Calculating rifle movement as bullet reaches muzzle

[jelenko]

truth was, everything mattered

ain't that the truth!
Sometimes it takes a lot of detective work to find the little things we do unconsciously. And frustrating when we find we already solved the issue more than once before.

 

[jackieschmidt]

Always remember the old adage that there are only a few things that will cause a bullet to take the exact same path to the target as the one before, but an astronomical number that will cause it not too.

 

[TaperPin]

I keep finding videos and discussions, mostly in the “tactical” community, where they are struggling to explain why shooting prone and aiming to the side moves the sight picture after recoil, but not the group on target. If the rifle is only moving 1/16” as the bullet leaves the barrel, give in the squishy part of the shoulder allows plenty of movement straight back before the shoulder pulls the buttstock to the side. Interesting.

I do like the videos showing normal shoulder pressure vs hard shoulder and it has me wondering if a big thick soft recoil pad has a benefit on a low recoiling rifle by smoothing out shoulder pressure in that short amount of recoil. We naturally have to hold the rifle the same way each time, but we’re human, and a soft recoil pad would seem to even out pressure the rifle sees, not just making it softer on the shoulder.

 

[TaperPin]

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

Small bore silhouette in my teenage years was more of a blessing than I knew at the time - the trigger pull and follow through that gets developed aids in all forms of shooting. I assumed all good shooters had a similar experience, so it was an eye opener that many adults skipped rimfire and could be successful with horrible fundamentals when shooting centerfire.

 

[dellet]

If you have access to quick load, the recoil calculator there probably has your answer. It will be load and rifle weight specific.

It does try to calculate rifle movement both before and after bullet exit.

 

[TaperPin]

I had hoped memory of a freshman physics class 30 years ago was correct, and I had hoped an engineer would bless the calculations, but of all things SAAMI has a discussion finding recoil velocity, but the formula shown can be used to solve for rifle movement once time drops out. I finally feel better about it, other than what happens to the air in the barrel as was brought up.

There must be a (mass x distance) value for air in the barrel, but it’s probably a very small number. (Mass of dry air is something 20k grains per cubic meter and a cubic meter is roughly 61k cubic inches or 1/3 gr per cubic inch. Pretty small number)

In my little primitive carpenter brain little guns recoil 1/16” and my 300 mag moves 5/16”. I‘ve wondered this for 40 years. 30 years ago I should have just used the equation in class. Lol

 

[TaperPin]

If you have access to quick load, the recoil calculator there probably has your answer. It will be load and rifle weight specific.

It does try to calculate rifle movement both before and after bullet exit.

I don’t have quick load, but that’s very interesting.

 

[dellet]

I don’t have quick load, but that’s very interesting.

This was a 30-30 with a 170 grain cast bullet at about 2260 fps

 

[gunsandgunsmithing]

I don’t have quick load, but that’s very interesting.

Did you look at the link in post 34? I think your answer is there, as much as it's feasible, anyway.
You can put gun and load data into that calculator and get recoil velocity. You'll need to calculate the in bore time of the bullet or, there's probably a calculator for that too, somewhere on the interweb.

Once you know the recoil velocity, is it not just a matter of multiplying that times the in bore time?

IOW, if the recoil velocity is say 5.12 fps and the bullet is in the bore for 1.7 milliseconds(.0017), 5.12x12=61.44 inches x .0017 = .1044.

It doesn't account for friction in either direction..but there's probably a calculator for that too! Lol!

In the end, it's a very small amount, in a very short amount of time and is of very little consequence as long as everything is consistent. Now, change something pretty significantly, like shooting uphill vs down and you affect velocity and in bore time...tune.

That should make this more interesting. Lol!

 

[gunsandgunsmithing]

This was a 30-30 with a 170 grain cast bullet at about 2260 fps

View attachment 1500937

Excellent! It's probably just not on that screen but I don't see bbl length for in bore time. Very nice, though!

 

[dellet]

Excellent! It's probably just not on that screen but I don't see bbl length for in bore time. Very nice, though!

Barrel length 26”, and bore time would be on the main screen with the load data.

 

[gunsandgunsmithing]

Barrel length 26”, and bore time would be on the main screen with the load data.
View attachment 1500945

I'm missing something that they are factoring in but I'll leave it up to others. I will say that I'm only worried about what happens while the bullet is still in the bore.

