average group size change with bullet depth change?[at short range]

[tim in tx]

I had asked this on another thread but it was the wrong time so I thought I would post it on another thread,I am trying to get an idea of how much your group size is affected by changing the bullet depth for short range .
[QUOTE FROM BOYD ALLEN] I ithink that the issue may be one of the muzzle's vibration pattern, and where within that pattern bullets exit, that seating depth may relate to timing in a way that is not a direct result of changes in velocity. This would probably be at one of a barrel's higner harmonics.

Putting a properly positioned weight at the muzzle has been reported by a top short range competitor as broadening tuning nodes. I think that this is because the weight reduces the amplitude of the higher harmonic, reducing the slope of the wave form. This may be a separate but related effect from what is done by moving the weight to tune a lower harmonic.

One may also take note that some very sucessful tuner designs incorporate materials that were selected for their vibration damping properties. It has been said that because of the speed with which vibrations travel in steel that vibrations that take place as the rifle is fired, that have mechanical origins, travel to the muzzle and back several times before a bullet exits. It has also been said that there is also a wave that is generated by the the pressure expanding the bore, that travels down the barrel independently of the bullet. It seems to me that getting all of these various factors into the most advantageous relationship is a problem that is complicated enough to challenge anyone, and that it can be attacked with more than one method.

With regard to your experiment, could it be that too small of a hole may be a problem because of the reason that you have postulated, but that enlarging the hole, while removing that issue may merely be letting something else that is not gas flow related show a positive effect. I will of course be interested in your results.

A friend, who does his own work, decided to develop his own muzzle brake design, took a rigorous approach, building and testing at least a half a dozen different designs. During that testing he noted that accuracy from the same barrel was different with different weight, and length tuners, and that this was probably very much like what is involved when configuring and adjusting a tuner. I think that it would be interesting to design a brake, for hunting applications, that had provision for adding small weights on the end. [end quote]

Sorry boyd I got back so late to that answer.What I am seeing is a random dispersion not due to vibrations.I know this because I have mapped my barrel for the differing vibration modes and the high frequency is at the muzzle when the bullet exits at 2 points which is at about 2830 and 2960 ,at those points of exit .I can redice the high frequency but not dampen it no matter what materials is used,It is reduced by simply using a tight fit neck sized only twice fired case , not completly eliminated but at least 90% is eliminated.With that said I ussually keep my velcoties well away from those exit areas and put them somwhere in between at 2880-2930 .I have seen some rifles groups effected greatly and some not and am trying to get a good reason for this problem and in bore yaw seems to be the best explaination so far .The deep hole muzzle brake is correcting the dispersion but a brake of the same exact weght without the deep hole will not correct the dispesion.This particluar rifle is know for groups to open up with certain bullet depth change to a fairly noticable degree so it was a good canidate for this type of tesing .This is not horizontal or vertical but a completly random dispersion so I wanted to see the average dispersion you guys are seeing on the ppcs and the 30 brs at 100yds where velocity changes in themself are of little effect.Thanks.

Tim in Tx

 

[Boyd Allen]

Do the brakes with and without the deep hole have their mass positioned exactly the same relative to the barrel's crown? I ask because slight differences in the position of a tuners mass are what are used to adjust group size. If your brakes do not have the same weight and distribution, relative to the crown, then you may be looking at a "tuner setting" difference. This is what my friend found. He saw marked differences in his groups based on the size and weight of the different tuner designs (weight, length), using the same rifle, barrel, and loads.

 

[tim in tx]

Boyd that makes perfect sense.But remember the rule of thunb at least in my eyes is what mode of vibration the tuner tuner or weight actually effects which is the vertical bending of the barrel,The mass by the physical properties does dampen the lower vibration mode which is after the bullets exit but cannot and will not dampen any high frequencyor should I say explosion to any real signifance on target but when I go to the source of the high frequency that is where the reduction in that kind of random dispersion can be had.but if you just stay away from that veolicty area you will never see the high frequency dispersion.so that is what I do to make sure I am not even close to the high frequency area. The tuner is first machined to exact weight to 1/100 of an ounce regardless of material and legnth l and then is adjusted and determined to be in the same position confirmed by bullet poi with multiple powder chrarges in a vertical pattern ,in other words the vertical is exactly the same with or without the deep hole with nominal bullet depth .but I can only change bullet depth and effect the random dispersion with the shallow hole brake.The deep hole brake is unaffected by bullet depth change. Tim in Tx

 

[Boyd Allen]

This is very interesting, but I am having trouble visualizing how you are getting the same weight position relative to the crown, with deep and shallow. Obviously, anything that reduces sensitivity to a tuning variable is a good (make that huge) thing, as long as the peak accuracy is not reduced. Have you looked at Varmint Al's videos of computer simulation of barrels' vibrations? If I remember correctly (and I will be reviewing them after I finish this post) tuners do more than affect barrel swing, although they do do that. What I am referring to is the relative positions of the crown and the nearest vibration node. In short range CF benchrest, this alignment may be out of reach, but it may be possible to achieve for long range, given the higher weight limits, and it certainly has been a hot topic in rimfire. Are you keeping the details of your work to yourself (understandable if you are) or would you care to share what you think might be a suitable hole depth and diameter for something like a typical 6PPC? I can hear it now..."Its not a tuner, its a gas flow bullet yaw reducer."

 

[tim in tx]

That is funny Mr boyd.And that is a great name for it . I was of the same opinion untill recent testing, what I have found is that it matters not how far a certain proportion in front of the crown but as long as a certain weight is in front of the crown the barrel does the exact same thing ,granted they both have to be fine tune adjusted.in other words one is adjusted with a movable weight and the other by machining and lightening and to set up a specific rate for the bullet trajectory correction. where the weight was proportioned was not relevent goes against my own beliefs in more ways then one. But that is what physicly happens so I am going with that .One tuner is 5 inches of steel and one tuner is 8 inches of aluminum but they weigh the same in front of the crown and then set .Now I knew in bore yaw existed as I read in many books and articles over the years .At one point in time I was to attemp to measure it with the help of Ken Oehler and his laboratory at his deer lease as it is close to mine but the critters were eating his cat 5 cable up,so it never happened ,I sure waundered if we could have first created in bore yaw and second to measure it with sensors from the muzzle to the target at 1000yds. Since we knew and Ken agreed with my beliefs that yaw was there but just could not put my eyes on it to prove it but since the advent of the new generation of tuners has come to light on in bore yaw .Some video footage has shown a slight wobble as it exits the barrel but to measure it to any degree would be dauntingly expensive[48000.00 per day] by use of doppler radar but I think a factory barrel with a steep leade [4 degree per side]and possibly not centered to the bore so this effect is more obvious or effecting ,but I do know on this particular gun the groups definately open up at the wrong bullet depth but I have some guns that there is little effecting of groups but most always there is some dispersion that is always there .I will post some pics in due time but want to make sure I am seeing what I think I am. Tim in Tx