Recoil and Semi-Autos (and maybe Revolvers, too)...

[carguychris]

Or it may be residue from prior shots still in the barrel being pushed out by the next shot...

I think it's residue combined with a small amount of combustion blow-by that occurred before the bullet fully engaged the rifling and created a good gas seal. This would then by blown out the muzzle by the column of air being pushed by the oncoming bullet.

I was more fascinated by the "wrong-way" shockwave formed by the combustion gases passing over the base of the bullet as it leaves the muzzle.

 

[Walt Sherrill]

I think it's residue combined with a small amount of combustion blow-by that occurred before the bullet fully engaged the rifling and created a good gas seal. This would then by blown out the muzzle by the column of air being pushed by the oncoming bullet.

That makes a lot of sense.

 

[mkiker2089]

Just one question. Assuming revolvers flip before the bullet leaves as has been implied the why can I aim so well with one? I do much better with revolvers and I have more trouble with muzzle flip on them. I say trouble but what I really mean is that I've stopped worrying about flip and just let the gun go up.

However with a semi I have to hold it darn tight to get any sort of aim. When I don't put the grip of death on my Sccy the bullets hit with a three inch per 20 yard variance in random directions. When I grab the front of the gun in a very impractical but safe-ish way to test muzzle flip I hit a bulls eye. Too bad I can't really shoot that way without giving my left hand a cramp.

 

[JohnKSa]

Assuming revolvers flip before the bullet leaves as has been implied the why can I aim so well with one?

It's not really implied, it's a fact. If you look at the slow-motion link I posted of the revolver, you can see the barrel rise between the time the hammer falls and the bullet exits the muzzle.

Also, you can prove it to yourself if you find a straight dowel that's a good fit for your revolver bore and a yardstick. Then put the dowel in the barrel of your UNLOADED revolver and put the yardstick across the top of the sights so it's just touching the top of the rear sight and the top of the front sight. You'll be able to see that the dowel is pointing down compared to the yardstick. If you think about it, you'll realize that the only way that can be true is if the muzzle rises during the time it takes the bullet to exit the barrel. If that weren't the case, the bullet would always hit well below the sights since the barrel points downward compared to the sights.

The reason you can aim well with one is because the muzzle rise is extremely consistent. It doesn't matter that it moves as long as it moves the same way every time--and for the most part it will.

I say trouble but what I really mean is that I've stopped worrying about flip and just let the gun go up.

If you're not concerned with time, then accuracy is generally better if you hold the gun fairly loosely--especially if the trigger is really good. So letting the gun flip is probably a good strategy if you're shooting slowly using single-action mode.

 

[mkiker2089]

Gotcha, so rise on the revolver is factored in with the sights. That's interesting because it can introduce another wrinkle as to why some revolver newbies have trouble with semi-autos. Turns out I'm not the only one. I can depend on the way the sights are set up, maybe?

 

[barnbwt]

The important/pertinent thing is that the revolver flip before the bullet leaves is consistent. Given consistent bullet weight/velocity (which it is) it should always be identical.

But it may be that revolvers exhibit more vertical dispersion across a range of loads than do semi-autos (compounded by the generally greater vertical height of the bore over the grip and the fact there is no take up in the action like there is on a reciprocating semi-auto)

It would also suggest that there is a tangible benefit to getting the bore axis lower on a revolver for accuracy reasons. Perhaps Ghisoni was onto something with all the 6:00 bore Olympic target revolvers he designed; might make dialing in a new load easier.

TCB

 

[Limnophile]

JohnKSa,

Here's one of a revolver fired in slow motion.

Great video. The muzzle rise prior to bullet departure is definitely discernible, and the angle of the line of sight relative to the bore axis, which is needed to compensate for that muzzle rise, is obvious.

Walt,

The muzzle velocity of handgun combustion gases are routinely assumed, as I recall, to be 50% greater than bullet muzzle velocity for purposes of recoil calculations. I assume this is why muzzle velocities are not measured at the muzzle -- by measuring 10 ft downrange the gases don't interfere.

We see some gases exiting the muzzle before the bullet because the bullet is obviously not tightly sealing the barrel. I suspect that some rapidly expanding gas sneaks around the bullet before it expands to fill the barrel, which is why most gases follow the bullet rather than precede it.

