Interpreting What You See From Testing

[williada]

Snippets from my 2003 Presentation Australian Palma Team Bendigo Practice with regard to test barrels, tuning for 1000 yards for the Winchester factory 155 Palma Ammunition. Some the information is duplicated from earlier posts.

Note : test barrels 2 & 3 were 35 inches, barrels 1,4,5,6 were 31.5, barrel 7 was 29 inches at test.



This test was based on the highest centre count at 1000 yards with factory ammunition
Note: 1-14 was 29 inches



Note: In this slide shows how a 29 inch barrel performed against the 31.5 inch barrels. It was cut short to see how the compensation would compare and out of a larger groove size to increase bore time a bit with the factory round. The test was done with relatively clean barrels to magnify extreme spread without warm up shots.




Below:
The slide below is with an unturned barrel. With a fundamental tuner weight it could be made to shoot better, but imagined what it would be like with reloads other than the factory rounds used for testing with a low extreme spread.











If your barrel is flexing in the vertical, a barrel weight at the muzzle will dampen it.
However, groups may arc, clump laterally with or without fliers or string diagonally. In the first case with the overtones they can be induced by the rifling and returning vibrations as per OCW tuning or could be from action/barrel joint and pressure placed on that by headspace variance. But rifling torque may be seen in slender barrels, resulting from a tight helix of rifling or a mismatch in engraving forces on projectile by bore and groove size or how the relative stiffness of the barrel is aligned in the vertical plane. Multiple flexes can be sourced from the interaction of all sorts of whips and overtones including bullet slap from concentricity problems. Diagonal strings can be offset a bit by re-indexing the barrel by looking how your groups form at 140 yards by the appropriate degree before 12 o’clock. Arcing groups a probably better solved by reducing charge or changing powder. Of course shorter thicker barrels can solve a few problems assuming the bedding is sound. In the old days, barrels could be less than 26 inches, so cutting off 1 inch may amount to a decrease in velocity of about 30 fps but cutting off long barrels (35 inches) only gave about 12 fps per inch. This demonstrates the marginal gains in velocity by addling length.














Below: Read the graph right to left. Its habit because that's how you set up drag charts.

The general trend of the barrel is negative. Edit: This was first determined by 25 yard tests with special loads then the subsequent free flight frequency test was done at 100 yards. But the factory load negativity is still evident. You may get positive compensation with .020” free flight, but probably with a fundamental tuning weight mounted so the weight extends past the muzzle.

Free flight at .040” is fairly neutral and is indicative of a tight nodal tune as well but could run a risk of falling off a node in bad environmental conditions or where moisture can corrupt your powder. Remember if you fly, it takes at least two weeks for your powder to settle as it takes on moisture in flight at 30,000 ft., unless it is sealed. On the other hand, if during the day soon after flying, and you are exposed to high temperatures, the moisture will not only come out of the powder, the atmospheric density changes will have you slipping steeply to the right off the node. You are in for a shocking shoot at the longs with elevation all over the place. That aside, there is no need for a tuner in this situation and you could comfortably sit back off the node @ .037”.

A light variable tuner would be very suited at .040" jump and you could easily get that slightly positive compensation.


After .040", you are in negative territory as low shots never catch up.

[DannyS]

Thanks David, very interesting.

Cheers
Danny

[williada]

Light can confuse your interpretation of test results, if you are testing at a longer range. At some point I may discuss this further, but generally speaking as the day gets hotter and moisture distorts the aiming mark, shots tend to go high in the 11-1 o'clock sector. Because your rifle may display a certain compensation characteristic i.e. out the top on a positive, out the bottom on a negative, and sometimes at long range out the bottom with a neutral with a low velocity shot. The light can mask what your group is doing. It just might be the case that a negative barrel can have a low shot held up by and apparent aiming mark shift upwards. So choose a light condition to test for elevation that minimises the apparent aiming mark problem.

Likewise test in a crosswind, rather than a head or tailwind. Head or tailwinds will have reverse elevation effects due to Magnus forces. Another reason to test before the overturning moment in the trajectory distance. Also try not to test in no wind conditions as this can cause 6 o'clock shots if there is a sudden drop off due to a lack of lift. Anything between 8-10 mph as a cross wind will give sufficient stable lift to test elevation and there is a minimal left wind lowers or right wind raises to corrupt the group testing at 500 yards. These conditions are probably closer to normal daily wind speeds anyway. Generally speaking you need to test in those conditions at 500 yards to make slight adjustment to short range tune.

I did mention on a previous topic about wind on cold mornings. Although there is a no wind condition which can cause lift problems for testing, another problem exists when cold air is sucked in from the sides of some ranges with vegetation along the sides or even on one side. This vegetation holds moisture and so the centre of the range is relatively hotter. The creates micro climates and willy willies that sometimes mirage does not show. It creates elevation problems and windage problems because wind can come from both sides. The atmospheric turbulence at 500 will distort your interpretation of results. No amount of tuning will sort it. Just beware. I like to shoot in late afternoon when the wind abates, the mirage abates, and willy willies have gone to sleep. David Edit: I keep finding typo's. I need speech recognition software.

[johnk]

Belmont range at 300 yards in August during the Queens can cause shooters to doubt their zeros. There's a combination there of a very flat mound (it once was the 200 yard Army mound before the stop butts were pushed back) and a drift associated with the relatively cooler low pocket behind SSAA results in shooters on the extreme right having up to 2 MOA left "error" on their TR sights & similar confusion with the F classers.