Hot barrel effect on fps?

[trauma1]

I am reloading berger 230 gr hybrid tactical with 73.5 gr 4831 for a 300 win mag. I use digital scale and measure to 0.02 gr each load. My last chrono data showed my rounds starting at 3030 fps. After about 10 rounds my fps started to drop by 50fps each shot. It leveled out at 2925.
The barrel was hot to touch. I gave it a rest and it bumped up about 40 fps. Then dropped back down.

How much effect does the barrel heat have on velocity? Thanks for you help.

 

[codemancode]

That seems like a big swing for only 10 rounds. I found the chrono to be the culprit for the strange data in most cases. How far is your chrono from the muzzle? Sunny or cloudy day? Intermittent cloud cover can throw your readings way off. I suggest firing 10 rounds at a 1000 yard target and checking the vertical dispersion. This method should give you a better idea about the consistency of your muzzle velocity.

 

[Name Enough]

I willl get weird readings when the Chromo is too close to the muzzle. So I try to place it 10 feet away and not to shoot it by mistake. So far so good.

 

[trauma1]

thanks for your response. How do you test a chrono

 

[Morale_Gear]

I willl get weird readings when the Chromo is too close to the muzzle. So I try to place it 10 feet away and not to shoot it by mistake. So far so good.

Ditto on the above 2 posts.

Also, if the barrel/chamber heat was effecting your velocity, it should increase slightly if you've been shooting many rounds in rapid succession. Probably not enough to really notice.

Changes as large as you are seeing are likely not a cause of a heated barrel or chamber.

 

[flashhole]

Your readings are exactly what I would expect from barrel heating. This is a classic physics problem. You know metal expands as it gets hotter. What happens to a pipe (rifle barrel)? Does the ID of the pipe get smaller or larger. Intuitively you might think it gets smaller but it actually increases in diameter. Increase in diameter = loss of pressure = reduction in velocity.

If you hunt, you now have direct relevant experience why you want to know where your first shot from a cold clean barrel is going to land. All my guns are dialed in to the first shot from a cold clean barrel. If you are a patient shooter and wait for your barrel to cool between shots you will get very good repeatibility with your loads. Some barrels "walk" more than others as they heat up. Light weight sporter type barrels will walk more than heavy bull barrels with rapid successive shots.

 

[Morale_Gear]

You could move it back further and check the consistency. 300WM is a pretty stout round, so putting the chrono further back may resolve the issue.

If not, try some other load in another gun. Even a 22 would probably suffice. However, the faster you go the more error you are likely to see, as even a 2% error would be a lot more noticeable at 3000fps versus 1200fps.

 

[trauma1]

Thanks for the data! Name enough & savage110nut, I am going to make sure the chrono is at good distance. I started to mske sure I tare between each load measurement. Flashole thx, it is a Les Baer barrel so pretty heavy. so i wil be patient and slow down the shot sequence. i appreciate everyone taking time to contribute.