OCW shows its teeth with my CZ550

[Pond, James Pond]

Finally the day arrived for me to try out my OCW loaded selection, a full 9 months or so after I prepped them!!

Anyway, I set up my targets and got the gun ready (CZ550 Varmint in .308, harris bipod, Bladetech soft cheek cushion, Burris XTR 312 zero’d for Norma Jaktmatch 155gr rounds). All my rounds were Norma cases, AMAX 155gr bullets, and Fiocchi rifle primers. Target was 100m away, cool day, still, damp.

Using a max load of 42gr dead, I had 3 sighting rounds between 37 and 39 grs , then 3 more at 40.3, 40.7, 41.2, 41.6, 42 and 42.4. I started getting flattened primers from 41.2gr, and I out this down to primer brand, but when the one and only shot at 42.4gr was a bit tighter on extraction, I decided that I would not shoot any more of those and so my max full 3-shot batch was the 42gr load.

I did not record velocities.

I followed the OCW shooting system, cleaning my barrel every 6 or so shots and letting it cool for about 2 minutes between shots.

I opted for prone because I wanted as little of my bad shooting to affect the results. I am a little recoil shy with .308 and I expected it to be worse, but I actually found this position very comfortable.

Attached is a picture of the two targets used. For me there are 3 loads that show the most promise. 40.3gr. Here the three holes are touching: an absolute first for me! The next group is #3 with 41.2gr. This group is actually spread out but that is because I made a mistake on my point of aim second time around. By comparing aim and impact points for all three, I can see that, had I aimed at the same spot, that second shot would have been in the middle of the first and third: all touching again. My last finalist is 42gr. Two touching one just off by under an inch.

All were shooting way above the POI for the Norma Jaktmatch. 5.5” high for the 42gr load, for example.

So now I will make up some more of those three loads but with differing OALs.

So far so good!!

In truth, I am really happy with today’s outcome. It shows that, between me, and my CZ550, we are capable of just under MOA accuracy, at 100m at least. In any case that is a lot better than before!!

[Unclenick]

Have you evaluated the mean points of impact (POI's)? If you know the horizontal and vertical differences between actual shot locations and the points of aim (POA's) used to fire them (this eliminates the aiming error you mentioned) it is easy to use Excel to spot trends in both group size and center location. Ideally you don't want the group centers to shift over a span of three or more load increments. Plotting a running 3 shot average of the POI quickly finds the best candidate. If you assemble the data and want help with this, let me know.

The reason for the above is overcoming the 3 shot group size's limited statistical significance. By itself, based on just the one three shot sample, any given 3 shot group predicts just over a 6:1 range of group sizes that fall within the 95% confidence limits for future groups. This goes from roughly 0.4 times smaller to 2.5 times bigger, meaning future groups are expected to be within that range 19 times out of 20, and outside that range (even smaller or even larger) 5% of the time. But by evaluating the average of three of your three-shot groups fired in a row (three charge weight steps in a row) as one group, you are looking at a collective 9 shot group, for which the 95% confidence limits are much tighter, with a smallest to largest ratio of about 1.5:1; from about 0.8 times smaller to less than 1.3 times bigger. It provides a much more trustworthy result. I assume it is the reason Newberry looks for three groups in a row with the same POI.

[Pond, James Pond]

Well, Unclenick, I am sure that is excellent advice but you lost me at "mean points of impact" in line 1!! Nothing to do with your description, just my my mental block with stats!!

You offered your help and I'd be grateful for it. If I take a measurement for each shot vertically, and horizontally in cm and mm, could you then advise me on how to input and manipulate that data effectively?

[Eppie]

Here is a good thread that will open your eyes.

[Pond, James Pond]

Here is a good thread that will open your eyes.

An interesting thread. Based on that I could also add the 41.6 load as two of those are also vertical and the third may have been my twitchiness. The 41.2 grain bunch are virtually stacked one on top of the other.

Truth is I am happy for members to assess my targets and advise on which they think are the groups to examine further

[Jimro]

Looking at your target I would have assumed that your "sweet spot" was 41.0 grains.

Your 40.7, 41.0, and 41.2 gr groups look to be about the same size, and about the same 8 squares above your point of aim.

Jimro

[Pond, James Pond]

Shameless bump.

At the moment, I am looking at group numbers 1, 3 and 5 for further investigation.

Anyone else care to evaluate my targets and give me pointers on a) which groups to choose and b) what to do with them after that?

[Eppie]

James,
There is such a large variance form your point of aim to the point of impact that it is hard for me to offer any advice. I don't know if the problem is your reloads or your rifle.

