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Old 03-31-2014, 03:22 PM   #1 (permalink)
Whiskey Hammer
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Join Date: Nov 2013
Location: Jacksonville, FL (and a lot of the time, Tampa)

Accuracy, Never Enough to Go Around

We've completed not one but two rounds of accuracy testing and we've hit a bit of an obstacle. Initially, I wanted to have the data out in a "one or two week" timeline from the posting date but, for reasons we'll get into below, I'm not comfortable releasing it yet.

The problem in my accuracy tests, is accuracy. I'm not getting as much as I should be getting. Take a look at the below comparison of X-component deviation* in the baseline groups from Punkworks and our first shooting session. Punkworks is getting a final result that is more than twice as accurate.
  • Punkworks: 1.725 inches
  • First Round: 3.6 inches
Obviously I was missing something... I came across the FS Centrifuge post here on M. Carter Brown and I thought that it was a very likely explanation for my accuracy problem. The paint in bulk boxes doesn't sit uniformly so the asymmetric mass distribution of odd settled paint throws off the accuracy. Seems legit right? I grabbed a round and forcefully tapped it against a level surface to settle the paint against the nose of the round. Along the same lines of how you would force a bottle of ketchup to settle at the spout. I visually inspected the round and the paint seemed to be pretty uniformly settled in the nose. To be safe I allowed the rounds to sit nose down overnight before the next round of testing and re-tapped them before firing. The comparison between Punkworks, the first round, and the second round is below.
  • Punkworks: 1.725 inches
  • First Round: 3.6 inches
  • Second Round: 3.55 inches
No statistically important difference. I racked my brain trying to explain the difference, and while looking at the 8 shot group distinctions in the Punkworks data set, I remembered that their methodology artificially centered each magazine's group. It follows then that the normalizing might also have a damping effect on the natural randomness of the rounds. The most likely way they did this was to average each groups X and Y values, and then subtract that average from the actual values. This results in three groups whose average values (i.e. centers) are all at the same (0,0) point.

Edit: Ha, I knew I remeber seeing a video about this: Tiberius First Strike Test Thoughts by cockerpunk. It touches on the damping effect that correcting the groups has.

I applied (ostensibly) (confirmed) the same mathematical methodology to my groups and it made a difference. We got about an inch off the group, which is pretty significant. Definitely closing the gap but certainly not all the way there.
  • Punkworks: 1.725 inches
  • First/Second Rounds: approx. 2.6 inches
Now we just need to shore up that last inch.

Thinking about it some more, it could be the barrel setup. There is a short stretch of smoothbore before the rifling in my MR5 setup that could be throwing off the accuracy. We know that smoothbores don't do much to the spin of the round; in some cases it can even rotate in the wrong direction. It's possible that the smoothbore presection is allowing some rounds to get a counterproductive spin before they hit the rifling and that those rounds aren't being stabilized as well as they otherwise would be.

It could also just be that my paint settling methodology didn't work as well as long-term nose down or centrifugally settled paint. I might have just grabbed and visually inspected a couple of rounds that were naturally settled right.

Another possibility is that I'm botching something about the mathematical theory here.

So basically I need to shoot a group that has been centrifugally settled, shoot a group from a system that doesn't have the smoothbore presection, and mess with the numbers more/shoot a larger group size/get confirmation on the Punkworks math.

Once I figure out what's going on with my accuracy I'll feel batter tackling my 121 accuracy/drag data.

The issue going forward is money. It's a finite resource for most of us, and I definitely include myself in that number. It would be a serious kick in the dick to shell out money for the only tested rifled barrel that works natively with the MR5. That would of course be the Hammer 7 threaded Tiberius/Lapco barrel. The other less dick kickish option is to buy the threaded Hammerhead back for Ion/Impulse threads and do a test group on my Ion. There are a couple of concerns there but it's a decent solution from a trouble shooting standpoint. Anybody have the Lapco barrel or a Hammerhead back in Ion/Impulse they'd be willing to loan out? I'd of course be willing to put up collateral through a third-party for the full replacement value. I can float the money easy as pie, but I'd rather avoid sinking it when I need to restock my paint. Other than that, I'm gonna move some gear on craigslist and buy paint as sales allow.

Oh and the studio/warehouse owner shot some behind the scenes stuff when we did our first shooting session. Bro did it gratis and I think it turned out pretty cool. Check it out here.

*I prefer to compare accuracy using just the X-component rather than the vector. I'll get into the specifc reasoning in another this post, but for now bear with me.

Last edited by Whiskey Hammer; 04-01-2014 at 06:47 PM.
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