Accuracy (group size) vs. barrel temperature

[trk]

I've noticed with a CZ (.30-06) that as it heats up the group size increases.

So to maintain a constant starting temperature I put a thermocouple inside the barrel to measure the temperature.

4 rounds will raise the temperature inside the barrel from about 72dF to 94dF.

Letting it cool, I know when it's the SAME temperature to start the next group.


I've got some ideas, but has anyone observed this or has an explanation?

[piwo]

I've noticed with a CZ (.30-06) that as it heats up the group size increases.

So to maintain a constant starting temperature I put a thermocouple inside the barrel to measure the temperature.

4 rounds will raise the temperature inside the barrel from about 72dF to 94dF.

Letting it cool, I know when it's the SAME temperature to start the next group.


I've got some ideas, but has anyone observed this or has an explanation?

Heat is a great detriment to accuracy on several levels. Immediately, the red hot barrel has expanded, which now is being squeezed by the stock, unless free floated. This dramatically changes harmonics. The heat also causes different friction internally, which will drag differently on the bullet as it travels down the barrel. Harmonics change, elasticity of the metal is changed.. and so on. Your observation is why benchrest shooters go to great length to get barrels of pure composition, Cryo freezing experimented with, and a whole slough of other techniques: maintain constant conditions and harmonics.

I recall watching "Vietnam" (based on the Stanley Kubrik book) and watching GI's in a rice paddy pounding away with an M60 machine gun. You could actually see the bullets hitting in the water not far from the barrel, while a guy was taking a helmets full of water on the barrel to cool it. With my 30-06, I notice that my groups start "stringing" on an angle when the barrel gets too hot.

[Mr Peabody]

Heck yeh, I've had most of my sporting weight barrels do that.
Recently I got one that doesn't. That rifle cost 3 times what I'd paid before.
It's a matter of stress trying to relieve itself. It might help to float the barrel;
and it might not. If it's a walk about hunting rifle; 3 good shoots is about all you get anyway. What do you want from it?

[PatMarlin]

My darn Mini-14 shoots cold number 1-3 shots 3" low, then put's em 1" after it warms up.. :roll:

[versifier]

One advantage of shooting cast boolits is that it takes more shots before the POI begins to drift upward, and left or right depending on direction of twist. With a typical "buggy whip" light sporter barrel and jacketed ammo at normal working pressures, three shots is about the limit, then I let the barrel cool before finishing a group for my records. With light cast loads, sometimes I can get the full ten with no problem. It depends on the barrel and the load. Most sporter rifles are made with hunting in mind, so that's no big deal as if you haven't hit it with three shots, it's in the next county anyway. A heavier barreled 98Mauser can handle five shots before stringing becomes noticable, but even if you shot ten rounds as fast as you could reload and work the bolt, you'd still be talking "minute of deer" out to 200yds or so without noticing anything wrong, even though you would see it on paper. That's the reason for heavy match barrels and for fluting, the first to provide more mass to sink the heat away from the chamber area and the second to get rid of the heat as quickly as possible by radiating it from increased surface area. Then there are those fanatics who will shoot one shot a day from a cold barrel over ten days to "really see" how it groups for "serious hunting". For me, if I've let it get too hot to lay my hand on the barrel over the chamber, it's too hot to be shooting any more right then. Offhand practice with a hunting rifle for me is three shots anyway.

[trk]

Good observations.
PIWO - you mentioned different friction internally, harmonics. I think I agree.

It's tough having certainty in assigning reasons, but behaviors are easy to document.

The different internal friction idea is consistant with the expansion of metal rings of small-ID/large-OD and small-ID/small-OD and the effect of heating - which would not occur as radically with a bull barrel.

The rifle is free-floated but not glass bedded.

As it heats up, the POI rises and the group opens up.

[357maximum]

Then there are those fanatics who will shoot one shot a day from a cold barrel over ten days to "really see" how it groups for "serious hunting". .


Only ten days?

[Castaway]

One thing not specifically mentioned is that the barrels are not consistent in how the metal cooled. When heated up, they tend to move in one direction because of expansion is not equal on all sides of the barrel. Cyro can help to a degree, but cooling the barrel between shots is what most mortals like me can afford.

[Bass Ackward]

I've noticed with a CZ (.30-06) that as it heats up the group size increases.

I've got some ideas, but has anyone observed this or has an explanation?


trk,

I can show you 06 loads that the accuracy INCREASES as the barrel temperature heats. In fact, most benchrest matches are shot with warmers to heat up the barrel so they can be shot .... hot and more accurately.

I have a summer time load for an 06 that shoots a 160 grain (cast) spitzer sub MOA at 2600 fps as long as it is above 85 degrees or if I shoot fast enough. At 50 degrees, it's a pie plate killer.

What's the difference? Lube.

You always hear Felix say that the lube must be taylored for the application. So if heating your barrel causes groups to open and the gun normally doesn't open with jacketed at the same temps, there is a good chance your bullets are too soft for your lube, or that there isn't enough lube on your bullet or that your lube is running out of viscosity.

And it's just the opposite in my examples, I need the heat to use or burn up excess lube or viscosity.