I use Federal Gold Medal Match 168grain HPBT ammo as my control group. That ammo shoots like a laser. If you can't it with that ammo the problem is in the rifle and until you fix that can't fix the re-loads.

After I've established that my rifle is still shooting well then I proceed to work on my re-loads.

For example this first target made it obvious to me that something had changed within my rifle and it wasn't shooting accurately anymore.



The second target showed that cleaning had improved the situation. But more work is still needed to bring it on target. The middle target labeled FGMM is the "control" ammo. The others are my reloads.

[Pond, James Pond]

The reason there is a difference in POI and POA is that the scope was zero'd for a completely different cartridge as explained in the opening post. I expect the reason is greater velocity and/or better BC (less drag) on the AMAX's.

I am not interested in accuracy in relation to the POA at this point.

I am seeking consistency of impact, and that consistency is evident to varying degrees, most notably, IMHO, in the 3 groups I mentioned in the OP and my penultimate post.

[Eppie]

My opinion is that the answer for you lies somewhere between 41.0 to 42.0 grains. With big jumps from 41.0 to 41.6 to 42.2 you could easily be skipping over the accuracy node altogether. As you saw from the other forum, those guys all use more refined measurements and I think that's wise.

So I would start buy loading nine rounds each of 41.0, 41.2, 41.4., 41.6, 41.8, 42.0

Then do the round robin at one paper with 6 targets. Clean the bore and then repeat at a 2nd target and again at a third target. Then you have 3 targets with the same exact loads to evaluate. At that point you'll know a lot more and leave less to chance. I've had many excellent groups that I couldn't repeat for one reason or another.

Good luck

[Pond, James Pond]

With big jumps from 41.0 to 41.6 to 42.2 you could easily be skipping over the accuracy node altogether.

The actual load ladder used was 40.3, 40.7, 41.2, 41.6, 42. This is based on the 1% increments derived from a max load of 42gr as per the OCW method.

These were somewhat rounded up. The exact charges should have been 40.32, 40.74, 41.16, 41.58, 42 and an over charge of 42.42 (loaded to 42.4) of which I only fired one due to over-pressure concerns.

nine rounds each of 41.0, 41.2, 41.4., 41.6, 41.8, 42.0

54 rounds!! You're going to keep me busy!

I took me ages to put together the 20 odd rounds for this shoot!!

I think I need to book a week on the bench in my garage....

[Eppie]

James,
I didn't mean to make things difficult for you, I'm just telling you how I approach the same problem and I'm getting closer and closer to my goal. I will attain my goal and then surpass it. However, I want to remind you not to make the mistakes of changing more than one thing at the time. Patience and perseverance are virtues that you must embrace.

Make sure your rifle is zeroed for the bullet that you're testing. Don't overlook the necessity of a "control/benchmark ammo" without it everything becomes relative. That's like navigation without a magnetic north, very difficult.

Best wishes for a Happy New Year to you and your family.

[Pond, James Pond]

No need to apologise!

I moan but with tongue in cheek...

The fact is all this kind of advice helps me work out how to proceed. I do confess to thinking that I was almost there, having done the OCW work-up. I knew there was more to come, but for some reason I thought I'd done a larger portion of the job than I actually have...

Still, I've started this, so I'll do it and have a good load at the end of it.

New Year greetings to you and yours too!!

[Unclenick]

First, for anyone confused, what James is evaluating is precision, not accuracy. The chart below shows the difference. Precision is group size, and accuracy is group placement. Unless the sight is loose, a good load for a gun will show good precision. Good accuracy can then be achieved by adjusting the sights.



James handed me the basic data in the form of how many millimeters (mm) from the point of aim (POA) that each bullet hole was vertically, and how many mm from the POA each hole was horizontally. I used the convention of up and up and right as positive and down and left as negative. Below is a plot of the average points of impact (POI's) for each of the 3-shot groups. You just average all the horizontal numbers for each group and average all the vertical numbers for each group and use this as the average POI. I also found the average POI for all 15 rounds combined (red X) and inserted a bullseye as the aiming center indicator. Since James was shooting at 100 meters (m) rather than the 100 yards we usually used, I converted these to minutes of angle (moa) which both ranges would have in common.

At: 100 m, 29.09 mm = 1.00 moa



My original intent was to combine the groups in sets of three to improve statistical significance, but with only five groups and the fact there is a 0.5 grain jump between groups 2 and 3 rather than the 0.4 grain step used in the other changes, that seems unproductive. Instead I used Student's t-test to evaluate significance of differences in group size and location. More on that in a moment.

