A Technical Question

[nemasis243]

Heres one for you wizz kids , if you shoot a cold bore shot and its 3000ft per second , if you keep shooting will the speed of the bullet increase or decrease due to the barrel getting hotter and expanding and the pressure increasing or decreasing due to this?

[col48]

Heres one for you wizz kids , if you shoot a cold bore shot and its 3000ft per second , if you keep shooting will the speed of the bullet increase or decrease due to the barrel getting hotter and expanding and the pressure increasing or decreasing due to this?

 

 

hi

 

what happens is when you keep shooting your rifle you will get your barrel and most important the chamber hotter,this will heat up your case and will create more pressure, the same as shooting in hot weather.

all so as your barrel heats up the bore size will decrease and this will all so create more pressures.

 

well thats how i see it

 

ATB

Colin

[Guest northernchris]

Col

 

I would say you are bang on about temp in the chamber area to a degree but i think you will find the bore size will increase as the temp rises.

[Ronin]

Simple answer is every gun is different.

 

Cold bore shot is usually a little away from the "usual" group area - which may only be 1/4 inch or 1/2 inch from 1st shot if your shooting groups you dont notice it much.

 

If your shooting one shot, precision at small targets, you need to know where the cold bore point of impact will be,

 

Will the velocity increase - yes, but not by much, in these days of high performance teperature insesitive powders there aren't affected much by chamber temperatures, unless its very hot.

 

I think the bore / chamber size would increase by microns only as the temps rise - too small to be any noticable effect.

[nemasis243]

Very interesting RONIN , I only posted this as I had an email from an old collegue who is now out in a far away land serving as a sniper , all that I can tell you is that they have been having trouble with unfired rounds(large cal) getting very hot and when being fired altering the point of imapct at long distance when engaging a target,

[col48]

Simple answer is every gun is different.

 

Cold bore shot is usually a little away from the "usual" group area - which may only be 1/4 inch or 1/2 inch from 1st shot if your shooting groups you dont notice it much.

 

If your shooting one shot, precision at small targets, you need to know where the cold bore point of impact will be,

 

Will the velocity increase - yes, but not by much, in these days of high performance teperature insesitive powders there aren't affected much by chamber temperatures, unless its very hot.

 

I think the bore / chamber size would increase by microns only as the temps rise - too small to be any noticable effect.

 

 

ronin

 

try leaving some rounds out on the bench, in the sun, and then crono them.

and then leave some in the shade and crono them, i think you will see a difference.just the same as chambering a round in a hot barrel and then taking your time to take a shot, the case warms and bingo a hotter round.

 

ATB

Colin

[nemasis243]

Youve hit the nail on the head there buddy

[Ronin]

Well

 

if youre talking 50 cals, they are not the most accurate round anyway, especially militry fodder.

 

Most of the chaps who make noises for a living I know of are using 338 rather than 50's now because of the inaccuracy, high recoil and high weight of the 50 cal platform.

 

 

I would hve suspected that your friend would have been trained to account for hot weather climates and been pre warned of the effect on the issued equipment / ballistics?

 

 

The heat will cause uplift winds, mirage etc that could account for the "off shots" as well as the hot ammo in hot chamber theories.

 

 

I'm not suggesting anyone is wrong, just that there may be more to it than just one factor.

[nemasis243]

Hi Ronin the gentleman in question is very good at what he does and now is working in the private sector

From what I am lead to believe the ammo is Handloaded in the States by the company he works for,

Will get more info when I can

[Ronin]

Nemasis

 

I look forward to the additional information from your friend.

 

If its a place that is really hot and sunny (40 / 50 degrees +), I would expect ammunition would be stored in a shaded area and the guys using it to be used to its tendancies.

 

Moving back to the original thread, ive shot quite a few rounds through the chronograph in both hot and cold UK weather and to be honest havent really noticed any disernable change in velocity in either with the same charge.

 

I did once have an issue with some 6.5x284 ammo that was great when I used it, then several months later it showed serious pressure signs, this I atributed to the cases "gripping" the bullet over time which caused increased neck tension / pressure.

[Vermincinerator]

It only takes one hot over presure round to put enough copper fouling up the barrel to destroy any good accuracy the rifle had.