 

[TaperPin]

This was a 30-30 with a 170 grain cast bullet at about 2260 fps

View attachment 1500937

That’s awesome!

 

[TaperPin]

The way I was trying to figure it used on your 30-30 to see if I’m even close to the .075” on the printout:
bullet travel 24.5”ish
bullet weight 170 g4
powder weight 32.4 gr
powder travel 12.25”ish
rifle weight 63,000 gr
x = [(24.5*170) + (32.4*12.25)]/63,000
= (4165 + 396.9)/63,000
= .072”
If I’m only off .003” that’s a win for this country boy! Lol
Ok - for all you physics professors getting a chuckle out of watching the struggle, I hope this gave you a glimmer of hope that the kid who skipped class a lot to go hunting can eventually figure out a problem. I still don’t regret changing my major to Econ after that horrible fluid dynamics class. Lol

 

[dellet]

The way I was trying to figure it used on your 30-30 to see if I’m even close to the .075” on the printout:
bullet travel 24.5”ish
bullet weight 170 g4
powder weight 32.4 gr
powder travel 12.25”ish
rifle weight 63,000 gr
x = [(24.5*170) + (32.4*12.25)]/63,000
= (4165 + 396.9)/63,000
= .072”
If I’m only off .003” that’s a win for this country boy! Lol
Ok - for all you physics professors getting a chuckle out of watching the struggle, I hope this gave you a glimmer of hope that the kid who skipped class a lot to go hunting can eventually figure out a problem. I still don’t regret changing my major to Econ after that horrible fluid dynamics class. Lol

Try it again with bullet travel 23.45”.
Note cartridge OAL of 2.55”. You used 24.5 for travel.

You might enjoy this question I had a few years ago, it shows why only knowing powder weight can be deceiving calculating recoil.
@gunsandgunsmithing was helpful as were some others.

Powder choice/burn rate and recoil. How are they related, How do you predict or measure effect?

After a few years of ignoring them, I started shooting my single shot rifles again. Two things have happened in that time. I've gained some knowledge about loading, and I have gained some years. It did not take too many shots to remember why casual competitive shooting was too many rounds in a...

forum.accurateshooter.com

 

[Will Henry]

Concern with what happens while the bullet is still in the bore is why BR stocks are made with the line of the buttstock as close to parallel with the fore arm as it can be, within the rules and why rear bags are made to be as stable as possible. It is also true that some rifles, and some shooters, might do better with some shoulder contact. WH

 

[wwbrown]

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

Momentum equals mass time velocity and it is not trivial in this case to get the movement of the rifle.

 

[TaperPin]

Momentum equals mass time velocity and it is not trivial in this case to get the movement of the rifle.

The value for time on both sides of the momentum equation are equal and can be taken out, leaving mass times distance.

 

[wwbrown]

The value for time on both sides of the momentum equation are equal and can be taken out, leaving mass times distance.

If only it were that easy, I would not have had to spend 6 years of my life getting my bachelors and masters degrees in physics.

For the conservation of momentum to apply the system must have no external forces on in the direction of motion, you are part of the system and there is probably friction between you and the seat you are sitting on otherwise you would slide back.

When we teach physics you apply conservation of motion and simple kinematics and we assume the net external forces on the system is zero. When I taught physics I would joke that we are turning reality off, the shotting system that I would define as you, the rifle, the ammo, etc. is hardly a system that is ideal from an analysis standpoint.

 

[TaperPin]

If only it were that easy, I would not have had to spend 6 years of my life getting my bachelors and masters degrees in physics.

For the conservation of momentum to apply the system must have no external forces on in the direction of motion, you are part of the system and there is probably friction between you and the seat you are sitting on otherwise you would slide back.

When we teach physics you apply conservation of motion and simple kinematics and we assume the net external forces on the system is zero. When I taught physics I would joke that we are turning reality off, the shotting system that I would define as you, the rifle, the ammo, etc. is hardly a system that is ideal from an analysis standpoint.

So, if that doesn’t apply to this situation, how would you propose to estimate the amount a rifle recoils at the time the bullet exits the muzzle?

Turning off reality isn’t an option in this case. Influences that are small can can be ignored as a practical matter, no?