Centerfire bolt-actions and rate of fire versus heat

mr00jimbo

mr00jimbo

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I know that guys who sight in their hunting rifles wait a while between shots to allow their barrels to cool. Alternatively, guys who used to use the Springfield 1903, Lee Enfield, K98, Arisaka and Mosin Nagant rifles in war were likely quite rapid in their follow-up shots.
And then there's modern varmint rigs, that have long, heavy barrels, I'm assuming in part due to rapid shots on nuisance gophers and the like.

Which makes me curious...what factor(s) indicate how fast you can shoot a bolt-action centerfire without overheating it? Accuracy suffers with a hot barrel and apparently it's no good for the gun either, is that correct?

But if you take two of the same gun in different configurations, like the Remington 700 CDL (hunting rifle) and the Remington 700 5R or Police, or a 700 Varmint rifle, is the former going to need more cooling between shots and the latter few going to be good to go to fire in rapid succession?

Does barrel thickness dictate how many shots you can take without overheating the barrel of the gun? Are there other factors involved when making a bolt-action centerfire gun right for multiple shots per minute?
 
As does how hot a load is, what the outside temp is, etc...
I still don't know if I believe in the fluted barrel theory, however some do.
 
The theory behind fluted barrels is more surface area to help with air cooling (think of cooling fins) and increased barrel stiffness compared to a round barrel of the same diameter.

Personally I don't like fluted barrels.

Round or octagon depending on the gun for me. I'm also ok with the flat sided ruger .22 pistol barrels and a few others, but I don't like flutes.

It's always possible to heat up ANY barrel on ANY firearm if you shoot them rapidly for long enough.
 
Fluted barrels are stiffer than a round barrel of the same weight, not the same diameter. For a given diameter, the round barrel will be stiffer.

The increase in surface area due to flutes is not sufficient to improve cooling significantly.
 
The heavier (because it's thicker) barrel simply has more mass of metal to heat up before it is affected, but eventually it will be if you fire enough shots rapidly enough. The thicker barrels of service rifles are that way so they can stand more rapid fire before the heat builds up enough to affect accuracy, but they also aren't designed to be as accurate as hunting guns in the first place.

And for any barrel, at a certain temperature the wear in the bore for each shot may be increased, so don't fire extended applications of rapid fire unless you have enough reason to do so that justifies to you the risk of increasing wear. But it's your gun, your money, use it the way you want to, just understand what you are using it for/spending it on.
 
I have a 700 in 308 with a barrel profile much like a 5R, but ending at 20inches that I regularly use for rapid shots. 10 rnd and try for 10 aimed shots in under 25 seconds. I used to use it in Service Conditions competition among other things. It gets hot, and you have to lube it, but it shoots really well.

If you want a bolt action rig that you can shoot rapidly, get the thickest barrel you can comfortably field.
 
Something that isn't being mentioned is the caliber/powder capacity.

A slower bullet like a 170gr out of a 30-30 will generate a lot less heat than a 140gr out of a 7mm RUM or similar overbored cases. As TheTooner states this is all fine and dandy as long as you understand what you are spending your coin on. Ripping out 10 rounds of a 30-30 isn't that big of a deal, but try slamming 10 rounds out of that 7 RUM without being able to boil water on the barrel afterwards. Most rifles are in between these two extremes.

I think to really get your answer you have to define what "overheating" means to you.

Do you want max accuracy? Then you should chart how many rounds it takes to open up and somehow measure the heat of the barrel, even if its just by grabbing it. You'll also want to know how long on average it takes to cool down.

Do you want to maximize your barrel life? That's a tough one too because there are so many factors involved that are poorly measured, but it is safe to say just keep it "cool" (non specific term again) and you can minimize throat erosion.
 
So the temperature of the inside 2mm of barrel on a heavy barrel is going to be a lower temperature than the inside 2mm of a featherweight barrel, ceteris paribus?

Or does the heavy barrel just feel cooler on the outside longer because you can't touch close to the inside diameter?

If solid metal cools cylindrical voids which contain explosions faster, then why do they put cooling fins on engines? Why not make 'heavy blocks'?
 
philthygeezer said:
So the temperature of the inside 2mm of barrel on a heavy barrel is going to be a lower temperature than the inside 2mm of a featherweight barrel, ceteris paribus?

Or does the heavy barrel just feel cooler on the outside longer because you can't touch close to the inside diameter?

If solid metal cools cylindrical voids which contain explosions faster, then why do they put cooling fins on engines? Why not make 'heavy blocks'?
Click to expand...

Yes the inner 2mm is cooler on a heavy barrel because that extra mass acts as a heat sink. It's doing the same thing as fins on a diff or block, but is less efficient, still functional.
 
cam1936 said:
Yes the inner 2mm is cooler on a heavy barrel because that extra mass acts as a heat sink. It's doing the same thing as fins on a diff or block, but is less efficient, still functional.
Click to expand...

But if you take into account surface area to the insulatory effects of volume, how does that work out compared to a 'heat sink' effect? Is a thin block of steel not going to cool faster than a thick block of steel?

Where are the experiments and the graphs? :D
 
The thin block of steel will cool faster than the thick one, but it will also heat up faster. Heating up faster will be a problem for accuracy with your thin barrel before cooling slower will be a problem for your thick one. With more metal to dissipate the give amount of heat (same number of rounds fired at the same frequency in each barrel), the thick barrel is less likely to get to the temperature that will affect accuracy or increase wear.
 
