I've been working with a newish .30-06 (CZ) and noticed that the first group out of a cold barrel is good (3/4" at 100yds) but as the barrel warms the groups open up.

Has anyone a good theory as to WHY? I've got a few ideas and done some measurement. With an abiant of 64dF the first 4 will heat to 94dF as measured with a thermocouple inside the bore. By letting it cool to 72dF before the next group I can keep groups down but if I don't let it cool the groups open up to 2 to 3".

Thoughts?

as the barrel is get hotter the metal is expanding so the projectile isn't cutting into the rifling as well as it should be and so you are going to get a veried grouping.when the barrel has cooled back down you will find that the grouping should be back to normal.i have seen on tv that snipers consider a hot barrel to be two consecutive shots and then have to start compencating for barrel warp i suppose you could call it although i don't know if that is it's proper terminanology. if you can start to see which way the grouping is pulling as the barrel is getting hotter then you should be able to adjust accordingly.i try not to get to caught up in groupings to much.the best grouping i have ever had out of my zastava 308 is 1.5" at 100 metres.most game that people hunt usually has a kill area of 8-10" so as long as your aim is good you shouldn't have to many dramas

Check to see if the barrel is touching the stock somewhere between the front of the action and the forend tip. If it is, float it. Be sure to put some sort of sealer on the wood after you float the barrel.

trk

Barrels will heat up but it shouldn't affect accuracy all that much. I am assuming that your barrel is of a light weight hunting barrel. In this case a thin barrel is going to heat up fasted than say a target barrel of heavier weight. But what a heavy barrel offers is a better heat distribution.

One thing to try, when shooting groups is to tape a piece of paper on top of the barrel the length of the barrel to just past the bell of the scope. This will keep heat waves from the scopes view which can cause changes of poit of impact. Your groups may be getting bigger because you are not aiming at the some point. The heat coming off the barrel will bend the light as you view thru the scope. With the make shift mirage shield in place it will keep the heat waves out of view.

Just a theory, as the barrel heats the molecules are in greater motion. This leads to a change in vibration of the barrel. The change in barrel harmonics equals different grouping.

The heavy barrel is stiffer and as already mentioned is better at distributing heat. which equals less effect on the barrel's harmonics.

muck

Barrel heating causes several things to happen. One effect is that the barrel is screwed into the receiver which provides a clamping action to hold the barrel tightly to the receiver. The metal is stretched to some degree. As the temperature increases, the metal expands and some of the clamping force is lost. This will allow changes in barrel harmonics.

There was an experiment conducted where they used Teflon on the threads to reduce friction. The receiver to barrel joint was tighter with more clamping force, which reduced point of impact changes as temperature increased.

When shooting a group, I usually just put the first two shots into the backstop to warm the barrel slightly. After that, I shoot the group, spacing the shots about 5 min. apart to keep the barrel at a consistent temp.

Spacing shots is OK if you are not in a timed match, otherwise it is impossible.

Not claiming to be an expert on the subject but i'm told that a properly " Normalized " barrel has very few of the fore mentioned problems.

Stepchild

WTH is a "Normalized" barrel?

I think "normalized" is another way of saying stress relieved.

muck

when you heat steel the particules expand.when the steel cools down the particules shrink and they will always shrink more than they expand.an easy way of seeing this is to get to get 2 pieces of steel,sit one on top of the other to form a tee and then weld the to pieces together down one side.you will see that once you have finished the weld that the 2 pieces will have pulled closer together on one side,this is the particules shrinking during cooling.normalising is the state of returning the steel paricules back to there original form through heat treatment

jimmy308
You lost me with that one. Please explain.

muck

the rifling is put into barrels using 3 different methods. 1. cut rifling. 2. button rifleing. 3 hammer forging. All barrels get stress releived i think. A hammer forged barrel is the one most likely to "walk" when it heats up. Some makers even hammer forge the chamber now. I have had 2 guns that "walked" the shots as the heat temp. went up. Not much you can do about it except rebarrel the rifle. High qualilty match grade barrels do not walk their shots with hot barrels.look here

i dont know about the group size changing, but one thing ive noticed is that if i fire a shot from a cold barrel its about 50fps faster than a shot from a hot barrel. probably because mine is a standard hunting rifle.