 

Ian.

[shrek]

Nemasis,

 

I wouldnt want to get on the wrong side of you with friends like that haha dont care if the shots on or off still wouldnt want it pointin at me from any distance!!!

 

Regards

Shrek

[silentsoulsleave308holes]

I have sat down for hours and hours in the field off my bench and crono set up, shooting for groups and testing one thing or another

 

what I can tell you is that the first few shots record the highest vel and providing the ammo is kept at a reasonable temp (ie out of the sun) then vel does go down the more shots fired, a fact, but that doesnt mean that the gun will shoot low because of the drop in vel

 

as the bore warms the i.d and o.d increase in dia and we use this fact at work to heat shrink parts into place, infact we have heated the boss (round bar with a hole in it) and shrunk the male part (round solid bar) with nitrogen

 

before the process the male part may have been say 42 mm dia o.d and the female ( the boss) may have been 38mm dia i.d

 

now when the boss is heated it becomes 40.5 to 41 i.d and the bar when shrunk becomes 39 to 40 i.d so the two can be put together and when the cool/or normalise the cannot be removed and are stuck for life but remember this is just an example, I do remember increasing a boss from 98 int dia to 106 in dia from heating it so you can see it is possible to increase it alot depending on how large the bore and how thick it is

 

this is how steyr bbl's are put on and this process shows what happens to your bbl when it gets warm, it increases in internal/external dia

 

you would think that because the vel has drop that the round will impact low well this may be the case for some guns but it is just down to the individual gun

 

all bbl's have a whippen affect when fired due to the bullet being pushed down a spireld bore and the bore would like to unwrap itself with the spirel

 

skinnies do it a lot, heavies do it less but as the temp and expansion of the bore increases usually the whip is reduced throwing the shot differently hence the change in inpact

[Jay Foxing]

Hi All,

 

Maybe this answers the question, taken from www.nordicsniper.com

 

TEMPERATURE, WIND AND HUMIDITY FACTORS.

For The highly trained Sniper, the effects of weather are the main causes of error in the strike of the bullet. Wind, mirage, light, temperature, and humidity affect the bullet, the Sniper, or both. Some effects are minor; however, Sniping is often done in extremes of weather and all effects must be considered.

Wind classification Wind poses the biggest problem for the sniper. The effect that wind has on the bullet increases with range. This is due mainly to the slowing of the bullet's velocity combined with a longer flight time. This allows the wind to have a greater effect on the round as distance increases. The result is a loss of stability.

 

Since the Sniper must know how much effect the wind will have on the bullet, he must be able to classify the wind. The best method is to use the clock system. With the Sniper at the center of the clock and the target at 12 o'clock, the wind is assigned the values: full, half, and no value. Full value means that the force of the wind will have a full effect on the flight of the bullet. These winds come from 3 and 9 o'clock. Half value means that a wind at the same speed, but from 1, 2,4,5,7,8,10, and 11 o'clock will move the bullet only half as much as a full-value wind. No value means that a wind from 6 or 12 o'clock will have little or no effect on the flight of the bullet.

 

Before adjusting the sight to compensate for wind, the Sniper must determine wind direction and velocity. He may use certain indicators to accomplish this. These are range flags, smoke, trees, grass, rain and the sense of feel. However, the preferred method of determining wind direction and velocity is reading mirage. In most cases, wind direction can be determined simply by observing the indicators.

A common method of estimating the velocity of the wind during training is to watch the range flag. The Sniper determines the angle between the flag and pole, in degrees, then divides by the constant number 4. The result gives the approximate velocity in miles per hour. If no flag is visible, the Sniper can use paper grass, or some other light material at shoulder level, then drops it. He then points directly at the spot where it lands and divides the angle between his body and arm by the constant number 4.

 

The most desired method of determining wind is with mirage. A mirage is a reflection of the heat through layers of air at different temperatures and density. A Sniper can see the mirage as long as there is a difference in ground and air temperatures. In order to see this mirage the Sniper must focus his spotting scope on an object at mid-range, and then place the scope back onto the target without readjusting the focus. As observed through the telescope, the mirage appears to move with the same velocity as the wind, except when blowing straight into or away from the scope. Then the mirage gives the appearance of moving straight upward with no lateral movement. This is called a “boiling mirage." In general, changes in wind, up to 12 mph can be readily determined by observing the mirage.

