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I've posted bits about this crazy reloading idea as semi-relevant mini-hijacks in other threads, but I thought I'd start a dedicated thread for the few people who might be interested in my latest geeky reloading project, and possibly to motivate me to do more work on the project.

I envisioned my Ten556 project as a way to shoot a fast 5.56mm bullet from a Glock 10mm handgun, maybe on the order of the performance from a PLR-16 or short AR-16 pistol. I'm not planning on any long distance accuracy. The main goal is just to see if it can be done, although if it's even partially successful, it should result in good penetration for whatever that's worth.

This is the CAD screenshot of the sabot I designed for my slowly progressing Ten556 reloading project. I used my resin 3D printer to print a plate full of these (32) in tough polyurethane. A 5.56mm bullet goes inside the sabot, which I will then load into a 10mm cartridge in place of a 10mm bullet.

Blue Product Cylinder Plastic Table


I'll need to develop a load with a powder charge that provides maximal performance with the very light-for-caliber projectile, without exceeding the maximum chamber pressure, while reliably cycling a Glock 40.

Potential Problems & Possible Solutions:

1) The polymer sabot may not work in the Glock 40's polygonal rifling. I have a 6" stainless barrel with convention land and groove rifling for my Glock 20 if needed.

2) The sabot may disintegrate in the barrel. I previously made the sabots from ABS using an FDM 3D printer that extrudes a thin filament of plastic but the layers don't bond very strongly. I would have been surprised if those sabots didn't break apart as soon as the chamber pressure reached 500 PSI. I have a much better feeling about the sabots that I 3D printed from a tough structural polyurethane on my resin 3D printer. If they aren't strong enough, I can redesign them with a thicker base, closed sidewalls, etc.

3) The bullets may not be spin stabilized, either because the sabots strip out and don't engage the rifling, or because the sabot is spinning but little rotational inertia is transferred to the bullet as the sabot spins around it. I can change the dimensions of the sabot to firmly grip the bullet and engage the rifling, as needed.

4) It may not be possible to make a load that reaches but doesn't exceed maximum SAAMI chamber pressure using such a light projectile. The usual 10mm powders probably won't work, but faster powders should work. I previously had good luck with Longshot powder when I developed a Mach 2 9mm SUB-2000 load using a 90 grain .380 Auto JHP bullet, but the Ten556 will be an even more radical departure. The good news is, there is more room in the 10mm case for powder. The 9mm was case limited, so I needed a powder that produced a lot of energy for a small volume of powder but also didn't have a tendency to produce a dangerously rapid pressure spike. I'll start the development with Longshot powder, but can also try other faster burning powders if needed, being very careful to slowly sneak up on the powder load, as the pressure increases are very nonlinear.

5) The light bullet and sabot projectile may not provide enough recoil energy to cycle the Glock 40 without producing dangerously high chamber pressures. I could always use a recoil spring that compresses more easily, but I'd like to have a Ten556 load that functions reliably in a stock 10mm Glock.

I 3D printed the sabots weeks ago. I'm going to pull them off the printer and post cure them tonight....

Here they are!

Tool accessory Games


The bullets, from left to right:
Hornady 33gr poly tip
Hornady 40gr V-Max
Hornady 52gr BTHP Match
Hornady 55gr V-Max

Unless the sabots are seated very deeply, it looks like 35gr may be the largest bullet I can get to feed from a 10mm magazine. I remember having that problem before, when I used the FDM printer to print the crummy sabots. I think I need to make the sabot 6mm deeper to use 55gr and 62gr bullets, even though that will substantially reduce the case volume that can be used for powder. I typically use 6.6gr of powder for 10mm and 26gr of powder for .223 loads, so this project may be totally impractical.

I need to chamfer the base of the sabot. The contact on the build plate grows because the light that cures the resin spreads at the build plate. Making the sabot chamfered at the base will compensate. Rather than having a wider flange, the undercut base will be easier to pry off the build plate with a scraper blade and will load into the case more easily, but these sabots weren't too bad for a first attempt.

The bullet cavity inner diameter is too large. The bullets drop in without any resistance. They don't rattle but they should be a press fit. The resistance fit diameter is 5.87mm so I need to reduce the inner diameter by .2mm.

The outer diameter of the sabot is 10.26mm, not including the base flange. It needs to be .1mm smaller.

The current sabot weighs a bit over 9gr. Lengthened, it'll probably be close to 12gr, for a total projectile weight of 67gr to 74gr with a bullet, which is much less than the typical 135gr to 220gr 10mm bullets. The Ten556 projectile is half the lightest 10mm load, so we're well off the load charts. There be dragons here!

I hope to head to the range some weekday in October when I can hopefully have the place to myself for some load development. I also have break-in loads for 6.5 Creedmoor for three rifles I've never shot. How sad is that? Unfortunately, Mom has dementia and we lost one of the sitters so I'm spending a lot of time with her every week until a replacement can be found, and that's cutting into my range time. I wish I lived in the country where I could shoot off my back porch.
 

