cooling the bore between shots

I did the same with my Tikka at the range.

I've seen guys keep a towel in a cooler with ice water. Leaving the rifle on the rest they drape the cold towel over the barrel between strings.

I generally take at least 2-3 centerfire rifles and a 22. After shooting all of the centerfires and the 22 for a few minutes the first gun fired is usually cool enough without any help.

On extremely hot days I have been known to keep the truck running with the AC on high and place just fired guns inside for 5 minutes or so.

I would be worried about warping the barrel, as some people say will happen if you pour water down the bore for example. If I'm shooting for groups I'm probably not in a rush, so I'd rather let the barrel cool slowly and, more importantly, evenly.

When I need to let my barrel cool down, I'll stand it up in one of the racks at the range. This lets the barrel act as a sort of chimney and pulls cool air in from the bottom and heat out from the muzzle.

I shoot 3 shot groups and then let it cool for 5 minutes of so before doing it again. I've never had problems with accuracy degrading. Taking more than one rifle helps so you're not sitting there waiting.

I take 2 or 3 guns and alternate when shooting for tiny groups on paper, but honestly if he gun walks vey much with a hot barrel I just trade it off and get another rifle. A properly manufactured barrel isn't going to walk much.

Shooting for groups, like load testing, I shoot 3-5 shot groups, then move on to a different rifle, or do pistol drills or something until it cools naturally. I've seen engine blocks and cast iron skillets crack when cooled too rapidly, so I'd not want to try it with a rifle barrel. Even if it didn't crack, (it is good steel, not cast iron after all,) it could still warp, which is no good. Better to cultivate patience than replace a barrel before you have to.

When doing normal practice, that is, shooting from field positions, I go by the rule, "If it's too hot to touch, it's too hot to shoot."

I should add that I don't spend a whole lot of time on load development. I pick a bullet and a powder, then find the charge that shoots best. If you've gotta try five or six bullets for each weight, and eight or nine powders for each bullet, youre likely to spend all your time on the bench chasing MOA unicorns, and wind up with half (or all) of your barrel's useful life gone before you do anything useful with it.

For field shooting, the difference between a.75 MOA load and a 1 MOA load isn't one you can really appreciate anyhow. Find a load that shoots, then crank out a bunch of 'em and get off the bench.

Just my speculation, but it seems to me the barrel would have to be very, very hot before rapid cooling would warp it and if it were that hot, it's probably shot anyway. Again, just my simpleton speculation.

One thing about steel is that it's a very poor conductor of heat as far as metals go. So the inner bore wall can easily be a good 30 to 50 degrees or possibly more hotter than the outside and you won't feel this new added heat for up to maybe a minute before the temperature evens out throughout the wall. The actual time will vary based on the thickness.

So it's possible that you can end up with a very hot inner bore wall inside that is trying to expand the metal while at the same time you're cooling and causing the outside metal to contract with the ice pack.

So first off it would be wise to wait a good 45 seconds to even a minute before you apply the ice pack. Ask your metallurgy friend about the forces that can be generated in the steel when a hot side and a cold side are existing in a wall that is 3/16 to 1/4 thick steel. Or even better put a piece of 1/4 x 1 flat bar in a vise and set up an indicator pointer at the free end attached to something else. Then heat the bar up with a torch to where it's just past being too hot to touch then apply your ice pack to the opposite flat side. I think you'll be shocked at how far the flat bar will curl due to the hot side to cold side expansion and contraction issue. And the length of time it takes before it equalizes and returns to straight.

So all in all if you want to use the ice pack idea I'd suggest that first off you wait for a good minute after the last shot before applying it. That'll reduce the extreme of the hot inner bore to cold outer surface tension in the walls of the barrel. And secondly that you apply the bag to take away the majority of the heat but then let it sit for a good minute or more to let the whole barrel equalize again. Being "just warm" on the inside while cold on the outer wall is just as bad for accuracy as hot.

Another issue that seems to be seldom addressed is the insulating properties of the fore end of the stock. And since you can't ice pack that area of the metal again it's going to take a good minute or more before the coolness you put into the exposed part of the metal can work it's way down into the depth of the barrel channel and the metal becomes a constant temperature throughout.

Another good "table top" experiment to demonstrate the thermal conductivity of steel is to get some steel and aluminium plate that are the same thickness and roughly the same size. On a range element set to high put the aluminium plate down and touch the top and take note of the time it takes before you can't touch it any longer. It'll be literally only a few seconds. Do the same with some steel and you can easily leave your finger tips on the metal for a good 4 to 6 times as long. Another way to test this is with 3 inch lengths of 1/8 diameter welding rod in aluminium and steel and also get a piece of 14Ga copper the same length. While holding the one end stick the other into a propane torch flame. The aluminium and copper will become too hot to hold within 30 seconds. The steel may not get too hot at all. It's such a poor conductor of heat that it actually dissipates into the air before it can get to your fingers.

