As a barrel heats up, does the bore expand or contract.Does the actual I.D. of the bore get larger of smaller. When a barrel heats up does the entire barrel expand or does the barrel expand and actually constrict the bore. I was having a discussion with another member And the subject could not be determined.
- Thread starter hogginit
- Start date
Re: barrel expansion
Logically, it should. Practically, it may not be enough to be measurable in a bore as small as a rifle barrel. In large bore canons, it is an accepted fact. Logically, it should have a bearing on bullet transit time. Practically, the difference may be so small that it is masked by normal variances.
Logically, it should. Practically, it may not be enough to be measurable in a bore as small as a rifle barrel. In large bore canons, it is an accepted fact. Logically, it should have a bearing on bullet transit time. Practically, the difference may be so small that it is masked by normal variances.
Re: barrel expansion
dL = L*Cte* dT
dL = change in dimension
Cte = Coefficient of Thermal Expansion
dT = temp difference
The OD and the ID both expand as it heats. One of the ways to fit 2 pieces together with an interference fit is to freeze one and heat up the other. This is especially useful when you want to stick a steel bearing in an aluminum bore.
If you want an idea of how much a bore is going to change, rough numbers on a 30 cal bore are:
L = .300 (bore, not groove)
Cte = 6.5x10^-6 in/*F
dT = 75 *F for a barrel after a few shots.
dL = .300 * 6.5e-6 * 75 ~ 1.463e-4 in
This means the bore goes from .300 to .3001463 inches
Not really an applicable worry IMO
dL = L*Cte* dT
dL = change in dimension
Cte = Coefficient of Thermal Expansion
dT = temp difference
The OD and the ID both expand as it heats. One of the ways to fit 2 pieces together with an interference fit is to freeze one and heat up the other. This is especially useful when you want to stick a steel bearing in an aluminum bore.
If you want an idea of how much a bore is going to change, rough numbers on a 30 cal bore are:
L = .300 (bore, not groove)
Cte = 6.5x10^-6 in/*F
dT = 75 *F for a barrel after a few shots.
dL = .300 * 6.5e-6 * 75 ~ 1.463e-4 in
This means the bore goes from .300 to .3001463 inches
Not really an applicable worry IMO
Re: barrel expansion
As Greg stated above, this is exactly what my friend stated. Artillery rounds strike longer as more rounds are fired. He says he was told in the corps that it, the bore contracted due to the expansion of the barrel. Now, Bohem's math shows different? Who is correct? Are both correct? Does it depend on the size of the bore and material density of the barrel?
As Greg stated above, this is exactly what my friend stated. Artillery rounds strike longer as more rounds are fired. He says he was told in the corps that it, the bore contracted due to the expansion of the barrel. Now, Bohem's math shows different? Who is correct? Are both correct? Does it depend on the size of the bore and material density of the barrel?
Re: barrel expansion
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: hogginit</div><div class="ubbcode-body">As Greg stated above, this is exactly what my friend stated. Artillery rounds strike longer as more rounds are fired. He says he was told in the corps that it, the bore contracted due to the expansion of the barrel. Now, Bohem's math shows different? Who is correct? Are both correct? Does it depend on the size of the bore and material density of the barrel? </div></div>
We're both correct. For a small bore like a small arms rifle such as the 308 the difference is meaninglyess.
If you rerun the numbers I showed abovve for a 155mm bore and get the barrel 150*F-200F hotter than ambient you'll see a large change in bore size.
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: hogginit</div><div class="ubbcode-body">As Greg stated above, this is exactly what my friend stated. Artillery rounds strike longer as more rounds are fired. He says he was told in the corps that it, the bore contracted due to the expansion of the barrel. Now, Bohem's math shows different? Who is correct? Are both correct? Does it depend on the size of the bore and material density of the barrel? </div></div>
We're both correct. For a small bore like a small arms rifle such as the 308 the difference is meaninglyess.
If you rerun the numbers I showed abovve for a 155mm bore and get the barrel 150*F-200F hotter than ambient you'll see a large change in bore size.
Re: barrel expansion
If the bore expands with rounds, it might make for less friction? Thus more velocity? Or, the heat of the entire cannon might heat the powder prior to ignition, making for more pressure?
If the bore expands with rounds, it might make for less friction? Thus more velocity? Or, the heat of the entire cannon might heat the powder prior to ignition, making for more pressure?
Re: barrel expansion
If the expansion is so small why do groups open up so much. After the third shot my groups seem to get wild and inconstant.I always blamed on heat, it must be something else. I am talking 8 or 10 inches.
If the expansion is so small why do groups open up so much. After the third shot my groups seem to get wild and inconstant.I always blamed on heat, it must be something else. I am talking 8 or 10 inches.
Re: barrel expansion
I have seen from experience (from the receiving end) that as canon barrels heat, rounds do not go further, they drop shorter. Probably saved my young butt on several occasions back in Cua Viet.
As bores expand, they present less resistance, so pressures do not build as high. The net result is less velocity.
