Different velocities when tuning for accuracy

[fgregorio]

Hi all, after putting in a new Bartlein barrel and taking off the old barrel (which finished off at 12070 rounds), I have been busy working out the loads for the new barrel.
Point of the matter was that after doing the basic load development, today I tuned the barrel tuner at 300 metres.
This tuner has got 8 graduations per turn (50 thou).
This was shot from a bipod and rear bag.
Interesting points: as I move the tuner by 1 graduation (1/8th of a turn), the velocity bracket changes and so does the time of flight to the target (in miliseconds).
The tightest groups (0.2 moa or less) were not the ones with the most stable velocities but instead, the groups that had the most even and lowest times of flight.
Is this a measure of positive compensation?
Velocities came in different brackets, per position of the thimble, and very repeatable.
After moving half a turn, the velocities fell back into the same bracket (position 1 to position 4 on the thimble).
Just a heads up. This ammo was loaded and developed according to the principles found out at the Houston warehouse.
Less than 1 thou runout. Less than 0.5 thou diff in seating depths throughout the whole batch. Cases batched within 0.2 gr. Neckturned, flash hole and pocket uniformed, annealed etc.
1 thou neck tension, projectiles seated by hand on a wilson micrometer die. HBC 155.5 gr with HBN. Batched within 0.1gr and zero tolerance on bearing surface length.
46.9 gr of varget. Load chosen by ladder test.
Check out the picture attached for a summary of plots, muzzle velocities and time of flight.
Food for thought, he says...

 

[stansrifles]

I don't by that reasoning at all.

 

[tonysnoo]

How did you measure time of flight....never heard of that ability. Sorry to sidetrack.

 

[fgregorio]

How did you measure time of flight....never heard of that ability. Sorry to sidetrack.

Our target system has that ability. A sensor measures the muzzle blast and the target sensors at destination time the impact. I see it as a qualitative tool, but nevertheless quite effective.

 

[LHSmith]

#1. Forget whatever results that (supposedly) came out of the Houston warehouse. IMHO it is pure folly.
#2. From Ratigan's book "Extreme Rifle Accuracy" under "One Group Evaluation?...It's impossible to evaluate your rifles performance with one three shot or five shot group......Possible causes for wandering bullets could be conditions, gun handling, component and/or mechanical problems........Shooting one group doesn't mean you have accomplished anything. You must shoot enough to properly evaluate the tune and setup."

 

[fgregorio]

#1. Forget whatever results that (supposedly) came out of the Houston warehouse. IMHO it is pure folly.
#2. From Ratigan's book "Extreme Rifle Accuracy" under "One Group Evaluation?...It's impossible to evaluate your rifles performance with one three shot or five shot group......Possible causes for wandering bullets could be conditions, gun handling, component and/or mechanical problems........Shooting one group doesn't mean you have accomplished anything. You must shoot enough to properly evaluate the tune and setup."

I fully agree Hence I am repeating the test but this time at 300 metres (again) and at 600 metres, a few days later.
I am really curious as what the results could be.
I ran some preliminary calculations, and so far the result is that at the point of most accuracy, the angle of departure of the projectiles coincides with the barrel time and both are constant. Different mv are due to different acceleration patterns WITHIN the barrel.
Yet, constant barrel time= constant angle of departure which means that low ES and low SD are important up to the point where they fall below say 15fps and 5 fps respectively.
How does this connect to the OBT theory?
Because this is brought about by the use of a barrel tuner, I reckon that Al Harral would be the shooter that would be the most qualified to answer this conundrum.

 

[walt willis]

Hi all, after putting in a new Bartlein barrel and taking off the old barrel (which finished off at 12070 rounds), I have been busy working out the loads for the new barrel.
Point of the matter was that after doing the basic load development, today I tuned the barrel tuner at 300 metres.
This tuner has got 8 graduations per turn (50 thou).
This was shot from a bipod and rear bag.
Interesting points: as I move the tuner by 1 graduation (1/8th of a turn), the velocity bracket changes and so does the time of flight to the target (in miliseconds).
The tightest groups (0.2 moa or less) were not the ones with the most stable velocities but instead, the groups that had the most even and lowest times of flight.
Is this a measure of positive compensation?
Velocities came in different brackets, per position of the thimble, and very repeatable.
After moving half a turn, the velocities fell back into the same bracket (position 1 to position 4 on the thimble).
Just a heads up. This ammo was loaded and developed according to the principles found out at the Houston warehouse.
Less than 1 thou runout. Less than 0.5 thou diff in seating depths throughout the whole batch. Cases batched within 0.2 gr. Neckturned, flash hole and pocket uniformed, annealed etc.
1 thou neck tension, projectiles seated by hand on a wilson micrometer die. HBC 155.5 gr with HBN. Batched within 0.1gr and zero tolerance on bearing surface length.
46.9 gr of varget. Load chosen by ladder test.
Check out the picture attached for a summary of plots, muzzle velocities and time of flight.
Food for thought, he says...