If you try these easy to do tests you'll quickly realize why I'm suggesting that even with a cold pack that you still want to wait that couple of minutes before shooting again. Otherwise you could have a barrel that is still hot on one side in the barrel channel and cool on the outside where you just removed the ice pack and if you measured it you'd find that it looked like a ski. But once the temperatures equalize it straightens up again. But that can take up to a couple of minutes.

There is actually a lot to be said for the "chimney" idea of shooting a string then rack the rifle with action open so the warmth in the bore can produce a simple chimney effect to let the warm air out the muzzle while drawing in the cool air. It's surprisingly effective.

I rarely shoot for groups. I zero, and move on to practical field positions.

When I zero, I'm going into it with the expectation that when I take a shot in the field, it'll be a cold bore. So I take my first shot, wait about 10 minute (or until the bore is cool again), then take the next, and carry on until I have three shots. Then I adjust, and repeat. If I'm doing that, I'll usually bring a couple other guns to plink with while I wait.

We use flame straightening and flame bending in metal work all the time.

If metal is hot enough and is rapidly unequally cooled it will definitely bend. We use this property to accomplish bending and straightening all the time.

But high temp is important, generally around the 1200 degree range, or when you can just barely see faint reddening in low light. Steel does not need to be glowing bright red hot to take a permanent bend. In fact good bending / straightening procedures prohibit this.

The US DOT has a great website the covers recommended heating and straightening procedures for highway / bridge members that are crash damaged.

In their document it is noted that It is generally considered safe to use accelerated cooling once the base metal has cooled to below approximately 600 F, but accelerated cooling from high temps is not done as it can lead to uncontrolled and unpredictable hardening.

A rapid quench from a high temp can lead to hardening, even on mild steel. For this reason I would be VERY cautious about cooling a gun barrel, and I wouldn't even think about speeding the cooling from a very high temp.

I shoot 3 shot groups and then let it cool for 5 minutes of so before doing it again. I've never had problems with accuracy degrading.

Click to expand...

Most of the year that works just fine. But in August and September when it is 100+ it takes a lot longer than 5 minutes. Without something to aid in cooling it can be closer to 20 minutes. I've seen several use the wet towel with no issues. I've never had a problem with placing guns inside my truck for 5-10 min with the engine and AC running.

I've never tried it, but have wondered about spraying canned air down the barrel. It won't leach heat like a cold liquid and seems like it could work. Any one tried it?
RT

^ Does the canned air come out cold? If so, condensation may be an issue.

Ever try using a thermometer?

Last range session, I tested about 50 rounds of 308 Win fired from my CZ-550 Varmint (heavy barrel) bolt action rifle.

I was using an inexpensive laser sighted IR thermometer, and I have not calibrated the thermometer

http://www.harborfreight.com/infrared-thermometer-93984.html

Measurements were made with the thermometer held about 12 inches from the barrel at a point immediately adjacent to the rifle chamber. During all testing, the barrel felt warm to the touch, but never hot or uncomfortable to the touch. Time between shots, bolt open, was used to unload, inspect brass and replace brass in the shell box, record shot velocity, record barrel temp, reload, and sight the next shot, typically about 2 min between shots.

F° Time Comments
79.4 Ambient air temp
85.4 13:48 "Cold" barrel temp
85.2
84.5
84.5
84.1 13:59 Cease fire, reset targets, adjust chronograph



84.5 14:06
84.1
83.9
84.1
83.6
82.8
83.1
82.8 14:21

I have additional temp readings for the session, the highest being 87.8, but I quit recording times with the temps. I believe my barrel was heating as much from being in the sunlight as from the rate of fire of 308 rounds, all about 2550 - 2575 FPS. The CZ-550 Varmint's barrel never got anywhere near "too hot to hold" with what I consider a normal rate of fire while testing various rounds. There was a light, variable wind, but no additional effort was made to cool the barrel between shots. The barrel temps were just about constant throughout the test firing.

it is my belief that while the rate of heat conductivity in steel is, perhaps " slow" in the metallurgical spectrum, that the overall mass of the chamber and barrel, and hence, the total heat capacity of the rifle dictate both the rate of heating and cooling, more so than other factors. At a reasonable rate of fire, with a heavy barrel 308 win, shooting at velocities around 2600 fps, barrel overheating isn't going to be a problem. There is good reason for the military calibers typically not being "over-bored".

