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Calculating rifle movement as bullet reaches muzzle

I feel Jethro Bodine and I have a lot in common on this subject but I can toss out a few what ifs I doubt the highly educated physic guys would have trouble keeping up with.

If the human body is involved an exact number from perfect calculations is nearly impossible.

I bet more than a few shooters have noticed top shelf shooters can shoot each other's guns with nearly identical results but even a top level shooter wont usually shoot to the same point of aim as a lower level shooter with the lower level shooters gun. It gets even worse when low level shooters try to shoot each other's guns.

Not sure what info the OP is looking for but when I have a non-braked rifle and also a brake equipped rifle the come ups stay the same but the initial zero will be different.

Of course I'll follow this to see if it's something I need to explore.

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TaperPin said:
I’ve seen that video before, but didn’t come away with anything useful other than a video showing the rifle begin to recoil. I could scale the movement, knowing it’s a .338” bullet, but without knowing the rifle weight and how it was held it doesn’t translate to smaller calibers as much as it could.
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There is no one set value that covers all of what you're asking for....too many variables. Suffice it to say that rifle starts it's movement when sear breaks and in microseconds the recoil starts long before the bullet ever even gets to the end of the barrel. If you can find Harold Vaughn's book,you really need to read it.
I think this subject could/would be included in the statement of Albert Einstein that said
"Not everything that can be measured is important and not everything that is important can be measured".
I hope you find what you are looking for. Good luck and stay safe.
 
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TaperPin said:
I thought you were joking, but it just hit me you weren’t, and that it makes sense that if a bullet spins twice going down the bore, the rifle spins the opposite direction matching the momentum. I may have skipped class and went hunting the week we went over angular momentum. That’s a lot harder to figure out for an oddly shaped object.

I don’t know if I can visualize the connection between angular momentum and linear, but that’s not my thing.
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I have experienced some significant torque, so much so that recoil wasn’t much at all (off bags). That rifle, 7mm08 at 33-34 ish inches of barrel (heavy Palma) probably weighed 15-16 lbs. I believe the torsional influence mitigated the rearward movement. That being an extreme example, I’m sure it would be present in all of them. Figuring out it’s quantifiable value is for a solid engineering math wiz, which I am not.
 
This thread below, came at this subject from a lot of different angles. I don’t know if this is a hyperlink or not but it’s at least the thread title.

There are as many videos where the gun doesn’t appear to move at all, before exit, as there are where slight movement is visible.

The rifles where slight movement is apparent, look to be sporters, not what we use. And a sporter probably has a 4 to 6 pound trigger being pulled back by a string, that by itself could move a gun, or disturb the outcome.

If fact, I am a little surprised that in the Litz video, he didn’t go out of the way to demonstrate the heavyweight guns that are typical of competition.

One of the postulates of the thread below is that a heavy gun -any gun - won’t move at all until the recoil force overcomes its resting inertia, friction, support, etcetera, and that threshold in many guns is not reached before the true separation of the expelled masses, bullet and gas. You can push up with a 185 pounds of force of a 225 bench rest bar, and it won’t move at all, so there is no linear relationship between force and movement.

Generally, something outside or leaving the system needs to trigger an equal and opposite reaction. Think firehose valve, rocket/jet engine, releasing a small child who is horizontal because you are spinning ;). In fact the gas being pushed 3,000 fps in front of the bullet out the barrel might be moving those lightweight rifles, which is not the bullet.

If the barrel’s muzzle was plugged solid and barrel was 6 inches thick, contained and stopped the reaction, would the gun recoil backwards when nothing ever exited at all? I don’t think it would. If that is correct, then exit is critical to recoil, the bullet is after all also attempting to pull the rifle forward. If you’ve ever tried to hammer a stuck bullet through the barrel with a rod, that’s apparent, and if a barrel was so long the friction stopped the bullet, it would either have never moved at all, or should be pulled forward from the bullet transferring momentum back to the gun, as far as it moved back, supporting exit or separation as being part and parcel to recoil.


 
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TaperPin said:
mass(gun) x distance(gun) = mass(bullet) x distance(bullet) + mass(powder) x distance(powder
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Conservation of momentum has mass X velcocity terms.

As a start, maybe a rough approximation would be start with [mass of gun] x [velocity of gun] = [mass of bullet] x [muzzle velocity] + [[mass of powder] x [ exit velocity of the burnt gases]]. This will give you the initial rearward velocity of the rifle.
Now you have to figure out the coefficient of friction of the stock on whatever it is resting on and multiple times the surface area of the contact. That will give you the force trying to stop the movement of the gun. You can then find the distance over which the friction force will case the gun to stop moving through
Force x distance = 1/2 [mass of gun] times {the starting velocity)*squared*.
 
jelenko said:
Conservation of momentum has mass X velcocity terms.

As a start, maybe a rough approximation would be start with [mass of gun] x [velocity of gun] = [mass of bullet] x [muzzle velocity] + [[mass of powder] x [ exit velocity of the burnt gases]]. This will give you the initial rearward velocity of the rifle.
Now you have to figure out the coefficient of friction of the stock on whatever it is resting on and multiple times the surface area of the contact. That will give you the force trying to stop the movement of the gun. You can then find the distance over which the friction force will case the gun to stop moving through
Force x distance = 1/2 [mass of gun] times {the starting velocity)*squared*.
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mass x velocity = mass x distance x time

Time is the same on both sides of the equation, so it drops out. No? Wouldn’t that leave the equation I started with?

Exit velocity of gases only comes into play after the bullet leaves the barrel, not at the moment the base of the bullet is at the end of the rifling.
 