 

[Walt Sherrill]

We see some gases exiting the muzzle before the bullet because the bullet is obviously not tightly sealing the barrel. I suspect that some rapidly expanding gas sneaks around the bullet before it expands to fill the barrel, which is why most gases follow the bullet rather than precede it.

I've been reading about ballistics as a result of our ongoing discussions here -- and picked up a book recommended by a participant in an earlier discussion. Understanding Ballistics: Basic to Advanced Ballistics Simplified, Illustrated & Explained, by Robert A. Rinker. Very informative book!

Then, too, Depending on barrel design and bullet shape a small part of the propelling gases may be passing around the bullet before the bullet moves forward enough to fully engage the rifling and create a seal -- if it can make a seal! (As you note, it may not be a good fit!) Then too, the powder doesn't go off all at once -- it's a "progressive" process, with ignition spreading and pressure building...and as the pressure builds, the action picks up/speeds up. It happens quickly, to be sure, but it's not instantaneous!

I hadn't read far enough into Rinker's book to even think about some of those factors when I commented earlier.

 

[JeffK]

We see some gases exiting the muzzle before the bullet because the bullet is obviously not tightly sealing the barrel. I suspect that some rapidly expanding gas sneaks around the bullet before it expands to fill the barrel...

I'm curious if a similar high-speed video of a modern revolver being fired would show less gas blowing out in advance of the bullet. That Webley Mk VI is probably 100 years old, and was made to fire .455 Webley rounds (0.454 mm bullet OD, 13K PSI, 12.2 mm base diameter, 31.2 mm length). In the video, it's probably firing .45 ACP (0.451 mm bullet OD, 21K PSI, 12.1 mm base diameter, 32.4 mm length), different enough to probably matter.

 

[mkiker2089]

Walt, doesn't the rifling allow air to pass pretty much no matter what? I realize that that the groves aren't very deep but the bullet can't fill them can it?

I realize it's a very small amount of gas but doesn't that explain how you can see a visible disturbance in advance of the bullet.

 

[Walt Sherrill]

Walt, doesn't the rifling allow air to pass pretty much no matter what? I realize that that the groves aren't very deep but the bullet can't fill them can it?

You're clearly asking the wrong person -- and I'm sure someone here can give you a more authoritative answer.

That said, some of the bullets are soft enough, even with a thin coating (as in FMJ) that they fill much of the rifling. How much? Darned if I know. Even some of that pass-by could certainly account for what is seen that we've not been able to explain.

 

[Limnophile]

mkiker,

Given that the default assumption is that the combustion gases exit the muzzle at a speed about 50% greater than the bullet, but that the majority of the gases seem to follow rather than lead the bullet, it seems the bullet does do a fairly good job of sealing the barrel after being hit by the peak impulse of the explosion.

 

[JohnKSa]

Yup, pretty good seal, though not perfect. IMO it's the little bit of gas that does escape through the corners of the rifling that makes the fouling buildup in that area hard to remove.

I'm curious if a similar high-speed video of a modern revolver being fired would show less gas blowing out in advance of the bullet.

It's hard to quantify due to differences in the powder, lighting, etc., but it might be less in a modern gun. It does appear to be there in virtually every slow motion video of guns being fired that I can find as long as the video shows the muzzle in reasonable detail and is slow enough to distinguish between gases exiting before and after the bullet.

http://www.dailymotion.com/video/xzgwut_slow-motion-gun-shots-from-ultraslo_tech

The first video on this page shows an ultra slow motion video of the muzzle of a semi-auto being fired.
http://www.kurzzeit.com/de/videos.htm

 

[Limnophile]

JohnKSa,

IMO it's the little bit of gas that does escape through the corners of the rifling that makes the fouling buildup in that area hard to remove.

I had never thought about that. I would have guessed that most fouling accumulates in the grooves rather than on the lands, but perusing a few photos of fouled barrels that accumulation can occur on both surfaces. My guess would have been based on assuming the lands experience more friction; thus, are essentially scraped better, and that the relatively small amounts of gas escaping around the bullet is predominantly in the grooves; thus, providing a cushion of lubrication there.

Your supposition presumably assumes the escaping gases, which contain uncombusted and poorly combusted powder elements, are compressed and effectively plated onto the barrel by the pressure of the passing bullet.