[Char-Gar]

shot stringing as the barrel heats is mostly a mechanical problem. Frequently is is due to poor bedding of the barrel in the stock. Sometimes is is due to a barrel that was not straight and has been straightened. When it get's hot it want to go back to it's original bent. It can be a variety of other mechanical factors.

The solution has been known to generations of shooters..just don't let the barrel get hot if you are interested in good groups or scores. You don't need a scientific thingagig..Just place you hand on the barrel. If it is too hot to hold comfortably, then let it cool.

[grumpy one]

Having no experience in the gun industry at all, I end up wondering why barrels aren't stress-relieved after straightening. It doesn't seem to be a very difficult operation. Has anyone any information on whether this is ever done?

[piwo]

Many High end rifle barrel blanks are stressed relieved after being cut, but then they are rifled. This adds its own stress points. That's why when the cryogenic freezing thing came about, it was experimented with widely after barrels were rifled, and chambers cut.
Charger says in his post:
One thing not specifically mentioned is that the barrels are not consistent in how the metal cooled. When heated up, they tend to move in one direction because of expansion is not equal on all sides of the barrel."
I think we were saying the same thing in a different way, or I didn't word my post correctly. When reference of "pure composition", I'm referring to high quality steel, which bench rest barrels are made of. The purity of the steel is the consistency of the steel, and that does dampen uneven swelling. The pressure points mentioned by castaway and others are the root of the problem. In order to shoot through the same hole at several hundred yards, many things have to be in harmony, and moving pressure points of any kind WILL prevent that from happening. A swelling and shrinking barrel is one such thing that will do that.

We have world champion BP shooters here that could probably write books on some of the techniques to dampen harmonics and provide consistency. I read "The Accurate Varmint Rifle, by Boyd Mace many years ago. Even though this is a different discipline, the impediments to consistency with a rifle are universal. Thin whippy barrels, when heated with high power cartridges, swell up and shrink with disastrous result.

[trk]

When I compare two rifles, one with a conventionally tapered (thin) hunting configuration and one with a (thick) non-tapered barrel, I notice that the bull barrel shoots groups of about the same size where the thinner barrel produces groups of increasing size. The question is why. What is the nature of the geometry of the metal to make the groups vary in size as it heats up?

Observing the nature of a steel cylinder with a hole in it as it is heated/cooled, I look at two examples - extremes perhaps.

In a high-school physics class the steel cylinder (about 3" in diameter) had a 1/2" hole cut in it just large enough so that the 1/2 ball bearing would just drop through it. Heated up (the steel cylinder) the metal expanded - the outside getting larger and the hole getting smaller - such that the ball bearing would not pass through - until it had cooled off.

In the automotive trade there is a 'ring gear' around the flywheel (that is engaged by the Bendix gear of the starter. To hold the ring gear onto the flywheel it is heated - because the steel is thin compared to the hole size - the steel expands and the inside and outside diameters of the ring gear enlarge, it is placed on the flywheel and allowed to cool - shrinkfitted onto it permanently.

Is not the first example analogous to the bore diameter near the chamber? And is not the second example analogous to the bore diameter near or at the muzzle (of a tapered but not of a bull barrel)? Hence, the barrel after shooting several rounds would get (SLIGHTLY) smaller near the chamber and larger at the muzzle - with the obvious results.

For me, at this point, this is theory. Measuring it would merely take measuring the bore before and after firing a few rounds to verify it.

[piwo]

The bull barrel, becasue there is more of it, heats at a slower rate. It would take more rounds to heat it to the same temperature of the thin barrel. Anything that bleeds heat, or slows it, will slow any potential detrimental effect of heat vis a vi wood/metal pressure points/harmonics.

[lovedogs]

Boy, have you guys gotten into a can of worms! There are so many variables in this POI change topic that we'll all be in the grave before it's all nailed down and understood. It may make your heads hurt. I take the simple route to success. If it's bedded correctly and shoots I praise God. If it's a walker I know something is wrong, usually bbl. stress from a variety of causes. I get rid of it and look for one that has a properly made bbl. If a bbl. is made right it won't make any difference if it's thin or thick. I've had several very light good quality bbls. that you could have shot until red and they'd still shoot tight groups. As in most things you get what you pay for.

[trk]

piwo - Good point!

lovedogs - sometimes I much prefer to take the Lil' Abner approach ("as any fool can plainly see"). But I'm using this one to learn some principles in making precision shooting ammo - which is forcing me to solve problems. The process is analysing the symptoms and trying alternatives. (It is in a way academic, as the first several rounds out of a cold barrel hit close and consitantly - which is all that's needed for a hunting rifle.)

[lovedogs]

Many moons ago I used to worry my poor little brain about this sort of thing. But as I aged I thought it was "much ado about nothing" and got tired of all the goose chases. There are so many variables in this controversy about bbl. and bullet behavior that by the time I got something like this figured out my gun would be worn out and it would no longer apply if I changed bbls., each bbl. being law unto itself. I then adopted the policy of keeping the good shooters and getting rid of the ones that didn't perform like I wanted.