Groups opening up generally mean that the rounds are getting heated in a hot chamber before firing. The energy in a cartridge includes the energy contained in the propellent's initial temperature. Cold propellent and warm propellent each heat up by the same number of degrees when they are combusted, but propellent that starts 100 degrees hotter also ends up 100 degrees hotter, so more overall energy gets released into the bore behind the projectile. My suggestion about your load is that it might benefit from a small charge reduction.
Greg
I have seen from experience (from the receiving end) that as canon barrels heat, rounds do not go further, they drop shorter. Probably saved my young butt on several occasions back in Cua Viet.
As bores expand, they present less resistance, so pressures do not build as high. The net result is less velocity.
Groups opening up generally mean that the rounds are getting heated in a hot chamber before firing. The energy in a cartridge includes the energy contained in the propellent's initial temperature. Cold propellent and warm propellent each heat up by the same number of degrees when they are combusted, but propellent that starts 100 degrees hotter also ends up 100 degrees hotter, so more overall energy gets released into the bore behind the projectile. My suggestion about your load is that it might benefit from a small charge reduction.
Greg
Re: barrel expansion
When the South rises again, it may have company.
When the South rises again, it may have company.
Re: barrel expansion
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Greg Langelius *</div><div class="ubbcode-body">I have seen from experience (from the receiving end) that as canon barrels heat, rounds do not go further, they drop shorter. Probably saved my young butt on several occasions back in Cua Viet.
As bores expand, they present less resistance, so pressures do not build as high. The net result is less velocity.
Groups opening up generally mean that the rounds are getting heated in a hot chamber before firing. The energy in a cartridge includes the energy contained in the propellent's initial temperature. Cold propellent and warm propellent each heat up by the same number of degrees when they are combusted, but propellent that starts 100 degrees hotter also ends up 100 degrees hotter, so more overall energy gets released into the bore behind the projectile. My suggestion about your load is that it might benefit from a small charge reduction.
Greg </div></div>
The latent heat required to actually make the propellant burn is lower when the propellant is preheated, it takes less time to get the power kernels burning and the preheated kernels burn faster as well. Similar to preheating the fuel in a rocket engine by running it around the nozzles to act as coolant, it makes the reaction more energetic compared to running it without this preheat cycle.
As a barrel heats and groups open up as much as the OP is mentioning I'd be hesitant to blame bore expansion, I'd blame the internal stresses of the barrel causing it to deflect in strange manners.
I do know that if I take my 208 Amax and RL22 loads from the 30-06 and let the round "cook" in the chamber I pick up several hundred fps sometims, I've seen cooked rounds come out at 3050+fps from a 26" barrel when the normal strings are in the 2875 realm. That extra 175fps causes less wind drift and a significant vertical dispersion at long range.
The barrel shoots very well both hot and cold, but if the ammo warms up significantly the temp sensitivity plays all kinds of havoc on the vertical dispersion.
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Greg Langelius *</div><div class="ubbcode-body">I have seen from experience (from the receiving end) that as canon barrels heat, rounds do not go further, they drop shorter. Probably saved my young butt on several occasions back in Cua Viet.
As bores expand, they present less resistance, so pressures do not build as high. The net result is less velocity.
Groups opening up generally mean that the rounds are getting heated in a hot chamber before firing. The energy in a cartridge includes the energy contained in the propellent's initial temperature. Cold propellent and warm propellent each heat up by the same number of degrees when they are combusted, but propellent that starts 100 degrees hotter also ends up 100 degrees hotter, so more overall energy gets released into the bore behind the projectile. My suggestion about your load is that it might benefit from a small charge reduction.
Greg </div></div>
The latent heat required to actually make the propellant burn is lower when the propellant is preheated, it takes less time to get the power kernels burning and the preheated kernels burn faster as well. Similar to preheating the fuel in a rocket engine by running it around the nozzles to act as coolant, it makes the reaction more energetic compared to running it without this preheat cycle.
As a barrel heats and groups open up as much as the OP is mentioning I'd be hesitant to blame bore expansion, I'd blame the internal stresses of the barrel causing it to deflect in strange manners.
I do know that if I take my 208 Amax and RL22 loads from the 30-06 and let the round "cook" in the chamber I pick up several hundred fps sometims, I've seen cooked rounds come out at 3050+fps from a 26" barrel when the normal strings are in the 2875 realm. That extra 175fps causes less wind drift and a significant vertical dispersion at long range.
The barrel shoots very well both hot and cold, but if the ammo warms up significantly the temp sensitivity plays all kinds of havoc on the vertical dispersion.
Re: barrel expansion
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: hogginit</div><div class="ubbcode-body">If the expansion is so small why do groups open up so much. After the third shot my groups seem to get wild and inconstant.I always blamed on heat, it must be something else. I am talking 8 or 10 inches. </div></div>
What kind of weapon and was it a light or heavy barrel?
If it's a free floated heavy barrel I would definitely look somewhere else.
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: hogginit</div><div class="ubbcode-body">If the expansion is so small why do groups open up so much. After the third shot my groups seem to get wild and inconstant.I always blamed on heat, it must be something else. I am talking 8 or 10 inches. </div></div>
What kind of weapon and was it a light or heavy barrel?
If it's a free floated heavy barrel I would definitely look somewhere else.
Re: barrel expansion
700p in 7mmRM
700p in 7mmRM