 

Conversion of wind velocity to minutes of angle All telescopic sights have windage adjustments that are graduated in minutes of angle or fractions thereof. A minute of angle is 1/60th of a degree. This equals 1 inch at 100 meters (1.145 inches). Snipers use min. of angle (MOA) to determine and adjust the elevation and windage needed on the weapon's scope. After finding wind direction and velocity in MPH, the Sniper must then convert it into MOA using the following formula. This formula is a rule of thumb only and is used as a starting point. Practice and keeping a good data book are the ultimate goals and tools.

RANGE (1st digit) x VELOCITY (MPH) = MOA (for a full value wind)

CONSTANT (for a half value wind divide this answer by 2)

The constant depends on the targets range:

100-500 meters "c"=15

600 meters "c" =14

700-800 meters "c"= 13

900 meters "c"= 12

1000 meters "c"=11

 

Effects of light.

Light does not affect the trajectory of the bullet; however, it does affect the way the Sniper sees the target through the scope. This can be helped by practice and a good data book.

 

Effects of temperature.

Temperature affects the firer, ammunition, and air density. When ammunition sits in direct sunlight, the burn rate of powder is increased, resulting in greater muzzle velocity and a higher point of impact. A general rule is that when a rifle is zeroed, a 20 degree increase in temperature will raise the point of impact by 1 MOA. A 20 degree decrease in temperature will drop the bullet 1 MOA. The key is consistency in shooting environments or knowing how the changes affect your shots when the consistency is off.

 

Effects of humidity.

Effects of humidity vary along with the altitude and temperature. The Sniper can encounter problems if drastic humidity changes occur in his area of operation. If humidity goes up, then the impact of the bullet goes down due to the drag effect and "weight" that the extra water adds to a bullet in flight. The book suggests a 20 % rule of thumb here. Meaning that if your humidity raises 20% from the day you zeroed your rifle, then the bullet will strike 1MOA low.

 

Correction to the above kindly pointed out to me by brad at JBM

1) On the page you talk about humidity, you say in effect that the air is heavier when the humidity goes up. This isn't true -- gaseous water is lighter than air (atomic weight of water is about 18, the atomic weight of air is close to the atomic weight of diatomic nitrogen which is about 28 -- I say about because the atmosphere is also about 20% oxygen which is heavier). Keep in mind that "humidity" is caused by gaseous water in the air, displacing air, for the SAME pressure. If you run some trajectories with varying humidity for the same pressure you can see the density of the atmosphere change -- it's only about 2-3%. I don't doubt that the bullet may impact lower, but I suspect it's because the changes causing the humidity rise may also raise pressure or lower temperature.

 

Hope this helps.

All the best,

Jay.

[brown dog]

Heres one for you wizz kids , if you shoot a cold bore shot and its 3000ft per second , if you keep shooting will the speed of the bullet increase or decrease due to the barrel getting hotter and expanding and the pressure increasing or decreasing due to this?

 

As a function of barrel heating alone? Decrease.

 

As someone pointed out earlier; the dominant effect of barrel heating is to increase internal volume, this in turn decreases pressure and thereby decreases burn rate and thereby decreases mv.

 

 

'Increase' answers are assuming the round is being loaded and left to sit for sufficient time that it heats to the temp of the chamber. ...for that to become the dominant effect, the round would have to sit for quite a long time in quite a hot chamber; one of the purposes (other than rearward obturation) of the cartridge case itself is to act as a heat sink between the chamber wall and the charge. Under 'normal' circumstances (we're not talking machine guns here) your chamber won't actually be that hot and you will have fired long before the chamber has warmed the case sufficiently to warm the charge by any significant amount.

 

Another effect that will cause the mv to change, rather unpredictably, from cold bore is that of 'bore conditioning'. The 2nd round down a squeaky clean barrel travels over a different surface to the 1st.....