I also found the difference in radial distance of each group average from the average location for all groups and plotted this below (blue line and points). The flat spot (least change between loads) then becomes exaggerated. The second plot (red) is just another way of showing the same thing. This time the line is the slope of change in distance from the overall average divided by change in powder charge at each group. It is plotted against charges between the two used to find the slope. This exaggerates the information even more. Since the OCW criteria is to find a spot where mean POI's change least, the nominal charge weight would be where this red line is nearest to zero in value. That turned out to be 40.95 grains, so 41 grains looks best in this evaluation.



The averages behave like an Auddette ladder in this case, and the first image is what such a ladder would look like, except we are using 3-shot averages instead of single rounds to plot the trendline. The flat spot is where barrel harmonics appear to be most in sync with bullet ejection from the muzzle's recoil deflection ringing phase. That is, a load centered on 41 grains is expected to change POI least with charge errors or changes in temperature and other change influences.

The t-test suggests the horizontal displacement differences between the average centers of groups 1-3 are not significant, so the fact group 3 went left of group 2, for example, has a very good chance of being just due to random error and might reverse the next time the same test was shot. The same holds true for the horizontal difference between groups 4 and 5. Only the jump between 3 and 4 is likely to be more than random, and that's enough that we can conclude there is a slight rightward trend as powder charge increases.

The t-test suggests the vertical difference between group centers is probably real to better than 93% confidence for all but the difference between groups 3 and 4. The vertical difference between 3 and 4 has less than 50% confidence, so the vertical change between 3 and 4 has a good chance of just being random and might go flat or reverse if the same test were run again.

The t-stest results also show group sizes for groups 2 through 5 are not different to a very good degree of statistical significance. In other words, with just 3 shots per sample, again, the difference in their size may just be random rather than load tuning differences. However, group one is statistically likely to be smaller, and that's unlikely to be random.

So, there are two tasks to straighten out. One is to fire the test with better resolution (more charge steps and uniform charge steps) so a 3 group trend analysis would be meaningful and get us around 3-shot uncertainty levels. The other is to get enough shots either side of group one to see if there's a genuine additional sweet spot there or to learn if the tight group was a random event (still possible; just not highly probable).

I recommend three rounds each in 0.3 grain increments. You will not need 9 or more of each if we have enough to combine three consecutive shots without stepping way outside of the accuracy bounds. The loads I would shoot would be:

39.8 grs
40.1 grs
40.4 grs
40.7 grs
41.0 grs
41.3 grs
41.6 grs
41.9 grs


Nick

[Jimro]

I always feel a little bit of relief when Unclenick's math comes to the same conclusion my highly calibrated eyeball does. I'm only joking about my eyeballs being highly calibrated, they haven't been calibrated in years now!

I honestly think that James could do 20 rounds loaded with 40.9gr of powder and just play with the seating depth. 0.005 off the lands, 0.010, 0.015, and 0.020 to see if he could dial in a sweet spot there.

Jimro

[Pond, James Pond]

Wow....Just wow.

That is an epic post.
Very informative, but I can see why my maths-averse brain opted for a "no... not listening....la-la-la-la-la I CAN'T HEAR YOU" approach when you first raised the topic of stats...

Can I be rude and beg you to do the same with the next lot of shots if I provide the measurements? Please!

It is also good as it seems to confirm what has already been suggested by other members. I do prefer 3 shot groups: I can make those all up in one afternoon and get to the range the following day, if all goes well. I could be loading my first ever tailor-made cartridge by the end of Jan!!

[Eppie]

Uncle Nick, you must be an engineer or a high school math teacher. It is very kind of you to take us back to high school and reminds us of basic math class that most of my classmates slept through.

However, by just analyzing the data you're not able to differentiate between the rifle or the ammo, it is an aggregate result. I know money is always a factor, but would you agree that a box of FGMM ammo would go a long way towards isolating some of the issues?

[Jimro]

Eppie,

During load development like the point is to tune the load to the rifle, so the rifle isn't a factor worth differentiating out. As long as the scope is in good working condition and the mounting is solid the rifle error will be the same for each shot.

As far as differentiating out the shooter, that either has to be done with a sled or rest, and even if it isn't then the circle probability of error for the ammo + shooter can factor out the shooter when comparing different charge weights if you assume the shooter introduces the same variability in each group (not always a valid assumption, but one you have to make).

Jimro

[Unclenick]

James,

No problem. But Jim's suggestion would be a good step, too. I think I'd run the next round robin series first, though, to better investigate the tight group for being a fluke or not and what happens just below it. If we can validate that flat spot centered around 40.95 grains (or get its center better defined or find another below), then we should start looking at tuning seating depth for sure.

