Swaging: Driving Bands and their Design

[Matt304]

Hey guys,

I am a machinist in my spare time and I have some questions for driving bands as I will be turning my own projectiles for a ~1"/25mm bore cannon.

First, can anyone point me towards some good info about the swaging process of applying the bands to the (in my case, steel) projectiles? Mainly I am looking for process details so that I can obtain proper data in order to create dimensions and machine the correct type of swaging dies for application of the bands.

Second, it would be great to have any other general info about the driving band design itself, such as proper materials used to create the bands, design as it applies to rifling engagement, or pressure and/or velocity, etc.

I can of course copy existing projectiles by sectioning and studying their overall design if that's what I have to end up doing.

I did spot something that threw me off when I was scraping the paint off of a 25mm TP projectile on the driving band. I expected to see brass when I started taking the paint off the projectile, but instead it appears that this target practice projectile does not in fact have a swaged-on band--instead the projectile body appears to be one piece, and the driving band actually looks to be made of the same thing the projectile body is made of, that being steel. The band is not the typical type as seen on say 20mm Vulcan TP design with a single raised plateau of band, rather the band has a ring groove turned in the middle to create two smaller bands (like rings) of engagement. If I have to I will make a cut to ensure it's all the same material as one piece, but from the small amount of paint so far removed it does look to be steel the band is made of. This threw me off obviously because one would think a steel band would tear the rifling up!

I appreciate any help I can get here, thanks!

 

[WestKentucky]

Slow down a touch. Driving bands are typically used on fixed ammunition. Your talking about a bore greater than 0.5 inches, and you mentioned rifling. That combination is a recipe for an extended stay in some cramped accommodations and food without flavor.

I get the question about driving bands, but what is it that your trying to feed, and are you certain that what you want to do is legal?

 

[AJC1]

Hey guys,

I am a machinist in my spare time and I have some questions for driving bands as I will be turning my own projectiles for a ~1"/25mm bore cannon.

First, can anyone point me towards some good info about the swaging process of applying the bands to the (in my case, steel) projectiles? Mainly I am looking for process details so that I can obtain proper data in order to create dimensions and machine the correct type of swaging dies for application of the bands.

Second, it would be great to have any other general info about the driving band design itself, such as proper materials used to create the bands, design as it applies to rifling engagement, or pressure and/or velocity, etc.

I can of course copy existing projectiles by sectioning and studying their overall design if that's what I have to end up doing.

I did spot something that threw me off when I was scraping the paint off of a 25mm TP projectile on the driving band. I expected to see brass when I started taking the paint off the projectile, but instead it appears that this target practice projectile does not in fact have a swaged-on band--instead the projectile body appears to be one piece, and the driving band actually looks to be made of the same thing the projectile body is made of, that being steel. The band is not the typical type as seen on say 20mm Vulcan TP design with a single raised plateau of band, rather the band has a ring groove turned in the middle to create two smaller bands (like rings) of engagement. If I have to I will make a cut to ensure it's all the same material as one piece, but from the small amount of paint so far removed it does look to be steel the band is made of. This threw me off obviously because one would think a steel band would tear the rifling up!

I appreciate any help I can get here, thanks!

Cool project. Any pictures for us slow people.

 

[Matt304]

I am very certain what I am doing is legal. What's up with the investigation?

 

[Riomouse911]

Since you didn’t specify what the projectile you were designing would be used for, and referred to the one 25mm projectile you’ve looked at for reference, I think the poster was referring to you possibly building projectiles for guns larger than the ATF allows:

https://www.atf.gov/firearms/firear...-firearms-national-firearms-act-definitions-1

Stay safe.

 

[Matt304]

I know exactly what he was referring to. I simply find it obnoxious that any time someone posts something that another reader can find some circumstance of illegality they can imagine applying it within, they whistle dixie and sound the bell saying "you can't do that!", and it happens almost inherently at this forum. It's as if these posters feel all of the other people who can legally make or build whatever it is which they cannot are locked into another universe where internet discussions of such things can never cross into their own universe to be seen. My gosh, really? Did it ever occur we all use the same version of the internet? Let's stop with the legality checking everyone; it sends off strange vibes when your peers are so paranoid.

 

[EricBu]

Good grief. "Teacher teacher, Johnny didn't do his book report". OP, your project sounds interesting. Are you using BP/Pyrodex as the propellant? What kind of cannon? You're own design, or are you basing it on some classic cannon, but trying a more modern projectile to improve accuracy? Super cool project!