 

 

 

The secondary "when my mate leaves his ammo in the sun" question is nothing to do with cold bore. That's being a....

 

....err, no. I'll try to word it as carefully as Ronin :

 

...that would reflect sub-optimal training.

[nemasis243]

As a function of barrel heating alone? Decrease.

 

As someone pointed out earlier; the dominant effect of barrel heating is to increase internal volume, this in turn decreases pressure and thereby decreases burn rate and thereby decreases mv.

'Increase' answers are assuming the round is being loaded and left to sit for sufficient time that it heats to the temp of the chamber. ...for that to become the dominant effect, the round would have to sit for quite a long time in quite a hot chamber; one of the purposes (other than rearward obturation) of the cartridge case itself is to act as a heat sink between the chamber wall and the charge. Under 'normal' circumstances (we're not talking machine guns here) your chamber won't actually be that hot and you will have fired long before the chamber has warmed the case sufficiently to warm the charge by any significant amount.

 

Another effect that will cause the mv to change, rather unpredictably, from cold bore is that of 'bore conditioning'. The 2nd round down a squeaky clean barrel travels over a different surface to the 1st.....

The secondary "when my mate leaves his ammo in the sun" question is nothing to do with cold bore. That's being a....

 

....err, no. I'll try to word it as carefully as Ronin :

 

...that would reflect sub-optimal training.

I dont think I said he left the ammo in the sun Brown Dog , from what he has told me the ammo is stores in vacpac foil strips and opened when needed as they have special types of heads on them

[col48]

you don't have to leave the rounds out in the sun, (that is just one way of heating them up) you could load develop in winter in near freezing conditions, and be right on the edge of pressure and then go to Africa or even experience problems in our summer.

 

ATB

Colin

[brown dog]

Nemasis,

 

Fair point. The move of the discussion to a consideration of charge temp in general, rather than the effect of post cold-bore barrel heating as per your original question was not of your making. Apologies!

 

Col,

 

Not clear where you're going with this:

 

you don't have to leave the rounds out in the sun, (that is just one way of heating them up) you could load develop in winter in near freezing conditions, and be right on the edge of pressure and then go to Africa or even experience problems in our summer.

 

It's universally acknowledged that, all other things being equal, raising charge temp raises burn rate...and hence, pressure and mv. Doesn't really relate to a consideration of the effects of post cold-bore barrel heating!

 

But for a fellow who's knowledge of internal ballistics started with:

 

all so as your barrel heats up the bore size will decrease and this will all so create more pressures. well thats how i see it ATB Colin

 

I think your attempt to offer advice on the subject is 'admirable'.

 

....or should that be 'questionable'?!

[col48]

Nemasis,

 

Fair point. The move of the discussion to a consideration of charge temp in general, rather than the effect of post cold-bore barrel heating as per your original question was not of your making. Apologies!

 

Col,

 

Not clear where you're going with this:

 

 

 

It's universally acknowledged that, all other things being equal, raising charge temp raises burn rate...and hence, pressure and mv. Doesn't really relate to a consideration of the effects of post cold-bore barrel heating!

 

But for a fellow who's knowledge of internal ballistics started with:

 

 

 

I think your attempt to offer advice on the subject is 'admirable'.

 

....or should that be 'questionable'?!

 

brown dog don't be a football, i know it comes natural, but fight it.

 

so i put something down wrong , with people like you on the board I'm surprised anyone try to answer anything.

 

grow up and stop being a key board football,

 

 

ATB

Colin

[brown dog]

Humble apologies if I struck a nerve.

 

[ds1]

Nemasis,

 

A couple of distinctions - which may not help at all......

 

. cold clean bore shot - I can send this high or low, depending what the last thing down the barrel before the last patch was. If it was kroil or a bit of G22 the shot will go high. If it was a de-greaser, it will go low - so I think friction is playing a part. That is ASSumption for what happens in 2 of my rifles.

 

. cold bore shot (not clean) - I don`t shoot well enough to notice it being different from the other shots in say a group of 5 shots. I believe Rifles Only did a study on this - doing cold bore shots, with a variety of rifles and shooters - a strong conclusion was as 308 Panther summed it up - it`s more a cold body shot, down to the shooter rather than the stick.