Toward that end, do you have a tool for measuring bullet runout?


Eppie,

Yes, an engineer. Though I confess six-sigma black belts and other statistical tools were not as popular when I was in school. I got through basic stats class, then took a quantum mechanics class where we were calculating particle position probabilities by triple integral form Eigenfunctions. A little like graduating from the wading pool straight to Olympic platform diving. I've had to scramble to fill in the middle ever since.

All the variables you mention are there, so we are attempting to hold all but powder charge influence constant. It's never completely successful, but with the statistics to help see through the noise as best they can, we should get a good sense of what the powder charge adjustments are contributing.

[Pond, James Pond]

James,

No problem. But Jim's suggestion would be a good step, too. I think I'd run the next round robin series first, though, to better investigate the tight group for being a fluke or not and what happens just below it. If we can validate that flat spot centered around 40.95 grains (or get its center better defined or find another below), then we should start looking at tuning seating depth for sure.

Toward that end, do you have a tool for measuring bullet runout?

Thanks. That will be a great help.

As for the OAL variation batch, it too is something I plan to do but only once I have the sweet-spot load that this next batch should help isolate. This evening I charged all the cases and will be seating the bullets tomorrow with the same 71.00mm OAL. With any luck I can shoot them this weekend!

Run out: no, I don't have a specific tool. In fact, I didn't even know what it was until about 4 days ago. My suggestion would be to roll a few cartridges on a mirror, 45 or 90 degrees at a time and measure the bullet tip to mirror surface hoping that the distances all come out the same.

[Jimro]

Does amazon deliver to Estonia?

http://www.amazon.com/Dial-Indicator...dial+indicator

http://www.amazon.com/VB-1-Capacity-...ywords=V+block

You can set up the V block to hold the cartridge, and use the dial indicator to measure runout on the bullet ogive. If you don't have machine table (like a mill or drill press table that you can mount the V block on) you should be able to make a base out of scrap lumber that will hold the V Block and dial indicator base in the same position. That could be as simple as gluing some strips of woood to a base at 90 degrees, driving in some nails, and using thick rubber bands to hold the v block and magnetic base stable relative to each other.

Jimro

[Pond, James Pond]

Those are nice, but such dials I could probably buy locally.

Failing that there is Amazon in a number of EU countries that should ship stuff like that all over this continent.

The problem with ordering direct from the States is that the Customs department get ready to feast when you come in to collect!!

[Jimro]

As long as you can get your hands on a dial indicator and a precision V block you will be able to measure runout in a more precise manner than rolling loaded rounds over a mirror

Although at 100 meters, bullet runout is not a huge contributing factor based on my experience. Much more important once you get out past 200 based on competitors who sort lots of ammo based on bullet runnout.

We've discussed why certain loads group better or worse at different ranges before here, but the bottom line is that less runnout is better for precision although you may not notice the gain in precision at short range.

Jimro

[Pond, James Pond]

As long as you can get your hands on a dial indicator and a precision V block you will be able to measure runout in a more precise manner than rolling loaded rounds over a mirror

I tried that earlier, you know!

I rolled one on the mirror, holding the edge of my vernier calipers to the bullet tip and there was no discernible movement in the tip.

Could it be that straight forward?! No, didn't think so....

[RC20]

James,

A couple of points from the ditch digger type (I shoot for fun and keep it fairly basic).

1. When development a load, don't worry about where it hits, what you want is how it groups (clusters).
Where it hits will change form day to day (weather, gun stock etc). If you can get good groups then where it hits is mostly a matter of sight adjustment.
Keeping in mind if it groups good and shifts a lot from shoot to shoot there is something wrong but its consistent, ergo a stock warp that is still solid. That gets into bedding issues.

2. 5 shot groups are the better take. Hunting rifles have hard time with that as they are intended to group good for a few shots but then can wander with their thin barrels.
A heavy barrel stabilizes that. A lot of heavy barrel in 308 are readily available (US) as its a popular long range shooter.

Me, I just shoot at 100 yards (range limit) have fun. If I get a good group I am happy. The experts will tell me I have to shoot 5 x 5 shot groups to get all the statistical norms and deviation (and all that six sigma stuff!).

Its very true but I am not in that league.

I load a variety of powders (Powder shortage and fun) and I do not worry where the current test load hits (as long as its on the target!) just how it groups with 5 shots.

I shot a round a while back and I got 3/8 (30-06 which sadly heavy barrels are custom only). 5 shots I had 1 1/4. I think that was me not the gun or scope but thats a better take on accuracy (or consistancy if you will)