A little off topic... trk, where in the mountains of Va. are you? I was stationed in Norfolk when I was a young sailor. I've always wondered what it'd be like to go back there and hunt a little. How's the whitetail hunting there? Here in Mt. we've got miles of prairie dog towns; I imagine there you've got 'chucks, huh?

[piwo]

Many moons ago I used to worry my poor little brain about this sort of thing. But as I aged I thought it was "much ado about nothing" and got tired of all the goose chases. There are so many variables in this controversy about bbl. and bullet behavior that by the time I got something like this figured out my gun would be worn out and it would no longer apply if I changed bbls., each bbl. being law unto itself. I then adopted the policy of keeping the good shooters and getting rid of the ones that didn't perform like I wanted.


All comes down to what you enjoy doing I think. Most folks wouldn't spend the time, money or mental energy and trouble learning how to cast, and the intricacies therein. Buying quality arms/components and applying a few stress relieving techniques is far more efficient, cost effective and rewarding for me then buy, shoot, and trade in if it doesn't shoot to my liking. In terms of accurate rifles, it's much to do about everything in my book.

But that's just me: there's no right or wrong, just what one enjoys.

[trk]

...
A little off topic... trk, where in the mountains of Va. are you? I was stationed in Norfolk when I was a young sailor. I've always wondered what it'd be like to go back there and hunt a little. How's the whitetail hunting there? Here in Mt. we've got miles of prairie dog towns; I imagine there you've got 'chucks, huh?

I live in a small town - Pulaski. (Hometown of Count Casmir Pulaski who died of wounds in our Revolutionary war - all kinds of things named after him in the nation). We're SW of Roanoke and Blacksburg (Virginia Tech); about 2000 feet above sea level. About 7 hours west of Norfolk - another half-hour puts one into West Virginia.

Lots of deer and groundhogs (I call 'em woodchucks) here.

[Kraschenbirn]

Just a few observations from my personal experience...

Many, many years ago, I had a Win. Mod. 70 "featherweight" in .270 Win. that would consistently put the first two or three rounds (factory loads, no less!) from a cold barrel into a half-dollar at 100 yards, then, begin walking any following rounds high and to the right in about 1" increments. I tried floating the barrel, bedding the action, changing scope mounts, etc...all to no avail. As I used the piece only for hunting, this never really bothered me all that much.

At the same time, though, I owned a Springfield 03A3 that had been brought up to Nat'l Match specs..."star gauge" barrel, NM sights, bedded action, trigger job...and it wasn't, at all, touchy about barrel temp. Rapid-fire groups off a sandbag rest weren't significantly worse than slow-fire. Go figure...

Currently, I've got a synthetic-stocked, McGowan-barreled, M700 that was cryro'd after assembly. Using my match reloads, the piece shoots consistent 1/2-MOA groups. I don't really know if the cryo job made any significant difference but I've shot the old S&W Masters International (Rifle) Course with it a number of times...40 rounds in 10-round stages-all rapid fire...and it was still hitting where I was pointing when I popped the last cap.

Bill

[trk]

I have several rifles that will walk the rounds in one direction or another, usually vertically, as they heat. But with THIS CZ the groups open up - that's the curiosity.

[BruceB]

Here in Nevada, from about May through September, barrel heat is a fact of life. Even if I set a "hot" rifle aside to cool before firing the next group, it DOESN'T cool worth a damn. Ambient temps in the ninety-plus range have that effect!

So...when I take a bunch of rifles to the range in Der Schuetzenwagen, I'm usually set up to cool those barrels in NO uncertain terms. I carry a one-gallon jug of ice-cubes/water, with a 1/4" plastic line and valve adapted to the spout which allows me to gravity-feed ice water through rifle barrels. The jug is placed on the roof of the van. Fifteen seconds of such water flow gives a COOL/COLD barrel, far below atmospheric temperature.

To clear the water from the bore, I also have an "air pig" under the benchrest, a 15-gallon air tank with 150 psi at the start of the expedition. The 'pig' has a hose and valve allowing me to direct airflow through the barrel. Fifteen or so seconds of airflow removes all visible water from the bore.

This treatment sounds harsh. My digital temp probe tells me that when my fingers believe a barrel is "hot", it is in fact about 180-200 degrees. That is not a whole lot of heat, when speaking of steel. In using this system for several years now, I have noted no ill effects on the barrels, and also the "dressing" of the bore from firing cast/lubed boolits through it seems likewise unaffected.

Since most of my shooting involves many different loads on a single outing, I can't say whether point of impact is affected very much by this routine. There have been enough occasions where I fired similar or identical loads before and after the cooling treatment, that I can say there's no OBVIOUS difference in points of impact from cold to hot and back, and this reflects experience with at least a dozen sporting or military rifles (no true target rifles, in other words).

For MY purposes, I don't worry much about varying barrel temperatures during load testing. I do fire at a regular cadence, usually spacing shots one minute apart, but that still means the barrel temp for the last shot of a string of ten is FAR hotter than that for the first round. Frankly, if my load won't group under those conditions, it's an also-ran.