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Nice. I've been interested in these sort of projects ever since I saw the guy load a .223 bullet in a sabot for the 7.62x25. But you're braver than I am. This will almost certainly always be academic to me as I'm likely too chicken to try a sabot in any center-fire metallic cartridge.

I think you will be able to figure out a safe pressure load. Cycling will almost certainly be an issue. The .22TCM had to replace the slide, barrel, and springs to get it to work right, despite being a (more-or-less) necked down 9mm case.

I also think you're right about the spin stabilization. Because the proper twist rate is determined by both the length of the bullet and the width, using a "thinner" bullet will almost certainly give you too slow a twist rate to get ideal spin. Maybe it won't matter at short distances or maybe you'll keyhole worse than What The Butler Saw.

I was also worried about feeding from a mag but I think you got that figured. I wonder if your resin will drag on the feed ramp and bind up just because it's not "clad" or "copper patched" maybe?

Peace favor your sword,
Kirk
 
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I consider keyholes to be a perfectly viable alternative to JHP.

I have no experience here. But why not neck down and buy a custom barrel instead of the sabot thing, which looks like trouble to me? 556 may be too much to neck down to, but there are loads of 7mm sized things to play with that might do if you are open to other projectiles. It looks like you are doing it the hard way (but the alternatives look like the 'expensive' way, which can quickly become prohibitive).

also, any concern about the sabot melting etc under fire?
 

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Discussion Starter · #4 ·
The recent 7.5 FK from FK BRNO is not a necked down cartridge but is for all practical purposes a necked down 10mm. The pistol even ships with a 10mm barrel.

https://www.fkbrno.com/field-pistol.html

The performance, including penetration, is impressive.
There are other necked down 10mm cartridges, and that's certainly a valid way to go, although with more development work required than 3D printing some sabots. The sabots are a tradeoff. Roughly 20% of the energy is used to accelerate the sabot instead of the bullet, and the sabot represents another component in the reloading process for added cost and complexity. Bottleneck cartridges are more complicated to reload and barring case neck annealing, neck cracks usually limit the number of reloads. I generally reload straight walled pistol brass until I lose the brass.

The 9X25 Dillon is a less extreme necked down 10mm and the 224 BOZ is essentially what you described, a 10mm necked down to .223. It achieved the near AR-15 performance from a pistol that I'd like to achieve, so that's encouraging despite the inherent problems of this project. I'd like to know which powder they used. :) There is also an effective 9mm necked down to .223 which did well despite the problem of case volume limitations.

There will probably be some melting of the polyurethane sabot in the barrel, but I expect that as a result of friction with the barrel more than the heat of the expanding gases from powder combustion. The entire process is over so quickly that there isn't much time for heat to transfer into the sabot. I may need to use a bore solvent that has solvents to remove plastics, similar to shotgun wads melting in the bore and leaving a plastic residue. That may be more pronounced with a rifled barrel.
 

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The 9X25 Dillon is a less extreme necked down 10mm and the 224 BOZ is essentially what you described, a 10mm necked down to .223. It achieved the near AR-15 performance from a pistol that I'd like to achieve, so that's encouraging despite the inherent problems of this project. I'd like to know which powder they used. :) There is also an effective 9mm necked down to .223 which did well despite the problem of case volume limitations.
I'd heard about the 224 BOZ but didn't know a lot about it. Some internet reading has set me straight. From what I read they had to lighten the slides to get reliable cycling, same as they had to do for the 22TCM. The article below does talk about powders:

"Propellant choice was varied, again dependent upon the projectile and desired performance, but all were conventional commercial double or single base such as Win 296, H4227/IMR4227, Viht N120 loaded to 105%, with Max/Min practical burn rates spanning Alliant 2400 to Rel7. In addition to the basic ball, there were AP tracer, frangible and blank designs. The two primary rounds were the 50-grain jacketed spire delivering (in the reworked Glock) 2000 fps and the 40-grin steel core achieving some 2300 fps."​

https://www.gunmart.net/ammunition/rifle-ammunition/case-histories-224-boz

Then there's this:
Ammunition Bullet Gun accessory


Peace favor your sword,
Kirk
 
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A few physics equations relevant to this and all other wildcat endeavors.

