New reloader / .308 load development questions

[GeauxTide]

I believe you're complicating the obvious. Get a chronograph that will give you Extreme Spread and Standard Deviation. My 46 years of loading tells me that the lowest ES, coupled with a SD below 10, prints small groups. I've never, once, worried about seating depth, even on my long throated 6.5-06 and 338-06

[Metal god]

Well with a ladder test all loads have a ES/SD of zero because it's only one shot of each charge . The next thing to do is to pick one of those charges to test further . Ladder test seem to be hit and miss for me . I've had great results along with impossible results to read . In this case there appears to be a 1gr -ish jump in stable zones to pick from ???

[Unclenick]

Originally Posted by Metal god

Note : it was pretty windy when I did this test so please understand my POA was not the dots.

Then what are the dots? Understand that all shots for an Audette ladder need to have common POA. Otherwise their position isn't relative to a common point, as is needed to interpret the ladder. I wouldn't compensate for wind, either, because wind from the right raises POI and wind from the left lowers it for a right hand twist barrel, and vice versa. If you supply the twist and velocity so the stability factor can be estimated, I have a chart that will compensate for that lift or drop without knowing the exact wind speed, but just how far left the shot landed.

[Metal god]

The dots were put on the paper as a intended POA . I quickly realized the wind was going to push my shots to far left with such a small over all target . The target would become very hard to read as seen by the second shot on top line of target . The dot was my POA and the POI was the on the dot to the left . It was then I started compensating for the wind . The wind was blowing hard at times ( over 10mph ) but not on every shot so where they landed left to right would have been very hard to compare to the last or next shot fired do to different wind speeds for each shot . The wind was pretty consistent as far as angle . Almost a 90* at 3:30-ish to 9:30-ish with a few times 4 to 10 Not sure which shots were different .

My understanding of a ladder test was it has much more to do with the vertical POI then horizontal POI . The OCW method takes both into account but I thought the ladder test only looked at vertical ???? This is why I made sure each POA was at least on the line that the dots were on . How would you look at horizontal spread with only one shot ? Assuming I charged the cases properly the vertical spread should be more consistent do to the constant of gravity yes/no ? How ever when varying wind conditions are introduced I don't see how one shot can be trusted to indicate much as far as horizontal POI ?

Was my test a complete waist of time in the wind ?

I'm shooting again this weekend and can run the test again with my POA being a specific spot and run them though my chrono at the same time .

Let me asks this though . Should I even be bothering running these test using virgin brass ? It will never be the same again . I check the case volume last night on the 14 cases I fired . AVG case weight was 179.44 / AVG case volume was 55.6 of a 14 cases sample

[Unclenick]

You'll have to determine whether it was a waste or not. It depends how exactly your remember your wind compensation. The reason is shown on the Hornady 4 DOF exterior ballistics calculator for some of their bullets (they haven't yet put any competing bullets in, but say they will) you'll see the effect that can throw your data off because of the wind effect on POI.

For example, the Hornady 168 grain match bullet at 2600 fps MV does the following in a 10 MPH wind that switches from left to right, assuming a right hand twist and a machine rest rifle that is a one-holer in zero wind:



So, your vertical numbers from a strong wind are off compared to those fired when the wind slackened. Only if the wind was steady, do you have none of that influence on vertical POI.

[Metal god]

OK lets for this discussion say I'm in 100% agreement . How much of an effect will this have at 300yards . I understand there are variables with each load to consider but are we talking 1/8" to 1/4" rise at 300yards or are we talking 1moa+ difference .

I ask because this seems like one of those things like Coriolis effect . Yes it happens but only at extreme distances and the shooter ability to adjust for the effect is strongly determined by how well he can call the wind speeds .

If it only effect my bullet by 1/4" or less then 1/10 moa at 300yards . I just can't shoot well enough for that to make a difference in my evaluation of the POI from shot to shot at 300yds .

[Unclenick]

MG,

Look at the graph again. It's for 300 yards from Hornady's 4 DOF calculator and I put in a scale of inches of rise and fall on the left that are for the actual named bullet at the named MV and rifling pitch. The 10" twist produces a high stability factor with that bullet (about s>2.5, IIRC) so you are getting almost an MOA change in elevation for every two MOA of wind drift. Most long-range match shooters are used to the idea that their rifle has a wind drift-to-rise-or-fall ratio and that they will have to come down an MOA for every so many MOA added left windage to bring the bullet back on POA, or up an MOA for every so many clicks of right windage dialed in (vice versa for lefthand twist barrel). If you have a lower gyroscopic stability factor (lower twist rate that still stabilizes the bullet well) then there is a lower rise or drop to windage ratio. Another example is below. Those are the actual angles of the line through the bullet holes off the horizontal line through the center of the target.



