When the bullet leaves the muzzle question

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06shooter

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It’s been a while since I posted anything, I hope everyone is well.

I’ve been working on a load for a youth 7mm08 ,went from a 120 gr flat base to a 140 gr BT with Big Game shooting 5 shot groups and adjusting seating depth.
The 140 gr @ 2800 COAL got a very tight group with 3 on the bull but 2 were 1”
Low but together directly under the bull.

This rifle is uncomfortable to shoot so it’s hard to be consistent.
So while I was driving the other day a question popped up , at what distance from the muzzle or how far out from the muzzle is a bullet still being pushed by the gas/blast ?
Can the path of the bullet be changed by the improper movement of the rifle as it begins it’s flight ?
Thanks, David.
 
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If you watch some high speed videos of bullets leaving the barrel you see the bullet long gone before the barrel harmonic vibrations and MUCH longer before the recoil muzzle rise motion, it’s basically disconnected. The gases expanding out of the barrel must decrease in pressure in relation to the expanding volume in a spherical volume as opposed to expanding into cylindrical volume, so it’s not very far from the crown...



Looked this up and learned something myself here, interesting topic.



https://en.m.wikipedia.org/wiki/Transitional_ballistics



http://aegisacademy.com/external-ballistics-part-i/

Meant to say Muzzle Rise above, obviously recoil starts immediately as pressure forces act against both the bullet and rifle, but rifle muzzle rise is a condition caused not by recoil but by a moving rifle acting against the support (shoulder/grip) causing rotation.
 
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If you watch some high speed videos of bullets leaving the barrel you see the bullet long gone before the barrel harmonic vibrations and MUCH longer before the recoil motion, it’s basically disconnected.
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Recoil motion begins the instant that the bullet begins moving. This can be seen in videos that are extremely high speed, but not in a typical slow motion video.

In this video, if you very carefully line up a post-it note with the silver washer/spacer where the barrel joins the suppressor before the shot, you can see it move before the bullet exits the bore. That's at around 2:17.

But even in this video which is extremely high-speed, the motion is quite small and hard to detect. This is to be expected. Since the bullet is around 400 times lighter than the rifle, recoil will move the rifle over a distance about 400 times shorter than the distance the bullet travels to traverse the bore.

So if the bullet moves 20 inches before it exits the bore, the rifle will move about 0.04 to 0.05 inches over the same timeframe.

https://youtu.be/7pOXunRYJIw?t=137

If you look at the slow motion shot around 3:40, the recoil prior to bullet exit is much more obvious. In fact, recoil motion is detectable prior to bullet exit in all of the slow motion closeup shots in the video.
So while I was driving the other day a question popped up , at what distance from the muzzle or how far out from the muzzle is a bullet still being pushed by the gas/blast ?
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I seem to recall seeing an article/analysis of this question at some point, but I can't recall where or what the specific result was.

Clearly the gas pressure behind the bullet stays higher than the ambient pressure for a little while, I just can't remember if the effect was measurable or how they tried to measure it in the article.
 
Once the bullet has left the barrel it is no longer has any influence from the barrel or the gases.
"...2800 COAL..." That'd be FPS.
"...Recoil motion begins the instant..." Yep. Simple physics if you leave the math out. Equal and opposite reactions. All of which happen in microseconds.
"...rifle is uncomfortable to shoot..." It would be being too short. Have the 'youth' do the shooting. Gets him/her involved in the load work up and gives him/her practice with the rifle. Makes it his/her rifle.
 
T. O'Heir said:
"...2800 COAL..." That'd be FPS.
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Nope, that would be COAL as he said. What he left out was the decimal point - 2.800. He was talking about arriving at the proper seating depth.
 
Thanks for the very interesting articles and videos.

Yes, I left out the decimal. I wanted to tune the hand loads to his rifle and then make adjustments to the way he shoots (scope wise ).
 
Getting two clusters in one group generally means you changed how the rifle was held. Were the hand, cheek or rest is placed as well as tightness of grip all have an effect.
 
"This rifle is uncomfortable to shoot so it’s hard to be consistent. "

Uncomfortable to shoot? If so perhaps the shooter is flinching a bit? Does the rifle have a recoil pad installed? If not this can help.

The other option is get a gun that does not recoil so bad. Something like a 222 Rem.
 
Sounds like two grouping. Trying to remember what caused that and I think the guy before me hit it. Changing to way the gun is held. Keep wanting to think it's a bedding problem and can't remember for sure what. In the action I think. Tighten down both action screws then loosen the front one slowly and watch for movement of the front of the action. Or maybe the recoil shoulder is a bit out of wack allowing either the action area or the rear tang area to move enough to change the bedding slightly. The rifle won't have to re-position itself much to throw off the shot's.
 
jamaica said:
"This rifle is uncomfortable to shoot so it’s hard to be consistent. "

Uncomfortable to shoot? If so perhaps the shooter is flinching a bit? Does the rifle have a recoil pad installed? If not this can help.

The other option is get a gun that does not recoil so bad. Something like a 222 Rem.
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Yes , it has a recoil pad with an extension for adult use. My son has grown a lot so he needs it now.
We use it for deer hunting.
 
Don Fischer said:
Sounds like two grouping. Trying to remember what caused that and I think the guy before me hit it. Changing to way the gun is held. Keep wanting to think it's a bedding problem and can't remember for sure what. In the action I think. Tighten down both action screws then loosen the front one slowly and watch for movement of the front of the action. Or maybe the recoil shoulder is a bit out of wack allowing either the action area or the rear tang area to move enough to change the bedding slightly. The rifle won't have to re-position itself much to throw off the shot's.
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In 2016 I loaded some 120 gr loads and it shot 1 1/4 groups without a stock spacer.
This hunting season I put the spacer on and shot the same load , totally a different group, stringing vertical on the bull.
Changed the load and was very happy. The two isolated groups I feel was on me.
 
OK, so you only need fire a few at targets to see it is sighted in, then one at a deer? Just live with it and love it.
 
Recoil starts immediately, but harmonic oscillation of the barrel does not, that is a result of the recoil motion being NOT directly aft of the barrel bore but reacting against your shoulder and grip such that it has a rotation as well, possibly effected by flash hider or muzzle brake. The bending moments on the barrel occur much slower than the initial recoil, and once the moment is applied it bends then oscillates, all long after the bullet has traversed the length of the barrel.
 
his rifle is uncomfortable to shoot so it’s hard to be consistent.
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not really anything to do with your question but I have nothing but praise for Limbsaver brand recoil pads. I had one of my rifles set up in a aluminum chassis and a .308 20 inch fluted barrel last summer, it weighed just a tad over 6 pounds. The Limbsaver was the only thing that made it bearable for more than 5 or 10 rounds of shooting
 
hounddawg said:
not really anything to do with your question but I have nothing but praise for Limbsaver brand recoil pads. I had one of my rifles set up in a aluminum chassis and a .308 20 inch fluted barrel last summer, it weighed just a tad over 6 pounds. The Limbsaver was the only thing that made it bearable for more than 5 or 10 rounds of shooting
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Oh , I didn’t mean uncomfortable as in pain , it being small it’s hard to be consistent.
 
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