I have been thinking about the "wall" a lot lately. Call it what you will, RPM threshold, Wall, that special place in a boolit's flight where everything turns to poopy diapers, or that thing that can't be named that is to be ignored at all costs......whatever.
I've seen it, youve seen it, even the folks that claim it doesn't exist are beating their heads over it.
The faster you shoot, at a certain point everything goes to the dogs. Some people can shoot faster than others, but at a certain point, the inevitable happens.
It seems that RPM is the force that acts on our projectiles to cause them to go haywire suddenly as speed increases. That's not a limit, it's just a fact of science. Like saying that your tires can take only so much torque until they spin on the pavement. You have as much effect on where your particular boolit quits being accurate as you do on how often your tires squawk.
The thing that I can't get my head around is paper patch and sabots. Seems that I can shoot much faster with those things than I can with a simple GC cast boolit.
Why?!?!?!
Obviously, the patch protects the boolit from something that the barrel does to it, but unfortunately, recovering a soft lead boolit that has been fired at 2400+ undamaged enough to draw conclusions from is a frightfully daunting task! I have tried and tried to come up with a way to do it that doesn't involve me moving to Alaska so I can shoot snow drifts.
However, it occurred to me that shooting at high speed could actually damage the boolit as it leaves the barrel of the gun in ways that you would never detect, even with a HS camera unless you were really really lucky, because the blast would cover up what was happening to the boolit, and by the time the boolit is out of the plume of flame, it's too late to see it.
So, I considered the conundrum. Instead of trying to solve the whole problem, I decided to see if I could just get some clues by "fingering the scale" to my advantage. I thought the most important thing is to recover boolits that had passed through the barrel, propelled by a powder blast, and make sure they were as unmolested as possible. Well, the easyest way to do that, would be to shoot them slow.......like really slow.
I've never messed with cat sneeze loads, so I called Larry Gibson (thank you sir) and he got me lined out in short order.
I loaded up my 30-06 Argentine with a .313 groove (hey it was janky and handy and perfect for my purposes) and loaded up some soft .310 diameter 110gr boolits that I got from milkman last year (thank you sir).
I paper patched a bunch of them, and I loaded a bunch of them on top of 2 whole grains of bullseye, and I fired them into soft, wet clay/sand that I dug from the side of the house.
What I saw was interesting, and I thought I would share.
The rifling smeared the driving bands into the lube grooves and left "fins" inside of them. What's more, they were not even all around the boolits. Even at very low speed, the boolit was not strong enough to center itself in the barrel and was effected in an uneven manner. The thought that I had as I looked at the closeup pictures of the boolits was that it would be very possible for the muzzle blast coupled with the rapidly leaving boolit lube, and also the centrifugal force caused by the RPM to break off some of those fins thereby imbalancing the boolit terribly!
Thus, I wonder if the trip down the barrel sets up a fragile condition but it's at the point that the boolit crosses the crown of the rifle that the damage is actually done to it.
Look at these pictures. One of each groove on one of my recovered boolits:
Think about if those fins were pressed into a lube groove full of lube and then blasted out of the muzzle at high RPM. There would almost certainly be damage to those delicate little features.
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SO what about paper patch?
I theorize that the patch eases the damage to the boolit (this is backed up nicely by the following pictures) which makes it so that material is displaced equally forward as well as backward on the driving bands (no skid effect) and more importantly, acts as a blast shield as the boolit crosses the crown and gas leaks around it. Also, since there is no lube in the grooves, I think it eliminates the effect of hydraulic plucking as the lube leaves the grooves at high speed.
Here's the pictures of one of the paper patched boolits:
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I apologize for the greater damage to the paper patched boolit, but it made a better seal on the barrel and consequently, was fired at much higher speed. Still, the damage is greatly reduced and I think these pictures have shown me something that I couldn't have seen even with a high speed camera.