The critical area for heat damage to a rifle is the chamber's throat and first few millimeters of the rifle's lands, if what I read is correct. The most potential damage is done by rapid firing of "hot" loads in an over-bored caliber rifle. popping off 3 or 4 quick shots in, say, a 243 win with loads in the 3000+ fps range very quickly builds high temps in those critical areas, with the forces of each successive shot acting adversely on those overheated metal surfaces.

Imprudent varmint hunters might "burn out" the throat of a barrel in a single shooting session. But a modest rate of continuous fire with even just a minute or so between shots doesn't worry me at all with the CZ550 Varmint in 308. I'd be much more worried about the potential adverse effects of shooting a relatively quick string, then trying to cool the barrel as quickly as possible to do it again, especially if the cooling were uneven, in a relatively light barreled rifle with hot, over bored caliber loads. That can't be good.

PS. Here's one of the 5-shot, 100 yard groups I shot that Sunday afternoon:

shekarchi said:

My experience shows that a hot barrel shoots erratically [this is a hunting rifle- not varmint or heavy barrel].

I came up with a solution that I would like to share. I carry a cooler of ice with some ziplock bags to the range. after 3-4 shots when the barrel needs to cool off I wrap a ziplock bag with ice cubes over the barrel and move it up and down the barrel. in a couple of mniutes I am ready to shoot with a nice cool barrel.

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Couple things here, which stand out to me:

1. You describe the rifle as a "hunting rifle". In my hunting experience (small game and varmint), a hunting rifle doesn't see enough action to significantly heat up a barrel between shots. So this really isn't a concern at all if the primary use of the rifle is for hunting. That said, if your goal is to tweak the most accuracy out of your rifle over several consesecufive closely spaced shots at the range, more power to you. After all...shooting is fun!

2. If barrel temperature increases are enough to affect accuracy significantly over the course of a few shots, as you describe, then the rapid localized cooling caused by applying an ice pack may also significantly affect accuracy, only potentially in a far more adverse way. Rapid localized cooling may cause barrel warpage which might become permanent. So I'd be careful of this.

Just my opinion.

I've also experimented with a chilled bore snake. It was slow cooling alright. Abandoned it, too messy and wet.

YZ said:

I've also experimented with a chilled bore snake. It was slow cooling alright. Abandoned it, too messy and wet.

Click to expand...

Hmmm...now that opens up possibilites, though...

Take a shell casing, fit it up with a nylon tube through the primer hole location, put an O-Ring around the base of the casing...then fit the shell into the breech and pump water through the barrel under pressure until it's cool!

Then swab down afterwards.

Of course, this might be carrying things to the extreme...

In their document it is noted that It is generally considered safe to use accelerated cooling once the base metal has cooled to below approximately 600 F, but accelerated cooling from high temps is not done as it can lead to uncontrolled and unpredictable hardening.

Click to expand...

In general, that sounds pretty safe to me with regards to cooling.

Making an educated guess, I wouldn't expect there to be any permanent distortion as a result of cooling a warm (not hot) barrel with ice packs, assuming the barrel is of decent quality.

Thermally cycling steel shouldn't result in substantial microstructural changes up to 200~250F would cause warping upon cooling. I certainly wouldn't expect any effect from short excursions to temperatures a bit below the tempering temperature, which is typically in the 750F+ range for 4140 depending on the desired hardness.

I would not expect a thermal gradient substantial enough to cause permanent warping from casual cooling using the ice packs, but stranger things have happened.

Of course this could vary depending on the alloy, the heat treat process used to obtain its final state (e.g. stabilization, existing stresses), dimensions/geometry, etc.

Let us know what the metallurgy professor says when you hear back. I would definitely be interested in hearing his thoughts.

I tried cooling my 308 by putting it over the duct in my house and running the central air.The rifle is very accurate,but after a couple of 5 shot groups,accuracy would fall off.After cooling it,the POI had changed by well over an inch.Tried it again...same result.I would conclude that this was caused by uneven cooling.The barrel was warm in the stock,but cool to the touch where the metal was exposed.In my early reloading days,I loved to shoot a lot of rounds and see the result of my work.Nowadays,I just take a couple of guns out at a time.Or even better have a buddy so there's somebody to BS with while they cool.Sporter barrels will do crazy stuff when they're hot,and if you try to sight one in hot,it'll most likely shift POI when it cools down,especially if not free floated.There's my 2 cents worth,even ifn it hain't worth 2 cents.