I’ve always had the best accuracy with pure lead. Yes, it’s a bit more difficult to get good ‘looking’ slugs but they do shoot better with air.
Dave
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Slugs & Molds - Designing, Making and Testing
- Thread starter caliusoptimus
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Dave
I just love the huuuuge hollow points, these gaping abysses that do minced meat with my pests....
WHEN can I finally buy slugs like that...?
Matthias
Note, that for the extreme expansion I'm hoping for, the lead needs to be as soft as possible: "Pure makes splatt!"
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Looking at the pictures and thinking of external ballistics only, a couple of things come to mind. First, your meplat diameter. For drag, you do not want a meplat larger than about 60% of the projectile diameter. Anything above that will cause drag to increase sharply, which will degrade your cross wind performance at longer ranges. Secondly, the first slug, the Mr Hollow Point, has rounded edges on the meplat whereas yours are quite sharp. This will affect the air flow over the nose of the slug and may affect the aerodynamic stability. The Kolbe calculator is based on the MacGyro software, which will not be able to cope with the large Meplats or the hollow parts.
Perhaps it may be better to forget the hollow point for the moment and concentrate on the external shape. A hollow point which is only slightly off-centre could have a big effect on your group sizes. Making solid slugs will get rid of this variable. If I were you, I would not use a conical straight sided nose shape, ogives are much better and have lower drag at subsonic speeds.
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Thank you! That all makes a lot of sense, and I'll definitely put it into practice.
I've been thinking about this for a while... you seem like the right person to ask-
Would a steady state wind tunnel style CFD simulation make sense for comparisons between boolit designs? I have access to software that can do this with precision at subsonic velocities, but I know for sure that there are certain things (like rotation) that I will not be able to simulate.
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There are things which CFD finds very difficult for accurate prediction. One is large blunt areas facing into the airflow, particularly with sharp edges around them. Another is if the said large blunt area has a hole in it. Then there is the question of the edges of the blunt area, where tiny changes in the shape of the edge can give large changes in the airflow. Unfortunately, at the front of a slug, you have all of them. Then you have the slug sides with unknown rough cuts in them from the rifling or in the bands supporting the slug in the barrel. Then there is the hysteresis effect of a slowing down projectile, where the aerodynamic changes tend to lag a little behind the change in speed and Mach number. And finally, the speeds which you are interested in, i.e. transonic, are the most difficult to calculate. It is very easy to get it wrong.
The good news is that if you already know the answer, then you can get a good representation of the airflow by optimizing your mesh sizes, boundary layer model and the number of iterations to carry out. So if you have a baseline configuration with good measured data, then it is possible to simulate the effects of small changes.
You can also produce very pretty pictures which make it look like you know what you are doing on videos, in reports or on forums.
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Hah! Guilty on all counts with previous projects. Gotta love the pretty pictures, and even more so when real world testing matches them.You can also produce very pretty pictures which make it look like you know what you are doing on videos, in reports or on forums.
In my previous projects, that also involved sharp edges and flow, I kept cranking the mesh up until the results stopped changing. Ended up being a two day long simulation but results were solid. Those projects were waaay more complicated... but definitely subsonic.
I'll give it a try and post some pretty pictures. I will NOT, however, pretend to know what I'm doing
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Exciting stuff, and with Miles chiming in, it gets even more so! 
Miles, your explanations what makes a hollow point slug aerodynamically BAD was a great summary.
Of course, what makes it BAD in the air — makes it GOOD in flesh for inflicting more deadly wounds.
➠ I keep coming back to this: Gun choices, scope choices, projectile choices — the centrail issue is purpose! Or rather, of balacing contradictory priorities.
Thanks for the enjoyable reading during my break. Now back to work. I might even get to advance a bit on my 25 yard gun range at home....
Matthias
Miles, your explanations what makes a hollow point slug aerodynamically BAD was a great summary.
Of course, what makes it BAD in the air — makes it GOOD in flesh for inflicting more deadly wounds.
➠ I keep coming back to this: Gun choices, scope choices, projectile choices — the centrail issue is purpose! Or rather, of balacing contradictory priorities.
Thanks for the enjoyable reading during my break. Now back to work. I might even get to advance a bit on my 25 yard gun range at home....
Matthias
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A quick design based on some photos I've seen on this forum like the winning rmac big bore projectile thread.
