#19 Postby williada » Tue Feb 05, 2019 4:48 pm
Group tunes
Sorry, Alex I have been busy. The details I cut the groups were posted a few years back on the forum when I was working with Tony on development. Such a barrel would perform better across the course on trough nodes. Each of the groups could be formed in .005" increments of the tuner thimble, having been established through development. The tuner increments could roughly equate 0.2 grains in a .308 for the purpose of the exercise if people went that way, rather than equate with seating depth. Competency depends upon studying your load development and knowing which way to move based on group shape in certain conditions, then sneaking around a node as the day warms in small increments. It’s all work, you could do this with competition notes over time if you are worried about wearing out a barrel. Got to learn somewhere and it’s not free.
The most reliable would be, Group 1. It’s nearly a nice round node suggesting shock waves are not distorting the bore on exit. This occurs when the muzzle oscillation both vertical and horizontal are equal or in the same 1:1 ratio. If it was just a tad further forward on the bottom node it may compensate mildly and effectively broaden the node. Graham is correct, if a low SD is generated, it will hold reliably out to 1000 yards. The only risk is if the temperature drops dramatically which happens more in the south, the group may form before the trough and give negative compensation which is exacerbated with distance. In this scenario primer fluctuations can be seen in six o'clock shots.
Group 3 is closer to a peak node and assuming the slow bullets are tossed higher would only really be suited for 1000 yards, but could be refined for distance with a tweak of the tuner. This would shoot ok at 300 yards but distances between the compensating distances, especially mid distance would express more vertical. Unless the tuner was tweaked that top left shot would repeat with distance and should be considered if groups are safe sided. Group shape is not random despite the crap that gets peddled.
I like Group 5, and with a tweak of the tuner the right shot could be brought into more vertical alignment. The group offers two advantages, one being it is the least wind sensitive and the other is that it could offer mild positive compensation which brings all ranges into contention across the course.
The groups clearly demonstrate there is no comfort in waterline shooting at long range when barrels can flick shots there. You need to have the least wind sensitive load possible, then you can say the waterline is correct. However, temperature and bullet slip in the atmosphere affect shock waves. When groups change shape it is a sure sign you are not on the selected test group position whether that it is a node, a positive compensation tune or an OCW tune. It can indicate slipping to a poor node where the ratios of vertical and horizontal maybe 1:2 etc. Still a node but the oscillation pattern is like the ABC logo. One minute you are on the tight intersection of the barrel oscillation path and the next a small temperature change can tip you off. Groups do hold shape with distance but merely get bigger.
So if you guys were shooting at 1000 yards and getting a waterline group like Group 4 what do you suppose would happen during the shoot?
What options would you have in moving the tuner?
Which one would be preferable?
How far would you have to move the tune?
Would you adjust elevation before shooting?
Why?
How much?
If you have a thimble that moves you .005 for every 5 graduations what would 1 graduation equal?
Suppose you had a tuner with 30 divisions you get a group like group 6 a week later, after a good shoot, how far would you move the tuner to get a group like Group 1 for the second shoot?
If the tuner thimble thread was 32 tpi and suppose you moved from group 6 to group 1, would it be more than a full turn of the tuner or less?
If you found a node in the morning and the group shape was starting to change in the afternoon when it was warmer, would you try and find the old node or chase another one? Explain.
Assume the incremental base for charge weight was .2 of a grain for the purpose of the exercise, and you were at Group 6, and your tuner was fixed because you did not know how to use it, how much would you reduce your load to get back to group 1? Mind you, these are extreme scenarios.
Now the big problem with beginners is that they confuse conditions i.e. light, mirage, wind etc with real grouping capability. Load development should take place, not only because the bullet has lost its fast precession (gone to sleep) but you can actually examine true group shape at 140 yards for F Class.
Sorry if it’s over the top, but top competitors know this stuff. If you are a team man, hopefully the coach can do it for you and all you have to do is break the best shot possible. David.