 

. Now to the question I think you were asking - shots between 5-20, or say 1-20 (I`m thinking of a particular comp where you get 10 shots in 10 mins followed by 10 shots in 3 mins at 300m). Barrels are very warm to the touch by the end - you do see some groups opening up, and others not so - forgetting the shooter, it`s down to the barrel, or rather stresses in the barrel - barrels do "walk" - hammer forged and thin long ones more so than cut rifled short fat ones.

 

David.

[col48]

Humble apologies if I struck a nerve.

 

 

 

one thing is for sure,brown dog, you don't bother me one bit ,iv met a lot of twats in my time,theres room for one more.

feel free to squeeze in,

[silentsoulsleave308holes]

ya learn a lot on the hide

 

disagreements are common place and funny to read most of the time

 

on this forum things can be taken a little serious sometimes and we need to learn to not hide it or remove it as this shows we as sportsmen/women hide things to hide and we dont

 

we are human and have a difference of opinion's and disagreements, so what, even the anti's do, sh!t everybody does its part of life

 

if it gets nasty like personal threats then yeh step in mods

 

to go on a forum and see bits have been removed here and there and for the site to want to pretend all is 100% and we are all like angels would put me off a forum

 

its funny as hell seeing/reading some of the arguments on the hide, makes me laugh, makes me feel normal, makes me aware I'm not the only ass*hole in life

 

I believe, for what its worth, is a forum thats over protective,,,well if even the slightest dig/bad comment is made etc etc we take it more personal than if it went on more regulaly

 

I dont know about the rest of you guys and girly guys but it would be easier going if we were more open about things, have a dig here and there, make it feel more normal then less offence is taken

 

the mods on here do an excellent job but sometimes in their quest to do their job as best as they can I feel the site can be a little over protective (just a little)

 

do you know what I mean/ am getting at????????

 

or am I just being a fookin blancmange?

[brown dog]

Silent,

 

Not sure I agree...but then again, this isn't the Hide

 

 

 

Col,

 

I'm unclear whether your playground offensive language is meant in good humour or not.

 

If nothing else; one thing to take away; calling me a 'football' and a 'blancmange' masterfully covers both ends of the spectrum.

 

If you'd applied the same technique to your concept of barrel heating; stating something such as 'it increases and decreases bore diameter" you'd only have been 50% wrong

 

One to ponder

 

I'd also offer that "oops I messed that concept up, I'd better read up on it" may have been a rather more professional or 'grown-up' response than "na na na you're a football / blancmange"; but each to his own.....at least it explains why you always stick up for Wireviz's posts

 

 

 

David,

 

Your points are spot on. (Again: assuming the bod in question isn't rapid firing his barrel upto 100degC chamber temp and then leaving a round chambered for a couple of minutes!)

 

 

All,

 

Out of idle curiosity, I looked up some 'rules of thumb' in the relevant RMCS publication:

 

Bore diameter increase in response to heating....for a 7.62 it's around 0.028mm per 100degC

 

How long to get to 100degC? At 50rds per min around 1 min.

 

Average differential between bore surface temp and external surface temp? 30degC

 

 

To put that in perspective, anyone care to hazard a guess what temp charge cook-off occurs at?

[col48]

Col,

 

I'm unclear whether your playground offensive language is meant in good humour or not.

 

If nothing else; one thing to take away; calling me a 'football' and a 'blancmange' masterfully covers both ends of the spectrum.

 

If you'd applied the same technique to your concept of barrel heating; stating something such as 'it increases and decreases bore diameter" you'd only have been 50% wrong

 

One to ponder

 

I'd also offer that "oops I messed that concept up, I'd better read up on it" may have been a rather more professional or 'grown-up' response than "na na na you're a football / blancmange"; but each to his own.....at least it explains why you always stick up for Wireviz's posts

 

please take my post any way you see fit. and i will let you have the last word, as it takes you an hole day to come up with.

well that crap above.

all so offering advise on what i should have done,try leading by example, or is it do as i say not has i do.

please read my post it clearly says don't be a football. if you choose to be one don't get up set when you are refaired to as one

 

see ya next time

 

Colin