JEFFPPC said:
This is a shooters forum not an engineering forum, maybe your in the wrong place for your skill set.
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TaperPin said:
I’m just a shooter of hunting rifles - in a group that measures weight to 1/70,000 lb, diameter to .0001”, velocity to single digit sd’s, and can talk at length about a dozen forces on the bullet in flight, I figured someone other than me was curious about how far a rifle moves in recoil? Lol
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shit, any talk about moving something down a barrel to hit something else is interesting to me.
 
TaperPin said:
Since google is not as much in the business of providing answers as much as selling products and keywords, it’s surprising difficult to find something as simple as how far a rifle moves before the bullet leaves the barrel. Where I’m going with this is just adding a good, even if rough, value to the movement in discussions - rather than explaining to a kid the gun moves an amount that nobody knows, it would be awesome to be able to say in free recoil it might move .0xx” and with a good two handed hold and shoulder pressure that changes it .00x”.

Engineers among us will find this quite simple, so humor me

From memory of physics 101 years ago (feels like 101 years ago), I’m tending to think it would be a simple conservation of momentum? Roughly, if powder expands equally, would the distance of the center of mass of the powder be about half the barrel length, ignoring volume of cartridge case above and beyond the bore diameter?

mass(gun) x distance(gun) = mass(bullet) x distance(bullet) + mass(powder) x distance(powder)

My gut says that’s the easy part and describes the rifle in free recoil at the moment the bullet exits. Has anyone seen, or wish to take an educated guess, as to what kind of range a two-handed hunter hold combined with pressure from the shoulder adds to the mass of the rifle, reducing movement?

Somewhere, someone has high speed camera footage of free recoil and different holds as the bullet comes out, but I’ll bet it’s been dumbed down into an average fudge factor in an equation in a footnote of a ballistics book.

Thanks for your thoughts! This is one of those things that’s been in the back of my mind for decades and I wait for something to come up, but I was either busy and didn’t notice the conversation, or it was too vague to be of much help.
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Enough to miss your target. Had it happen with me 4 times. 3 on Pronghorn and one about two weeks ago on a deer. This only happened in the field and not from a bench. Two contact points on a rest and bags offers a bit different recoil that a one contact point in the field. The very light gun and muzzle lift when shooting in the field is much different than off a bench rest. My heavy guns never miss the same shots I missed with the ultralight. No muzzle lift occurs and hits are exact. The bullet hasn't exited the barrel before the muzzle rises in the carbon barrel rifle. In the end, it's about learning the proper technique to shoot each rifle. Being use to shooting heavy rifles, I didn't take enough time shooting from field positions to overcome and master the shots that were presented to me in the field with that light rifle. User error and entirely my fault. Won't be this way next year.
 
TaperPin said:
Since google is not as much in the business of providing answers as much as selling products and keywords, it’s surprising difficult to find something as simple as how far a rifle moves before the bullet leaves the barrel. Where I’m going with this is just adding a good, even if rough, value to the movement in discussions - rather than explaining to a kid the gun moves an amount that nobody knows, it would be awesome to be able to say in free recoil it might move .0xx” and with a good two handed hold and shoulder pressure that changes it .00x”.

Engineers among us will find this quite simple, so humor me

From memory of physics 101 years ago (feels like 101 years ago), I’m tending to think it would be a simple conservation of momentum? Roughly, if powder expands equally, would the distance of the center of mass of the powder be about half the barrel length, ignoring volume of cartridge case above and beyond the bore diameter?

mass(gun) x distance(gun) = mass(bullet) x distance(bullet) + mass(powder) x distance(powder)

My gut says that’s the easy part and describes the rifle in free recoil at the moment the bullet exits. Has anyone seen, or wish to take an educated guess, as to what kind of range a two-handed hunter hold combined with pressure from the shoulder adds to the mass of the rifle, reducing movement?

Somewhere, someone has high speed camera footage of free recoil and different holds as the bullet comes out, but I’ll bet it’s been dumbed down into an average fudge factor in an equation in a footnote of a ballistics book.

Thanks for your thoughts! This is one of those things that’s been in the back of my mind for decades and I wait for something to come up, but I was either busy and didn’t notice the conversation, or it was too vague to be of much help.
Click to expand...
A big factor is how the rifle moves on the bags or against your body. Recoil amount is just a small part of how the rifle moves while the bullet is in the barrel. Many of the YOUTUBE videos discuss rifle setup on the bench. It's not just how much the rifle moves but how you deal with it. Repeatability of correct movement on the bags.
 
@TaperPin - In the example on the first page, post #19, , are you saying that for free recoil, ( i.e. not holding the rifle at all), that the rifle will travel less than 0.1"
 
TaperPin said:
You’re right - that looks better.

Luckily it still drops out I think.
Click to expand...
it doesn't drop out when equating momentum from different objects. Otherwise, you get something like
velocity of the gun = the velocity of the bullet + the velocity of the gases.
That would have the velocity of the gun moving at ~ 3000 fps.
 
There are indeed a ton of variables involved here; bullet weight, powder charge, rifle weight, stock shape, friction and more. All of this ignores shooter influence. Today, most of my shooting is off-hand, so the rifle is always moving (in my case, it's moving a lot! ). Back when I was shooting more BR, I always felt the biggest influence was what I did between the time that the trigger tripped and the rifle fired, rather than during the bullet's travel up the barrel. The truth was, everything mattered. I can remember one year when I started out, testing a new rifle, and was having a heck of a time with the occasional high shot. I finally figured out that I was relaxing at the moment the trigger tripped and letting the butt drop (I was squeezing the rear bag for elevation). My shooting style was such that I had to follow through consistently; sometimes, I didn't! Even if I did hold until ignition, I could still get a shot which was an eighth inch high. Bench technique has changed a little over the last forty years or so but I think consistency after the trigger breaks is still just as important; even more so when shooting rimfire or BPCR. WH
 
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