Three shots in a hunting rifle pretty well sums up the saying . . . one shot is a hit, two shots is a maybe and three shots is a miss.
There is little concern in my opinion when firing three shots. My procedure is to single load, spot my shot, reload, etc. and take about two minutes for three shots.
When you rip off three shots in rapid succession, a spotter is required.
Competitors firing ten to 15 rounds in a timed event is different. and there cadence would not normally be rapid.
 
My theory has always been to buy more guns then you don't need to shoot ONE so much you have to worry about heating, throat erosion or any of the other maladies associated with not having enough guns. JMHO ;)
 
Barrel thermodynamics aside for the moment stop and consider how the different types of guns are used in actual practice.

  • The hunting rifle is carried a lot and shot once or maybe twice to take down the animal. The all important, and hopefully ONLY shot, will always be through a cold bore.
  • Military rifles are often going to be shot multiple times during any engagement at moderate to rapid fire rates. In such use the barrel will become very hot.
  • Varimint rifles are reasonably often shot at moderate rates where the barrel will heat up but likely won't get overly hot.

For the hunter sighting in their rifle the cold bore first shot becomes the ultimate test. A proper sighting in should see the scope be sighted then let the gun sit for a good amount of time. 30 minutes or more to fully cool would not be out of the question. Then the next shot would truly be a cold bore shot for that day's conditions. And ideally the hunter would do their sighting in and cold bore follow up on some cold and wet days that would mimic the conditions where they go hunting.

A varmint shooter clearing out groundhogs or similar should likely take note of where their first cold bore shot goes and during an active varmint session they can shift the sight point a little for the first shot of the day. But generally they are going to set up in a fairly target rich environment so much of their varmint control is going to be a case of fairly regular shooting. On the other hand those that go after bigger varmints and farm pest animals might be in the same situation as the hunters and rely on that important first cold bore shot.

Something to consider is that steel is a TERRIBLE conductor of heat. So a bull barrel can actually hold the heat in and result in the bore walls being hotter for a given firing rate than a thinner profile. The thinner barrel might have less surface area but the thinner walls will let the heat reach the outside surface that much sooner where the air can carry it away. The bull barrel will take a little longer to get up to temperature but once the firing rate is high enough the bore walls of the bull barrel will actually end up hotter than the slim barrel at the same continuous rate of fire.

Adding flutes does two things for the heat transfer. First off it increases the surface area so the air has more room to work on. But likely more importantly is that the flutes reduce the distance that the heat has to pass through the metal to reach the outside surface. In effect the flutes make the big bull barrel act a little more like a slim barrel.

Some barrels use a composite makeup of aircraft aluminium outer with a slim steel liner. If done right this can be the best of both worlds. You've got the good steel liner that has thinner walls than even a slim one piece barrel and you've got the aluminium outer jacket to add stiffness and which is an excellent conductor of heat. Then on top of this if the aluminium outer is also fluted then the fast thermal transfer rate through the aluminium makes the flutes act more like the cooling fins that most folks consider them to be.

The key to this composite setup working well is that there must be a good contact between the liner and the jacket. This can be either a good metal to metal contact or it can be a thin layer of a good heat conductor such as the thermal paste used in electronics. Some makers have missed this last bit. Currently Ruger makes the Mk III LITE with a tensioned liner in the aluminium outer frame. But they included an air gap around much of the liner which only touches the outer frame at the chamber and muzzle. I've read reports about leading that appear to be due to the bore becoming overheated during long term rapid use. Filling the gap with heat transfer grease would most likely make this issue go away.
 
TheTooner said:
The thin block of steel will cool faster than the thick one, but it will also heat up faster. Heating up faster will be a problem for accuracy with your thin barrel before cooling slower will be a problem for your thick one. With more metal to dissipate the give amount of heat (same number of rounds fired at the same frequency in each barrel), the thick barrel is less likely to get to the temperature that will affect accuracy or increase wear.
Click to expand...

Not sure i believe this. The rigidity and accuracy part is fine, but the thermal part leaves me questioning.
 
philthygeezer said:
Not sure i believe this. The rigidity and accuracy part is fine, but the thermal part leaves me questioning.
Click to expand...

It's a function of something called specific heat:
http://en.wikipedia.org/wiki/Specific_heat

Basically, a mass of any given material requires a certain amount of heat energy to raise its temperature by a given amount. One pound of water requires one BTU of energy to heat it by one degree Fahrenheit, for example. Increasing mass correspondingly increases the amount of energy required to increase the temperature of the heavier object by the same amount as the lighter one.

Compared with a lighter barrel, the heavier barrel has to first absorb more energy to get to a certain temperature and then dissipate that same larger amount of energy in order to cool back down to ambient temperature.
 
tjhaile said:
It's a function of something called specific heat:
http://en.wikipedia.org/wiki/Specific_heat

Basically, a mass of any given material requires a certain amount of heat energy to raise its temperature by a given amount. One pound of water requires one BTU of energy to heat it by one degree Fahrenheit, for example. Increasing mass correspondingly increases the amount of energy required to increase the temperature of the heavier object by the same amount as the lighter one.

Compared with a lighter barrel, the heavier barrel has to first absorb more energy to get to a certain temperature and then dissipate that same larger amount of energy in order to cool back down to ambient temperature.
Click to expand...

Yes, but doesn't this assume you are raising the whole barrel temperature evenly? Aren't we interested in the first 1mm off the rifling if barrel life is the concern? I'm still not convinced a lightweight barrel doesn't cool at the rifling quicker. Also, what is the bore temperature of a heavy barrel given the same temperature on the outside of a .920 and a .565 diameter bbl? Is it higher than the soda straw?
 
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