If you didn't see my previous thread about this, here's the link,
So, Hornady says if you shoot enough shots per charge, you'll find that the group sizes will be essentially the same. IOW, charge does NOT affect group size if you shoot enough shots per charge. So, I can't resist it any longer, I'm going to shoot enough shots per charge to see if charge weight...
In a nutshell as they say, the purpose was to see if I shoot enough rounds per charge, will the groups wind up about the same size for all charges. Hornady says they will. So let's see what I got.
Mossberg MVP patrol with Timney trigger
Vortex Viper PST 6-24x (In hind sight, I should have used a 32x scope)
Handloads, 77 gr Sierra MK with Varget powder.
I used five charges in incremental steps of 0.3 gr. The scope was set for a POI above the dots so I wouldn't obliterate my aiming reference. I didn't quite get it high enough and kinda wiped out that top left dot. That may have contributed to group spread.
So let's start with the targets there are 7 of them at 3, 5, 10, 15, 20, 25, and 30 shots.
A few comments:
- First, I apologize for the tiny labels. I can't control the size. But I'll post a pic of a table of the group sizes below.
- It should be clear that the groups did NOT all come out nearly the same size! Of course this is for my gun, my loads, and my shooting procedure.
- If you're used to seeing 3-5 shot groups, the 30 shot groups are gonna look really nasty. However, notice that my 3 and 5 shot groups are pretty good. Without getting into the top tier guns, manufacturers specify accuracy guarantees of "3 shot sub-MOA". There are reasons why they limit groups to 3 shots. Anyway my worst 3 shot group is 0.781 MOA and my worst 5 shot group is 0.888 MOA - both sub MOA, so I'm really happy about that. And, of the 30 shot groups, one, the top right, was 1.092 MOA, and another, the bottom middle dot was only 1.2 MOA. For 30 shots both of those are excellent as far as I'm concerned!
- Also, there is one 30 shot group I should come clean about. It's the top right dot, a hit identified with a circle and the text "All me". That was my fault and I knew it as soon as the shot broke. I did not include that shot, because it would not accurately represent the precision of the rifle for that load. I must say also, that out of 150 shots, fired over three days, that's the only one I muffed - that's not bad.
- I was concerned that there could be a POI shift from one day to the next but I don't see that, that happened based on the tighter groups, top right and bottom center.
Here's the table that lists group size vs shots fired for each charge. Those "diff" columns shows some impossible results. Groups can NOT get smaller than a previous group size and those numbers in red indicate that's what happened. Well, that didn't happen - that's impossible!
It's challenging to measure group sizes at 3, 5, 10, etc. shots on the range - lots of walking and tying up the range. So my plan was to video the whole set and then convert the appropriate video frames into pics and then use OnTarget
to measure the group sizes. It sounds easy when you say it fast like that. Unfortunately I discovered between the scaling and hole designation process, some measurement error can creep in, and it did. Thankfully the errors are small for the most part, and what it really indicates is the group size didn't change with subsequent shots.
And then the chart of the table which clearly shows trends. The dotted lines are the trend lines.
While the group sizes seem irregular for the most part, the trends are all the same. the trendlines clearly show the more shots we shoot per group, the larger the group size gets. And that is the expected result.
So in summary, I'm inclined to say that all charges do NOT produce essentially the same size groups. It may be that top tier guns with custom chambers used with matched die sets would produce similar group sizes with various charges.
Here's a little challenge for anyone that believes their rifle/load will shoot sub MOA "all day long", go shoot 30 shots of one load at one POA and see if it will.
And I'll throw the following in for free.
You know all we hear about ES in the teens and SDs 10 fps or better? That may happen from time to time with low sample sizes but it's pretty much not reliable, repeatable, or consistent. I watched Eric Cortina shoot his custom rifle with custom handloads to see if a barrel speeded up as more rounds are shot through it. Hence he had to chronograph all the shots. I forget how many rounds he fired at a time, but out of about 7 sets, he had about three ESs in the 50's - just like I get BTW! Anyway...
I thought you might like to see how ES and SD occurs as we shoot more and more shots. So here's a graph of how my 150 shot ESs accumulated.
Let's look at the green line first. Notice for the first 5 shots the ES remained at 17 fps! That's excellent! I do need to explain that a bit. You have to shoot 2 shots before you get an ES. So of the first 5 shots I fired, my ES was constant at 17 fps. So, here we have that small sample size leading us to believe we've got something special. We don't.
All we really have is the lucky outcome that our tightest velocities came within the first 5 shots. If I had stopped shooting at 5 shots...
But look what happened as I continued to shoot - the ES keeps growing until it reached close to 80 fps. Also notice that all the charges went well past that mystical ES in the teens.
Also, I need to say something about that blue line. That was a strange one! A couple of shots were low and one was exceptionally high, 2357 fps. Mostly that exceptionally high, generated that abnormal ES of 97 fps.
The important thing to take away from this is that low sample sizes are really, really, poor predictors of consistent performance. It's just really the luck of the draw so to speak. If we happen to shoot 5 shots and they're really tight, then we get a low ES. Likewise, if we happen to shoot a high velocity followed by a lower velocity, then we get a high ES - and that generally will be indicative of performance.
And lastly, still for free, is how the SD develops.
It's interesting that once you shoot two shots, the ES can never go lower than those first two shots no matter how many rounds you shoot after that. The ES can grow larger, but it can never be less than the first two shots.
OTOH, the SD typically starts out high and develops downward in magnitude. But SD can go up, go down, or even stay the same. Take a look at the SD plot vs the number of shots fired. This just three of the charges.