Accuracy and bullet transit time

big ears

Well-Known Member
I have an accurate load in my 308 with a 150gr sierra. Thinking of trying 165gr. I have started playing with quickload so was wondering would the accurate transit time with the 165 be the same as the 150 i.e. Is the accurate transit time rifle specific or bullet specific?

cheers

BE
 
sorry i don't know feller but i use the same in pro hunter it ticks all the box's for me, i'd be interested myself if you get a result ,
 
I have an accurate load in my 308 with a 150gr sierra. Thinking of trying 165gr. I have started playing with quickload so was wondering would the accurate transit time with the 165 be the same as the 150 i.e. Is the accurate transit time rifle specific or bullet specific?

cheers



BE

By 'transit time' you mean time of flight?~MUir
 
Sorry for not being clear, I mean barrel transit time. The question is whether this is the constant that makes the round accurate despite the weight of the bullet?
 
Big ears, put Quickload away and get out shooting, it will f**k up your mind and have you worrying about things that bare little relevence to what you are trying to achieve in the field

Ian.
 
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There was an article on this in Airgun world oddly enough. A specific projectile will generally be accurate if it leaves the muzzle whilst the barrel is at the maximum displacement ( in either the positive or negative direction) from its resting position. This is because the barrel with vibrate with simple harmonic motion when the shot is fired. Towards the maximum displacement the acceleration of the barrel due to this movement is tending towards 0 and so it has the least effect on the path travelled by the round. The fact that the barrel will pause very briefly before returning in the opposite direction (like a pendulum in a grandfather clock) will also reduce the effect caused by slight differences in projectile weight or propellant efficiency. You should in theory be ale to 'tune' your barrel to work with a specific round by the addition of weight to the free floating section of barrel so that this effect occurs for a different projectile speed and mass. There is no reason why this effect shouldn't occur in firearms as well as air rifles, after all the physics is the same, its just that the magnitude of the values increases. However, to achieve any reasonable gain in accuracy is extremely difficult and so just finding a round that the barrel likes is rather simpler. In terms of transit time it depends on the charge behind the round, but the heavier projectile should take longer (all else being equal) to exit the barrel as it will take longer to accelerate ( more inertia) and it will travel more slowly (muzzle velocity decreases as round mass increases in general). Both may be accurate if they both fall on maximum displacements, but not necessarily the same one if that makes sense. You can't really tell until you try it and as for whether it is even worth changing, well I'm not really qualified to say...
 
Thanks Alistair, this is what I am asking. The vibration of the barrel is in part due to the blast from the charge passing along the barrel and back again. This velocity is fixed as it is a function of the metal and thickness of the barrel. My question is how much of this vibration is due to the bullet travel itself? If this is negligably in theory a charge could be calculated for different weights of bullets if an accurate load is already known by adjusting the charge so the bullet exits the barrel at the same time interval as the known one. Hope this makes sense.
 
Yeah that makes sense. However, I wouldn't even know where to start I'm afraid. I would guess that the bullet travel will be negligible in and of itself, but the effect of it travelling along the barrel effectively makes it a variable damping force on the whole system? I suspect that such a charge could be calculated, but without some serious mathematical knowledge and a powerful computer I would have thought that trial and error would be a more time efficient method. There are just too many variables- rifling rate, land depth, barrel length, the interactions between the projectile and the aforementioned and the effect of changing velocity on this etc. Even if some can be eliminated, inconsistencies in bullet composition and weight distribution would screw your calculations completely. However, if you do work it out, please do publish your formula, it'll save us all plenty of cash testing loads. Sorry i can't help...
 
The dynamics of an air rifle and its barrel are totally different than a centerfire rifle.
A centerfire rifle has a wave running from chamber to muzzle and back at the speed of sound in steel, about 18,000 FPS, which manifests itself in different types of vibration, different frequencies, etc.

To your question, No, a slower 165 or 180 grain bullet will not have the same time in the barrel as the faster 150-gr.

A .270 Winchester with a 150-gr bullet at 2,850 fps will kill the biggest red deer all day long. So will a .308 at 2,800 or a .30-06 at 2,950.
 
The dynamics of an air rifle and its barrel are totally different than a centerfire rifle.
A centerfire rifle has a wave running from chamber to muzzle and back at the speed of sound in steel, about 18,000 FPS, which manifests itself in different types of vibration, different frequencies, etc.

To your question, No, a slower 165 or 180 grain bullet will not have the same time in the barrel as the faster 150-gr.

A .270 Winchester with a 150-gr bullet at 2,850 fps will kill the biggest red deer all day long. So will a .308 at 2,800 or a .30-06 at 2,950.
Point taken but out of my 20" barrel I struggle to get such speeds. The most accurate load is 2700. Hence the move up a weight.
 
A 150gr at 2700fps will do fine for Red, provided you do your bit.
You will get a nose bleed from a brain burst if you pursue this course.

P.S. you're better off working on fitness if you're going on the hill
 
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Point taken but out of my 20" barrel I struggle to get such speeds. The most accurate load is 2700. Hence the move up a weight.

I wouldn't worry about the speed. 2800fps MV 150gr Sierra Gameking (#2125) is 2700fps at 35 meters.

And at 2700fps MV GK carries 2000 Joule to 225 meters. 2000J is the Finnish 100m energy limit to hunt moose (there's two limits and I used the more appropriate one even though it's technically wrong).

Although it's true that 165 GK at 2600fps MV carries the same energy to 275 meters. If your MV for 165gr is 2500fps, the energy figures are identical to 150gr at 2700fps.
 
red deer are massive softies, i think even with your 20" barrel you'll be more than fine. learn your drops out to the limits of what distance you want to shoot at and get on and do it.

happy shooting
 
Good Lord. I've hunted north American elk with a 30-40 Krag-Jorgensen using 180 grain RN bullets at 2300 fps! I never fussed about whether or not a good shot would put them down. ~Muir
 
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