a reaction by the gun, as the bullet exits the cartridge some reaction will take place.

Yes, the net effects will be pretty much imperceptible to the shooter, all of them having taken place in a few milliseconds, but they are all still there. At three yards there will not be much in the way of shot displacement, but a fraction of a minute of angle becomes more significant the farther the bullet travels, though still not much of an issue at "normal" handgun ranges.

The STI animation is also interesting because it shows the spent cartridge ejecting straight up. I wonder how they do that.

[FlynJay]

The problem is with all of these animations is that they show the slide starting to go back only after the bullet is well clear of the barrel.This isn't how it really works. The slide starts going back at the same time as the bullet starts seperating from the brass. The slide of course is going to accelerate much more slowly than the bullet. Both the slide and the the bullet achieve max velocity while there is pressure in the barrel. The secret in a good gun design is to not allow the barrel to tilt until the bullet has cleared the barrel. Some designs do this better than others

I agree, the slide retraction is going to start happening at the same time the bullet starts to travel forward (Newton's laws of physics). But conservation of momentum tells us the bullet is going to be moving much faster than the slide since the bullet weighs much less than the slide and the slide also has a spring working against the motion. You might notice that the first millimeter (or more depending on barrel length) or so of barrel motion is locked with the slide. This maintains barrel alignment long enough for the bullet to exit the barrel. If it doesn't, it is a poor design.

I think inconsistencies in ammunition will cause larger groupings than barrel movement.

[KBCraig]

I agree, the slide retraction is going to start happening at the same time the bullet starts to travel forward (Newton's laws of physics).

That's only true in a straight blowback system. In a delayed blowback system (most modern semi-autos), slide movement is hindered by leverage and/or friction until it overcomes the delaying mechanism.

[boomerang]

Animation is entertaining and conjecture is interesting but high speed videography would show what really happens with real pistols in the real world. Unfortunately I can't find any that show enough detail.

There are also plenty of stop motion strobe photos of bullets hitting objects, but google isn't showing me any pics of bullets leaving the barrel of a semiauto handgun. Did anyone else have better luck?

[youtube]http://youtube.com/watch?v=B-ToCK4C7ig[/youtube]

[CHLDave]

I too did some searching on youtube but couldn't find any good slow motion video showing the bullet leaving the barrel. But even in the video that boomerang just posted, the muzzle flash occurs before the barrel seems to move by any perceptible amount (that I can tell anyway).

Anyway, thanks for the responses!

[Liko81]

These animations are a *little* misleading, but for all practical purposes that's how it works.

The problem is that Newton's third law doesn't involve any delay. The instant the secondary ignites, bullet and slide are imparted equal force, and both start moving. The gun is designed, therefore, to move a certain distance backwards before the barrel and breech separate (and I define that as the moment the barrel and chamber begin to move downwards, not when the barrel stops moving backward).

This works because the bullet weighs something like a hundredth the weight of the slide and barrel. Therefore, because equal energy is imparted it gets a hundred times greater acceleration. So, the bullet does exit the barrel before the barrel moves relative to the slide; BUT the barrel and slide are moving at the same time as the bullet, and thus the gun has come out of "lockup" and if the barrel is very loose at that stage it can affect accuracy.

[mr.72]

it's not milliseconds.

In a typical pistol it'd be like a couple of hundred microseconds between the time of ignition and the time the bullet exits the barrel.

The barrel and slide are locked and will move back, but so will the frame. Since the frame is allowed to move independently from the slide+barrel, then the frame will move less than the slide+barrel during this time period but the distance may be very small. Of course the shooter's hand will also move, as will their shoulder and upper body and the whole works will move some measurable, albeit small, amount. I think in a typical semi-auto the barrel+slide will move only a couple of mm before they are unlocked. There is spring tension between the frame and slide that causes the slide to be able to move more than the frame, and then the tension of your muscles and joints causing your shoulder to move less than the frame and just about the only thing that doesn't move some amount is your feet on the ground.

[FlynJay]

it's not milliseconds.

In a typical pistol it'd be like a couple of hundred microseconds between the time of ignition and the time the bullet exits the barrel.

The barrel and slide are locked and will move back, but so will the frame. Since the frame is allowed to move independently from the slide+barrel, then the frame will move less than the slide+barrel during this time period but the distance may be very small. Of course the shooter's hand will also move, as will their shoulder and upper body and the whole works will move some measurable, albeit small, amount. I think in a typical semi-auto the barrel+slide will move only a couple of mm before they are unlocked. There is spring tension between the frame and slide that causes the slide to be able to move more than the frame, and then the tension of your muscles and joints causing your shoulder to move less than the frame and just about the only thing that doesn't move some amount is your feet on the ground.

I don't really know the amount of time, my point is that it is very, very small. Also all the movement of the gun and bullet happen at the exact same time since the forces all originate from the same place, the burning of the primer/propellant in the cartridge. I could do the math for us all but I am not that motivated.

[mr.72]

yes you are right everything must move all at once.

However if you allow the frame of the gun to move excessively (like if the gun were firing on its own while floating in air) then the slide will not move apart from the frame except as much as it is allowed to by the static inertia of the frame of the gun (extremely small amount).

Anyway this is all very interesting to me and I find it fascinating that semi-automatic handguns are so frequently reliable when they depend 100% on the grip and technique of the shooter for correct function.

[WildBill]

Question for the gun scientists (or maybe it's just a dumb question and I don't know).

I think what you are asking is if the accuracy of your gun is degraded by the motion of the barrel, I believe the answer is "yes." I believe that when semi-automatic pistols like the 1911 are accurized they install match bushings and barrels for this very reason. I am sure one of the many 1911 shooters can provide more details.

That said, I wouldn't worry about it. You still have an accurate self defense pistol. No offense, but the your XD is probably more accurate than it's shooter.

[CHLDave]

That said, I wouldn't worry about it. You still have an accurate self defense pistol. No offense, but the your XD is probably more accurate than it's shooter.

No truer words have been said hah! And I really am happy with the XD.