I have an approximate formula somewhere that I worked out as a curve fit to Harold Vaughn's graph of the slant angle of a line through the bullet holes vs. stability factor, but I can't lay my hands on it at the moment.

[Metal god]

Wow I had no idea the spin of the bullet and cross wind could effect vertical POI that much . It's interesting because I've read a few things and watched a few videos on ladder test and I've never heard anyone talking about evaluating how the spin and wind act upon the bullet effecting the vertical POI .

Although I can adjust for wind ok when shooting . I really have no idea what that wind actually is . It's really just experience that has taught me where to hold . If the flag of mirage looks like this , I hold here type of thing .

This is somewhat disappointing . I just started trying the ladder test method last year . This is my third try at it with one resulting in a fantastic load , one with absolutely no pattern to judge from and this last one . Before last year I had always done a traditional load development . Shooting 5 shot groups moving up in .4 or .5gr increments at 100yds .

I must have gotten lucking with that first ladder test I did . I just don't think I'll be able to judge the wind that precisely to calculate that vertical stinging to effectively evaluate a ladder test correctly . That first ladder test I did must have been in no wind and these last two had enough wind to effect the results . I'll add that this last one I know had more wind then any of the others but still had what appeared to be a readable pattern . My last one had shots all over the place ( high and low ) with no discernable pattern to judge a stable zone .

I'm shooting again on the 9th . I'll try it again with a few changes . I'll chrono the shots and shoot much earlier in the morning which should reduce wind deflection quite a bit .

I'm still concerned about doing these test with virgin brass . I already know on the second and subsequent firings my case head space will be .003 longer and bullet hold will almost for sure be less . Is it likely my sized brass will have a completely different load they like then the virgin brass or is more likely I'll just have a POI shift ?

[Unclenick]

If you know the length and weight of the bullet, the spin of the rifling and the direction of the twist and the muzzle velocity, then you don't need to know what the wind is. All you have to do is shoot at a target large enough to stay on paper using the same POI. We then estimate the gyroscopic stability factor either from the JBM site's calculator or from Geoffrey Kolbe's McGyro-based calculators, which are on his site. From that gyroscopic stability factor we get the angle of the diagonal. We draw a line at that angle through every bullet hole and see where it intersects a vertical line through the POA (or any other vertical line on the paper) and where the diagonals cross a vertical line is their relative POI height. That's the thing you are trying to find for an Audette Ladder.

[Metal god]

OK thanks UN . I'll apply fewer dots on the paper giving me more room to work with . The fact I know I'm safe pressure wise through out the ladder range . I'll drop a few of the lower charges I know I'll never use based on my intended use of this load . I'll then load a few more at higher charges to find my max pressure seeing how I don't feel I hit it yet .

I'll update some time after the 9th

Thanks again

[Road_Clam]

MG,
Another good read (scroll down to "Gyroscopic Spin and Coriolis Effect") it's a free .pdf file.

http://appliedballisticsllc.com/ball...rces/articles/

I was introduced to spin drift calculations when I took the Sig Reach For a Thousand instructional class. I've tried SDC's at 600 yds and it's not really a substantial factor. SD does become a factor at 800+ yds.

[Unclenick]

MG,

No problem. You should only need one dot if you can see through the scope which hole is made by each shot. That way you don't have to shift the gun on its rest. But you can shoot multiple dots as aiming points if you prefer. Just don't change the sight setting during the test or try to hold off for wind, and this will work out OK.

Averaging the estimates of gyroscopic stability factor for the 175 grain TMK at the muzzle for 2600 fps and after it has arrived at the target, I get a -21° line from left to right as the diagonal, so if you draw a line with that much drop through each hole and see where it intersects the vertical through the POA dot, you will have the adjusted height above POA for zero wind conditions.

Click below for a 21° angle you can print and cut out with scissors to draw the diagonals.



Road Clam,

While interesting, at 300 yards using the same bullet over a range of velocities that is about 10%, we probably won't see more than about a quarter of an inch of horizontal difference due to Coriolis effect. My assumption is that he starts with his sights zeroed for this bullet and doesn't touch them during the shooting.