I've been thinking about internal ballistics of my various air rifles for a while now and have had some ideas and some revelations. Some of it seems within my capabilities to improve... or at least test and experiment.
One of those things is how the leade interacts with the projectile. At the very rear end of the barrel, for several of my air rifles, there is a straight section that has been machined larger than the bore and the projectile that I'll call the freebore. Seems this is there solely to allow the bolt to close easily, with no additional force required to seat the projectile into the rifling. All of my rifles that have this feature have it machined quite a bit larger (.003-.010") than the projectile diameter. This allows for the projectile to tilt, move out of bore centerline, and also to move back and forth. I can't think of any reason at all that this diameter should be more than slightly under the groove diameter of the rifling, since this would eliminate all of the movements I've mentioned. I have implemented this change on my most recent barrel. If the projectile is the exact same diameter as the grooves, and the freebore is .001" smaller than the bore, very little extra force on the bolt is required, but all movement and most misalignment is stopped.
Next in the barrel after the freebore is the forcing cone. I read that (I think it was jeff siewert's book "ammunition demystified") that the forcing cone angle can have and effect on both on engraving force and, maybe more importantly, engraving force consistency. The data suggested that a forcing cone angle of less than 1.5deg has the highest force requirement, but also the greatest consistency. This may make a measurable improvememt in velocity ES and SD. I have incorporated a 1deg cone into my most recent barrel.
That said, one of my rifles bypasses the engraving force issue entirely by forcing the projectile into the rifling with the bolt probe. This sounds like a better solution, but wasn't practical to implement on the current project. This may also have it's own set of drawbacks in deforming the projectile. Not sure.
Sorry... no pictures of the barrel. Too hard to see inside, can't show the important details. More details on the rifle here:
https://www.airgunnation.com/threads/hp-ss-max-357-project.1307714/ Now that the barrel has seen a few improvememts... time to think about the projectile. I have quite a variety to choose from in this .357 cailber, but none have been specifically designed for long range, high BC, and low dispersion. Those are the goals.
I've done a variety of experiments with my flow simulation software, and although it's not tailored to this kind of project, I've been able to find some strong correlations with real world data that lead me to believe it's giving moderately accurate data. With "moderately accurate" in mind, I set out to make a projectile specifically for this rifle, for this barrel, with this leade and these goals in mind. Through this testing I've found some general ideas about what causes more or less aerodynamic drag. Some of the big observations were:
1. A longer, more slender ogive reduces drag.
2. Sharp transitions often (but not always) cause more drag. Examples are big flat meplats, lube grooves, sharp flat bases.
3. A boat tail will likely reduce drag over a flat base. My testing shows longer is better for less drag, and increasing the angle will also reduce drag up to 12 degrees. That said, I just don't see anyone using a 1.5 caliber long, 12deg boat tail. It seems there are some very good reasons for this that I don't fully understand, so I've kept my boattail design within reason.
4. A longer projectile with a given nose and tail will have a higher BC.
If I were to maximize all of these factors the projectile would be so long it wouldn't fit in the rifle, and so heavy that velocities would be painfully slow. Among other problems. So compromises were made, and a balance was found.
There are a few other factors that were also taken into account. Engraving force was a significant concern. I did my best to minimize this by using a bore riding nose and a narrow driving band at or slightly under the barrel's groove diameter. The fit can be adjusted with a sizing die. I've been wondering... how short can I go with the driving band? At what point is it not longer enough to handle the torque applied by the rifling?
Another concern was inconsistency in the friction of the projectile riding the barrel. The bore riding nose provides very little friction, so variance here will be minimized. The driving band has the potential to cause some significant variance. I'm currently working with the idea that: keeping the driving band slightly smaller than the groove diameter will reduce the overall friction, and therefor minimize the effect of an inconsistency. This is compared to a projectile or driving band that is the same or over the groove diameter, which will have significantly more contact with the barrel.
Yet another factor.... bourelette (spelling?) length. In other words, how long is the section of the slug that is touching the rifling? I did my best to find a good combination of a long slender nose, a long bourelette length, and a moderately long boattail. My experience so far is that slugs with a short bourelette length show terrible dispersion... I think this is a very important factor that many airgun slugs are lacking. I get the feeling that this design may still be lacking.
Boolit lube is another part of this. Was previously using heavy silicone oil which definitely reduced leading in the barrel, but may not be the best lubrication. Currently trying dry moly D from a spray can.
After taking all of this into account, here is what I've got:
Here's the mold:
Here's the slug after being seated into the barrel qith the bolt, then pushed back out of the breech for examination. The rifling marks show where the projectile is sitting when it is ready to be fired. 0.001" engraving (per side) on the nose, and the driving band has just started to engage the forcing cone. This is exactly as intended:
Haven't been able to simulate anything related to projectile stability.. other than a twist rate calculator. Because of this I have tried to stay within the boundaries of other designs that seem to work, and to avoid any design flaws mentioned here and elsewhere. Hopefully that's enough.
Anyway... will be testing this over the weekend. If I don't post groups then it's safe to assume all of this was for nothing.
It's the journey, not the destination, right?
