The Potassium Chlorate needed for Boomershoot 2011 Boomerite was delivered to my home in Idaho a few hours ago. Assuming the ground dries out enough to get it to the Taj Mahal by the end of April we just need some more target boxes, a few gallons of ethylene glycol, and the mystery ingredient (readily available from Costco in 50 pound bags) and we will have all the supplies we need for the next event.

On our recent trip to the Boomershoot site I walked the 150 meters from the road to clean up some left over targets while Ry waited in my vehicle. My size 14 shoes were close enough to snowshoes that I didn’t break through the crust on the snow and it was like walking on a sidewalk for me while Ry broke through and went in half way to his knees.

The first target I detonated was composed of the spill from another 850 gram target, a complete 850 gram target and a 200 gram target. Assuming half of the spill detonated that was about 1500 grams or about 3.3 pounds of Boomerite. This is a little larger than usual but not all that much. I shot it from 25 yards away and when it went off blobs of mud landed all around me and I was impressed with the thump I received from the explosion. Previously we had been setting off 200 gram targets from 25 yards and I was concerned that it didn’t seem that there was as much thump as I had expected. The only previous 850 gram target detonation had been from 470 yards and Ry reported it had the biggest flash he had ever seen and the thump seemed to be typical.

When I got back to the vehicle Ry reported the first explosion was quite remarkable. It was very thumpy. He also felt it before he heard it and he counted at least six echoes from it. He felt it before he heard it? Hmm… the shock wave had to be transmitted through the ground for that to happen. I did have that target on the ground as opposed to on a stake so the spilled Boomerite would get the maximum shock from the target so the ground coupling would be greater than normal. But how much faster does the shock wave travel through the wet ground than through the air? Would the 150 meters be far enough for the difference between the air and ground waves to be discernible?

A quick Bing search didn’t pop up anything for the speed of sound in water soaked earth but the speed of sound in water is about 1.5 km/s. The speed of sound in hard rock is between 5 and 7 km/s and the speed of sound in the air is about 330 m/s. Assuming the speed of sound in the wet earth was no faster than the speed of sound in water then the time to Ry from the explosion would have been about 0.1 seconds through the earth and 0.45 seconds through the air for a difference of about 0.35 seconds. This is easily discernible by a human. If the speed of sound is greater than the 1.5 km/s the difference increases.

Years ago when I had time to shoot targets at Boomershoot events I would shoot from the ground and it always seemed I could feel the thump from the explosion in my chest through the ground but I wasn’t quite sure. The thump through the air was quite noticeable too. This new data point confirms my impression from years ago.

I have been sick for days but Ry came over to Idaho with me on Friday night and yesterday we went to the Boomershoot site to do some tests. Boomershoot is May 1^st and time is getting short to make any last minute changes in the event. Ry has his own thoughts about the day on site and other ideas of potential here. I thought I would add a little more from my perspective.

At Ry’s suggestion, via conversations with http://www.ultimak.com/ (who blogs here), I found and purchased 100 each of some plastic containers to use for targets. These containers are actually made of a corn starch resin and are compostable. I was hoping that “compostable” would also mean “biodegradable”. Further research has revealed this is not the case:

PLA is not biodegradable in its “as-extruded” state. Instead, it must first be hydrolyzed before it becomes biodegradable. To achieve hydrolysis of PLA at significant levels, both a relative humidity at or above 98% and a temperature at or above 60°C are required.

I expected that UV light would break it down but again further research proves (from the previously linked document) that hypothesis is also incorrect:

PLA is also highly resistant to degradation by ultraviolet radiation, with no loss in elongation after 100 hours in Xenon arc testing, and minimal losses at longer exposure. Amazingly for a polymer that is readily consumed by microbes in composting, PLA does not support bacterial growth before composting.

This means that if we use these target containers we will have to pick up all the pieces of the targets that were broken but not detonated. As Ry pointed out in his post from yesterday the detonated targets are reduced to very fine shard less than 2 mm long and perhaps 1/10 that diameter. We were only able to find them because the paper labels we used to secure the lids retained a few of them throughout the explosion.

Another downside of the targets is that if you get an hit that does not detonate the target the back side of the target is blown completely open and the contents are dumped as shown in the picture below. But on the other hand it was easy for the shooter and spotter to see the target was dead.

This was a hit from a 50 grain Hornady V-MAX^TM traveling at about 1625 fps. Another target detonated from a similar hit (I had forgotten the satisfaction of seeing the view in the scope suddenly and silently completely fill with smoke followed by the ground shaking thump as the sound reached you 1.25 seconds later). And a third target received an edge hit without detonating. This is consistent with other tests on other target containers.

From the manufacturing and storage viewpoint these targets are awesome. They can be filled in probably 1/4 the time and with increased storage density.

We tested two different size targets.

One target size tested was the “32 oz Clear Rectangle Deli” which held about 850 grams of Boomerite. This compares well to the current seven inch square targets which contain about 900 grams of Boomerite. However the “active area” of these targets is significantly less. To be considered “a solid hit” the active area is only 4” x 5” compared with the nearly 7” x 7” of the previous cardboard box targets. We can get 16 of these targets into each of our storage crates which is a 33% increase over the 12 of the 7” x 7” cardboard targets.

The other target we tested was the “8 oz Clear Rectangle Deli” which held about 200 grams of Boomerite. This compares to current 3” x 3” cardboard boxes which hold about 215 grams of Boomerite. The active area is 3.25” x 4.25” which is a little larger than the current three inch targets about the same than the four inch targets. The current four inch targets contain about 385 grams of Boomerite. We can get 48 of these targets into each of our storage crates which is the same as the current 3” x 3” cardboard boxes compared to 18 of the 4” x 4” cardboard boxes.

On our way back to Moscow we talked about the test results and our major concern was that we expect there will be more spilled targets. An edge hit or failure to detonate results in a massive spill. This material must be cleaned up at the end of the day because it can spontaneously combust and result in unpleasant interactions with the neighbors. Later that night I figured out a way to easily spray the dumped targets with water and I had convinced myself we were a go for the new targets this year. Then I did the research on the UV resistance and biodegradability and am now mostly convinced we should not use them.

As of this minute my plan is to research some other plastic boxes (Ry found some and sent me links) before making the final decision on new target containers.