F= M*A or force equals mass times acceleration

1. If force is increased and mass is constant, then the acceleration (and thus the final velocity) is increased.
2. If mass is reduced and the force is constant, then the acceleration is increased
( lighter bullets go faster)

F= p*a or force also equals pressure times area (of bore diameter)

1. Necking up the caliber increases the area, thus increases force, thus increases acceleration( if mass is constant )

With regard to the Boz round, loaded to the same pressure and using the same bullet, this sabot round will achieve much higher velocity.
Remington had the Accelerator line using sabots to fire .223 bullets in various .30 caliber chambers. Problem was, twist rates in most .30 cal rifles was marginal for stabilizing the 55 gr bullets they were using. Other issues hampered the accuracy as well. Elsewhere I have read people poo-pooing the idea of using these sabots in the 300BLK. They say "Why not just shoot 5.65?" With the larger bore diameter, Similar
loads
with the same bullet, the 300 with sabot should match the 5.56 with less pressure; and without another upper. Plus, the higher twist rates used in BLK barrels to stabilize the subsonic 190-220 grain loads means that it is well suited for heavier 5.56 bullets. Again, other factors may hamper accuracy.

My biggest caveat is the difference in bore diameter( .22 to .40.) I fear the thicker petals may not upset as easily and may drag the bullet. However engaging soft targets at relatively close range, this may not matter.
 

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Discussion Starter · #7 ·
F= p*a or force also equals pressure times area (of bore diameter) Necking up the caliber increases the area, thus increases force, thus increases acceleration( if mass is constant )
True enough. However, mass isn't constant. A sabot generally trades plastic for lead, and lead is 11 times denser than plastic, so the combination of sabot plus bullet will be significantly less massive than a bullet of the same size. The Ten556 total projectile mass will be 1/3 to 1/2 that of a 10mm bullet.

In general, the pressure bearing area on the base of a projectile increases with the square of the diameter and the mass of a bullet increases with the cube of the diameter, assuming the bullet is scaled in all three dimensions equally and not kept the same length while the diameter increases. In practice, rifle bullets have a higher aspect ratio, so a rifle bullet in a sabot to replace a pistol bullet is the same length but a smaller diameter, so the mass decreases approximately by the square and not the cube.

Sabots aren't magic, but they do have an advantage in allowing a larger area exposed to chamber pressure to increase the force accelerating the projectile while keeping the mass low to increase the acceleration. The lower projectile mass increases acceleration linearly. Half the mass accelerates twice as fast. The larger sabot diameter increases the acceleration exponentially because the area increases with the square of the diameter, so twice the diameter increases the accelerating force four times. Those two effects combined confer a large benefit to sabots, although there are many disadvantages as well.

There's an invalid assumption in the above analysis. It assumes the chamber pressure is constant. It isn't. Not even close. We can't even assume that the varying pressure curves are the same profile because they're not. The powder burn rate is dependent on the pressure and the lighter projectile accelerating down the bore more quickly increases the volume and therefore decreases the pressure, which slows the burn rate, which further decreases the pressure. The relationship of projectile mass to burn rate is very nonlinear, and tends to offset the sabot advantages described in the previous paragraph. The burn rate versus projectile mass is extremely difficult to model, so the ammunition companies develop loads the same way that reloaders develop loads... by starting with a low powder charge and gradually increasing the powder mass while monitoring pressure and muzzle velocity. The ammo manufacturers have better pressure monitoring capabilities and heavy barrels that can safely withstand dangerous pressures, but the basic process is the same.



With regard to the Boz round, loaded to the same pressure and using the same bullet, this sabot round will achieve much higher velocity.
In theory, yes. In reality.... :D

Murphy has a way of messing with my dreams, even when I think I've calculated all of the details, and I'm far from that assumption for this project. But that's part of the fun.



My biggest caveat is the difference in bore diameter( .22 to .40.) I fear the thicker petals may not upset as easily and may drag the bullet. However engaging soft targets at relatively close range, this may not matter.
Shedding the sabot usually isn't a problem at rifle velocities. Sometimes, there is a delay in shedding a wad in shotguns. I'm more concerned about the sabot not engaging the rifling, or the sabot spinning in the rifling but not transferring the spin to the bullet. I can adjust the ID and OD of the sabot so I can fine tune the fit of the sabot in the rifling and the fit of the bullet in the sabot. 3D printing is awesome like that.

Taofledermaus just released a very interesting video of a solid brass 12 gauge slug that's modeled after the Lehigh Defense Xtreme Penetrator pistol bullets. The slug is loaded in a plastic shot cup which is essentially a sabot, and it's shot in a rifled shotgun barrel. Jeff discusses my sabot concerns related to bullet spin stabilization.

 

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Shedding the sabot usually isn't a problem at rifle velocities. Sometimes, there is a delay in shedding a wad in shotguns. I'm more concerned about the sabot not engaging the rifling, or the sabot spinning in the rifling but not transferring the spin to the bullet. I can adjust the ID and OD of the sabot so I can fine tune the fit of the sabot in the rifling and the fit of the bullet in the sabot. 3D printing is awesome like that.
I believe this will be a non issue, after some tweaking. The S.L.A.P. rounds made by Winchester work well enough for the military to use them. Since you have your own printer, minute changes will be quick and easy. Look forward to hearing about your results as you progress!
 
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