Did some refinement of the nose and boat tail based on the results of the sim... ideal tail angle was about 10deg (per side) and length of the tail was about 0.5 calibers. Interestingly the radius on the base improved the aerodynamics by about 5%. I've been putting this radius on various designs based on a hunch that it would eliminate aerodynamic effect of an imperfectly cast base... and that might actually be valid.
And turns out a longer and larger ogive radius improves aerodynamics as well... but I suppose that's kinda obvious.
Anyway... no clue if this simulation is real world accurate. Just playing for now, but I'll try to validate the the sim with a known projectile.
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10 degrees is too high an angle for the Reynolds numbers of airgun slugs. Wind tunnel data suggested a maximum angle of around 6 to 8 degrees. The boat tail on the round in my avatar was too large, though some did manage to fly successfully with a BC value of .057 for a 14.5 grain .22 slug for use in 12FPE guns. Any more and the airflow will be likely to detach and then reattach at different yaw angles, which will mess up the stability. The base of a boat tail should also always be a sharp corner. If you have a curve, such as you have on the base, the air will again detach at different points on the curved section depending on the yaw angles, which will cause stability problems. This problem has been seen in live firings on other projectile designs, which is why you are unlikely to see radiuses on large calibre projectiles.
As for the longer nose, in reality anything more than a hemisphere gives very little drag reduction at airgun speeds. It is only if you are getting over 1000 ft/sec that you may see larger differences. A longer ogive will also increase aerodynamic and inertial destabilizing moments, needing faster twist rates, which will increase the likelihood of dynamic stability problems.
Don't forget, nothing flies pointing directly into the airflow, there is always a yaw angle on a spin stabilized projectile.
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I did design a variant of the avatar round with an ogive nose and the modified boat tail, the main reason because it looked cooler. It came out very similar to your design in its proportions, but obviously very different in its construction.
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Thank you again for your help.
Is that projectile 2 different materials? Avatar looks like a lead core with 3d printed ends.
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3d printing didn't exist back then, it was 35 years ago when those were being made by Gerald Cardew for testing. It is basically a lump of lead for the mass and to be in contact with the barrel, with plastic aeroshells front and back. It was a way to get the different moments of inertia and the aerodynamics how I wanted them for stability and better ballistic behaviour. There are two ways of improving group sizes, first you can try to make the projectile leave the gun with very little or no initial disturbance, secondly you can accept that no projectile will leave the barrel perfectly and try to minimize the effects on the projectile's reaction to the yaws and yaw rates. Pellets and many slugs use the first method, this projectile was trying to use the second.
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Working on validating my simulations this morning... looking really decent so far.
G7 standard projectile, 0.5" dia. Zero yaw Cd result was 0.117 at mach 0.5. Atmosphere was NTP dry air.
Same parameters, but this is at mach 2. Cd .309
Seems very close. Certainly close enough to mess around on a hobby level.
Also, if anyone else is interested in this stuff I found a good book that is free in pdf form if you google the title: Modern Exterior Ballistics
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The G7 projectile has a small meplat which makes it easier for the CFD to get right.
Bob McCoy's book is known for containing a lot of mistakes due to him sadly dying before it was finished. Later editions have a large number of corrections, but I think there are still some errors in the equations in some places. So use with caution and try to follow the derivation of an equation before using it. It is a pity as it is probably the most comprehensive and easily available source on modern external ballistics.
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Nice. Can you show it for Mach 0.8, 0.9, 0.95, 0.98, and 1.0?
That’s where it gets interesting.
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Solidworks.
If I figure out how to automate that, definitely. Was thinking I should run the whole range from M0.1 to M5 or something... just to see how the graph compares to real word data.
Beyond that... I don't really know what to do with the data. If I can get some decent subsonic comparisons between known good designs and my own tinkerings I'll be satisfied. Ballpark BC estimates might be helpful too.
Edit: I figured out how to automate that. Will give it a try.
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Oh it's interesting alright. This is a G7 sim. Blue is sim data red is data from JBM.
I'll turn some knobs and push some buttons... maybe it can improve.
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I ended up making this design into a 357 mold. Not that I need even more 357 molds... but I needed one that dropped at .3595"
Left is 10deg boat tail, right is 6deg tail. Pure lead is 175gr and 178gr. My mold design is dropping extremely consistent weights, with >90% being +/-0.2gr. They both shoot pretty great with no obvious issues at 100yds out of a 1-20" twist.
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