You are a man that understands science! Thanks for sharing that information. I look at the resonate frequency as I do my HAM radio antenna tuned is done.
I learned this at the Marine MTU at Stony Point, NC when the coach told me about a shooter that would have his weapon crowned every day until he got the frequency matched to the issued ammo he was given. Many dollars have been spent working on the gun parts and not the simple things that can help the most.

 

[fgregorio]

You are a man that understands science! Thanks for sharing that information. I look at the resonate frequency as I do my HAM radio antenna tuned is done.
I learned this at the Marine MTU at Stony Point, NC when the coach told me about a shooter that would have his weapon crowned every day until he got the frequency matched to the issued ammo he was given. Many dollars have been spent working on the gun parts and not the simple things that can help the most.

If it helps, having tuned many rifles, when a rifle goes in tune, the recoil changes more from a kick into a push. I've emailed Varmint Al

 

[GSPV]

I tune by shooting shooting three shot groups of increasing powder charges to form a graph. I'm looking for a sine wave to develop and, specifically, for the portion of the graph where increasing charge weight starts shifting the POI down. I don't want it looking like a shotgun pattern, but I'm not terribly interested in minimizing group size during this exercise.

I capture the charge weight and velocity of the start and end of that part of the graph.

I also capture the temperature at the time that I'm shooting. Now, I'm kinda lazy, so I do this durimg normal shooting temperatures...not, say, ones that are freezing cold if I shoot in the summer.

After that, I produce two data sets. One is the change in velocity as a function of charge weight change under constant temperature. I'm typically able to capture that when I'm shooting the groups. The other data set is the increase in velocity as temperature increases under constant charge weight.

After all that, I will get on what seems like a the right charge for the current temperature and twiddle with the seating depth till I find what I want.

Since we mostly know from history what's going to shoot, this doesn't really take as much work and time as it sounds like.

The other idea is to use a tuner. Some people get very savvy with using them. I like to have one to be able to shift the part of the graph that I'm interested in...say if the barrel naturally likes to shoot with a load that is uncomfortably warm...I've got a 284 Shehane that is like that.

 

[gunsandgunsmithing]

I tune by shooting shooting three shot groups of increasing powder charges to form a graph. I'm looking for a sine wave to develop and, specifically, for the portion of the graph where increasing charge weight starts shifting the POI down. I don't want it looking like a shotgun pattern, but I'm not terribly interested in minimizing group size during this exercise.

I capture the charge weight and velocity of the start and end of that part of the graph.

I also capture the temperature at the time that I'm shooting. Now, I'm kinda lazy, so I do this durimg normal shooting temperatures...not, say, ones that are freezing cold if I shoot in the summer.

After that, I produce two data sets. One is the change in velocity as a function of charge weight change under constant temperature. I'm typically able to capture that when I'm shooting the groups. The other data set is the increase in velocity as temperature increases under constant charge weight.

After all that, I will get on what seems like a the right charge for the current temperature and twiddle with the seating depth till I find what I want.

Since we mostly know from history what's going to shoot, this doesn't really take as much work and time as it sounds like.

The other idea is to use a tuner. Some people get very savvy with using them. I like to have one to be able to shift the part of the graph that I'm interested in...say if the barrel naturally likes to shoot with a load that is uncomfortably warm...I've got a 284 Shehane that is like that.

Good post and info, Greg. Most of us aren't nearly as methodical.

 

[GSPV]

What can I say? I'm an uber geek. In truth, it's not anything that I came up with, just picked up from some good shooters.

It's a bit tedious, but if people have their tune go to crap when the temperature changes...or when the humidity drops out if they are throwing by volume...it is great data to have around.

 

[chevytruck_83]

Though I do enjoy the theory and science that effects accuracy I've decided long ago my time is better spent practicing my wind reading and shooting ability.