Wandering Shots as Barrel Heats Up

[Scorch]

All barrels have the initial "hole" gun drilled through the bore, even hammer forged barrels BUT the hammer forging process WOULD probably make the best barrels in regard to consistently centered bores IMHO

Hammer forged barrels vary in manufacturing processes. Some start out as a spud approximately 6"-8" long that is hammered and formed around a mandrel until it is the length of the finished barrel, the metal is heated by the hammers and is quite malleable buring the process. Others start out as a tube and are hammered down onto a mandrel. Some barrels are even chambered during the hammer forging process.

About 1970 started the use of cold hammer forged barrels and almost all are made that way now.

Hammer forging was invented in Germany about the time of WW2 and used extensively there. In contrast, here in the US we used broach cutting for barrel making at that time, which was slower and much more equipment intensive. It wasn't until the 1950s that hammer forged barrels started showing up in European sporting arms here in the US (FN, Husquvarna and Sako were first, I believe).

 

[warbirdlover]

Straightening if done correctly will not have too much affect. If the barrel is extremely warped it should not be straightened but thrown away. In that senario the straightening could weaken the barrel. Modern straighteners no longer rely on operator skill. The only bend what is necessary to get the barrel straight. If a barrel is too bad to start with the straightening will put extreme stresses in the barrel (form of cold working). Some companies still use the manual straighteners which are okay with skilled operators but that skill is hard to learn. Believe me, I've done straightening.

 

[wyop]

Given that the barrel was probably turned on a lathe between centers to get it from the blank OD (1.255 or so for many rifle barrel blanks) down to the final OD dimension, you're not going to see the barrel wobble unless you really spun it up in RPM - perhaps over 1,000 RPM for thinner barrels.

At that rotational speed, you're going to have to figure out how you indicate the runout... mechanical indicators will just vibrate pretty wildly, over-shooting on either end of the indicated range. Electronic indicators will not be able to display anything useful.

 

[Jimro]

Wyop,

Use a dial indicator with a dimension stop. That way you get a "maximum runout" measurement, no matter how violent the wobble.

Also, the closer you get to centers the the less the wobble will be (stabilized) if the bore is concentric. If the wobble is at the ends near the centers, then you have an off axis bore.

Jimro

 

[Bart B.]

I asked the manager of the machine shop where I worked about this. He said to bring in a barrel (no action or any other part of the rifle) and he would show me something. So I brought in a 30 caliber Obermeyer cut-rifled blank heavy match barrel and a factory Win. 70 .30-06 hammer forged sporter barrel.

Ater work hours, he laid the Winchester factory barrel ends on surface plates atop a granite gauging table. Each end of the barrel was on a very smooth, perfectly flat surface. The barrel slowly rolled until one side was down. Removing then replacing the barrel with that down part up to one side or another ended up with the same point down on the surface plates. He said the heavy side of the barrel made it move down from gravity. That barrel's bore was enough off center in its profile that one side had a weight advantage.

Then he did the same thing with the Obermeyer barrel. It barely spun after laying it on the surface plates at each end. That master machinist said it was very well made with the bore well centered on the barrel's profile and there wasn't enough mass on its heavy side to overcome the surface friction between the barrel and the surface plate to let gravity have its way and pull its heavy side down.

He also said there were "friction free" bearings that could be mounted such with centers in them that would show the same thing on any thing. Its heavy side would always move down. Such would be better for rifle barrels as their surfaces tension due to minute roughnes at their edges would have enough friction that exact unbalance amounts may not be visible.

 

[big al hunter]

Bart, the table testing you described will also show a bent barrel. Did he do anything else to check for straight? Was the win 70 barrel on an action? If so how did it shoot? If it shoots well that would show us if slight differences affect accuracy much.

I would expect that larger differences in lighter barrels would make more of a change in accuracy, particularly as the barrel heats up.

 

[1stmar]

Interesting Bart, I wonder if gs perform any similar tests before installing... It would be preferable to have the heavy side at the top (imo) when mounted. This would likely cause vertical stringing as oppose to horizontal.

 

[Bart B.]

Big Al, the Obermeyer barrel not shanked, chambered nor crowned when tabled for the roll test. It was later chambered for the .30-.338 round and easily shot about 1/2 MOA at 1000 for 20 shot test groups with each shot fired about 25 seconds apart. So it got pretty hot from cold to whatever its temperature was.

The Winchester factory barrel was not in an action. It looked and tested pretty straight with only a few thousandths runout in its middle with each end in a V block. Looked straight, too, but the three tapers on it made it hard to check.

Regarding lighter barrels that are bent. All the M1 and M14 match conditioned rifles used for service rifle matches have the barrel bent down a bit by the stock ferrule's fit to the barrel's lower band. Their receivers are bedded such that the barrel in it rests about 3/16th to 1/4th inch more up from the stock ferrule than where it is when the hardware's put on the barrel and it's assembled into the stock ferrule when put together. This pulls the barrel down at that point at about 20 to 40 pounds of force depending on the rifle type. Barrels so set up do bend down relative to their chamber axis. But its only a few thousandths of an inch at that point becaus the stock fore end gets bent up at the same time. They all drove tacks when tested for accuracy, so their bent barrels heating up in a 20 to 30 shot string didn't suffer any point of impact change.

Gosh, that happened decades ago. 'Tis hard remembering the details of such tests back then.

 

[Erno86]

A gun barrel...that is placed in the hot sun ---will heat up the barrel on one side --- which will cause a different point of impact, to a gun that was not placed in the sun.