When shooting a long string of say 12 to 23 shots, has anyone noticed a regular change trend in their velocity reading from the first few shots compared to their last few shots.
Thinking that as the barrel heats up and expands it should have an effect on muzzle velocity?
Velocity variation in a string of shots
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AlanF
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Norm,
Yes, I see it with most barrels. The extent of it depends on the thickness and profile of the barrel (heavy barrels less susceptible). If it doesn't change suddenly, than in F-Class you can usually follow the changing POI, although a big velocity change may take you out of your accuracy node. The worst problem is when you have a barrel that suddenly changes its POI as it heats up I remember a 6.5 barrel that did it around the 8th shot. Sometimes I would see it start to happen and anticipate it, but it usually resulted in 5 at 12 o'clock. I believe this was a stress relief problem.
Alan
Yes, I see it with most barrels. The extent of it depends on the thickness and profile of the barrel (heavy barrels less susceptible). If it doesn't change suddenly, than in F-Class you can usually follow the changing POI, although a big velocity change may take you out of your accuracy node. The worst problem is when you have a barrel that suddenly changes its POI as it heats up I remember a 6.5 barrel that did it around the 8th shot. Sometimes I would see it start to happen and anticipate it, but it usually resulted in 5 at 12 o'clock. I believe this was a stress relief problem.
Alan
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Norm
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Ok Alan so the next question is how is the best way to deal with it and stay within the accuracy node.
Now would it be possible to batch ammo so that a set number of rounds were loaded in a particular order so that the powder charge is increased or decreased slightly to compensate for the rate of change in velocity and so maintain a constant muzzle velocity as the barrel changes temperature?
Thinking this change is maybe small at <20fps so only a slight change in charge would be required.
Say start with 55gn of powder for the first few shots, 55.1gn for the next X number of shots and then up to 55.2 for the remaining number of shots if you get my drift?
The shooter would have to keep track of his loaded rounds and what cartridge contains what charge but that is no big issue.
This may avoid having to correct for heat induced changes in elevation and leave the shooter to consentrate on reading the wind.
Now would it be possible to batch ammo so that a set number of rounds were loaded in a particular order so that the powder charge is increased or decreased slightly to compensate for the rate of change in velocity and so maintain a constant muzzle velocity as the barrel changes temperature?
Thinking this change is maybe small at <20fps so only a slight change in charge would be required.
Say start with 55gn of powder for the first few shots, 55.1gn for the next X number of shots and then up to 55.2 for the remaining number of shots if you get my drift?
The shooter would have to keep track of his loaded rounds and what cartridge contains what charge but that is no big issue.
This may avoid having to correct for heat induced changes in elevation and leave the shooter to consentrate on reading the wind.
Its my opinion that the best way to deal with it is know your barrel and when POI starts to lift come down on your elevation accordingly.
I don't think batching ammo is a good idea, I have often stopped mid strong for 5 mina waiting for conditions to change etc. this would lead to all sorts of problems with batches ammo!!
I don't think batching ammo is a good idea, I have often stopped mid strong for 5 mina waiting for conditions to change etc. this would lead to all sorts of problems with batches ammo!!
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AlanF
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Norm,
One way this thing manifests itself is where the velocity increases gradually then starts to come down to lower than it started (after foulers). You'd need to know these characteristics of each barrel. I think it'd be interesting to try the load variations and see if it kept the spread lower.
Alan
One way this thing manifests itself is where the velocity increases gradually then starts to come down to lower than it started (after foulers). You'd need to know these characteristics of each barrel. I think it'd be interesting to try the load variations and see if it kept the spread lower.
Alan
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Norm
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I have noticed that some shooters report that their rifles hold better vertical group spread at one distance than at others.
If say a rifle groups slightly higher on a target at 100 yards as the barrel heats up. While at the same time the velocity drops off as the barrel heats up. Then at a particular distance the two will cancel each other out and the rifle will appear to perform very well at that distance and not so good at others.
So by recording what is going on at 100 yards as the barrel heats up and also recording the velocity changes, it should be possible to come to some interesting conclusions that can then be applied at long range.
I think that I will do some testing along these lines when I get some spare time.
If say a rifle groups slightly higher on a target at 100 yards as the barrel heats up. While at the same time the velocity drops off as the barrel heats up. Then at a particular distance the two will cancel each other out and the rifle will appear to perform very well at that distance and not so good at others.
So by recording what is going on at 100 yards as the barrel heats up and also recording the velocity changes, it should be possible to come to some interesting conclusions that can then be applied at long range.
I think that I will do some testing along these lines when I get some spare time.
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RDavies
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Last year I had two barrels which reacted totally differently to each other. My 284 was not real great for the 1st 5-6 shots or so, but as it heated up it went GREAT, a good barrel for long shot strings.
My other 7mm barrel, a short mag, was GREAT for the 1st 12 shots, then started to lose accuracy. This one was good for 10 shot strings, but when I used it for 15-20 shot strings in USA, the last few shots in the 15 shot strings were a gamble and in the 20 shot matches were torture.
I really think there must be more to velocity changes which affect barrels as they heat up. As I have seen, we really just need to get down and shoot some long shots strings, which we often don't get to do in Australia. The longer shots strings used in Matchrifle competitions, such as Coonabarabran next month will be a great way to check what your barrel actually does on target.
My other 7mm barrel, a short mag, was GREAT for the 1st 12 shots, then started to lose accuracy. This one was good for 10 shot strings, but when I used it for 15-20 shot strings in USA, the last few shots in the 15 shot strings were a gamble and in the 20 shot matches were torture.
I really think there must be more to velocity changes which affect barrels as they heat up. As I have seen, we really just need to get down and shoot some long shots strings, which we often don't get to do in Australia. The longer shots strings used in Matchrifle competitions, such as Coonabarabran next month will be a great way to check what your barrel actually does on target.
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