Generally, I like to work with 30 fps average below and above a node velocity average to give an average range of 60 fps to induce that velocity variation. This was originally based on typical velocity spreads of selected factory ammunition and environmental changes we experienced in the fullbore days.
Now in terms of Denis’s problem, he’s found a good node and he’s at full powder capacity, I’m sure he wants to know his barrel is not a negative compensator and so use it at 1000 yards. Assume the node is now the fast charge and make up other test rounds, one that gives a charge about 30 fps less than the nodal group and another one that gives 60 fps less than node charge. Then go about your tests. Then refine them again if you have to.
Compensation depends on bore angle and bore time. It is just so important to have your barrel curvature oriented in the vertical plane as discussed on another post. Of course altering bore time can be induced by free flight distance. There are many ways to alter bore time. But free flight is an interesting one. It is an art to set the best free flight and only after doing many barrels do you pay attention to the patterns as they fan, hollow, round, elongate vertically and laterally, and form triangles do you know how far to take out your throat or when the lift will reverse, trend up, neutral or down. This can happen anywhere between -.015”(into the lands) and .085” jump to the lands. The free flight is very much linked to compensation characteristics.
Fig 1 Changing free flight alters sine wave seen with Winchester Palma factory ammo. Source: Williada. Tests conducted at 100 Yards from a machine rest with factory ammunition using a universal throating reamer. Jump taken out in .010” increments.
Fig 2 Incremental Load by ½ grain. Source: williada. Machine rest, reloaded 155 Sierra, Lapua cases at100 Yards.
These days at home, I do a charge test at 140 yards by firing incremental charge weights (about 5 rounds for each charge) in round robin style with each group separated by an aim off in the horizontal will reveal the sine wave pattern which you can tap into. This has several effects, one is to show the node for a nodal tune; two for a positive compensator look for the fast bullet usually to the left and down from of the peak of the sine wave and the slow bullet near the peak. By looking at all of the incremental charge groups in the horizontal rather than individual peaks you can see the macro trends of the barrel. Are sections level, pointing up or down? Which one could let you down if your reloading or environmental conditions change? So compensation tuning can help if you slip off the node. In figure 2, I would further test load between groups 3 and 4 knowing I would be into the node and close to neutral compensation if in the afternoon temperatures raised velocity. In the morning a slight positive compensation would exist if it was cooler because we know a cool barrel and particularly the first shots out of a cool barrel can go low. I find the horizontal test better than the ladder test from a visual perspective.
Also note that bullets have to be travelling faster than the speed of sound at the target distance. There was a tendency for factory Palma loads to fall below the speed of sound at 1000 yards with barrels under 31 inches and enter the transonic range which destabilises the projectile. In other words they cross back over the sound barrier. People don’t realise the speed of sound changes with temperature and groups can fall apart at long range when the cartridge is at its velocity limits. A good place to be is past the sound barrier before the sonic curve gets steeper. Lower velocities require faster spin rates to keep them stable. That is another topic.
Fig. 3 Transonic Range
This introduces the coning or transitional yawing effect for short range testing. So I test when the bullet has gone to sleep at 140 yards.
Yawing effects. IF YOU WANT TO LEARN ABOUT BALLISTICS check Ruprecht’s site. Oldy but a goody.
http://www.nennstiel-ruprecht.de/bullfly/
Fig. 4.
The cross section of the yawing effects demonstrates that a small sample size can mislead you to think that future groups will be small if the shots happen to group tightly by random. Your tests need to be repeatable.
The barrel lift which affects bore angle can also be altered by changing a fundamental weight on the muzzle as mentioned in the sticky post on recoil and stock design. There are many ways to change barrel lift.
The importance of round robin testing cannot be understated when you see some graphs of velocity, barrel temperature and pressure shot at strict 45 second intervals. I will leave detailed comments for a later date. But think how long your string of centres will last in a long string before you can expect smaller sine waves to naturally develop in your velocity spreads given that cartridges are as identical as you can make them. Extend to a really long test string and you will see how rhythmic they are. Learn to expect them and adjust on them, and not on a perceived environmental change. Note that it takes until the fourth shot for the barrel to settle and that it is the second sighter that can mislead the shooter, not the first assuming your sights have been set correctly. I don’t start group refining until I have at least 3 warm up shots, something that F Open used to practice as they used to be allowed 3 sighters.
Fig 4. Velocity, Barrel Temperature, Pressure.
Coaches might find this stuff worth reading to assist them with analysis and correction of groups. Would knowing your setup is the best it can be and your ammunition is as uniform as it can be assist you to relax?
Menu
Newest members