 

[WestKentucky]

I simply find it obnoxious that any time someone posts something that another reader can find some circumstance of illegality they can imagine applying it within

All it took was a simple, “yes I know it’s legal”, it didn’t take a rant. I apologize on my own behalf and on the behalf of all the other guys on here who see crazy questions from time to time that are leading somebody down the trail to a conviction for doing something dumb. I fully apologize for a simple question trying to make sure that your not one of the guys doing something dumb and trying to go to prison for a felony. Now, go have your fun and enjoy prison if that is in fact where you end up. And by the way, I find your ridiculous defensive post obnoxious. That is telling much in the way that my 10 year old tells on herself when I ask her what she’s doing.

 

[Matt304]

And here we have it. Another thread bites the dust. Thanks

I'm using a straight wall 20x102mm Vulcan case with shoulder blown out to make a straight wall. As it turns out 25mm falls into the sweet spot of bore sizes for something known as the 4-bore, a round with sporting exemption. A larger version of one of my previous projects. The reason why I bought up all of the 25mm TP-T projectiles remaining I could find. When I run out, I will be making them, though probably making them well before I run out at the rate things are going.

Take it easy on the 10 year old.

 

[Matt304]

After some research, some info surfaced I thought may be suitable in the discussion of driving band design.


There's a notable phenomenon occurring in large bore cannons (though it is a phenomenon always occurring in propellant-driven firearms, nonetheless) in which reduction of barrel bore life is exacerbated by the total amount of propellant increase, enlargement of the grain size of the propellant, as well as the effects pressure-induced flow paths during combustion create in conjunction. Some may think of the term referred to as "overbore" when describing cartridges known to have short barrel life, and the term most generally refers to cartridges with a relatively large case capacity for the size of bore used.

As some get used to seeing what these overbore cartridges look like at the smaller levels of scale and bore sizes, I.E., rounds like the 220 Swift, 22-243, 7mmRUM, the Lazzeroni's, the Allen Magnums--a general image may start to form in the head of the kind of size relationship needed between case and bore size to qualify as an overbore cartridge having short barrel life. In other words, one can start to take quick glances at various cartridges and instinctively know the barrel life won't compare to the more "average" rounds such as 223 Win, 308 Win, 30-06, or something thereabouts of capacity/bore relation. However, this simple skill to develop for spotting overbore cartridges can begin to fool you as the bore size increases! Or what I should say is, as the bore size increases, overbore criteria, as it would commonly be imagined, tends to be a situation which develops much more easily. In other words, short barrel life develops with less of a size ratio difference between case capacity and bore size as it would with smaller calibers.

I could probably generalize and say that this is somewhat due to our perception of things when we look at cartridges; we tend to view them in a 2-dimensional representation from pictures. The case width; the bore size--things often appear 2D in form, but what is going on physically is occurring in 3 dimensions. That might seem like a, "well duh", sort of thing, but what I mean is that the forces grow 3-dimensionally as the cartridge scale increases. This would mean things like the energy involved in powder granuals slamming against the throat of the bore when leaving the case; when the powder grains have 2x the cross sectional area, they grow with a weight factor of a cube, and hence the energies involved scale exponentially instead of linearly. These forces tend to do brutal things to barrels as the scale of the cartridge becomes larger, since the forces have grown exponentially inside the combustion area.

This all boils back to when I had originally been somewhat perplexed that the driving band of a projectile would be made from steel. It is in fact made of steel, the same steel as the projectile body on the 25mm TP-T, but when I began to do some research and found how short the barrel life can actually be on some of these cannon barrels--simply from the combustion events of the powder--I learned that the friction from the projectile body is really small potatoes in that comparison. This is likely why, along with a few other factors, things uncommon at smaller cartridge scales are not uncommon at larger ones. As it turns out, the 25x137mm cartridge used in the Mk242 Bushmaster chain gun actually used a grease ablative paste inside the M919 round's case shoulder, to aid the throat life from the large-granule propellant that moves that projectile out at over 4,500FPS. The powder charge itself is the devastating thing it would appear, rather than any projectile friction with the bore.

Just thought I would share what I thought was an interesting bit on the bore-breakdown process.

At 4-bore levels, using less than 1/2 the propellant charge as the 25x137mm case would allow, things would be much less severe during firing, so it would seem that machining projectiles using a steel driving band will probably be quite alright for me. However, what I will most likely do is come up with a 2-piece projectile design, so that the rear portion containing the driving band won't require the whole length of projectile body to be machined (turned) away to create the step where the driving band would protrude out from. That would be a lot of metal to remove in order for the body to be the correct diameter--would be much easier to simply start near the correct diameter with 1" bar stock, for the primary portion length, then either thread or swage-on a rear portion of larger diameter to create the drive band segment.

 

[Matt304]

For reference this case will be straightened, and driving the bullet above it. See attached.

I'm working out the chamber final dimensions to be used, then I will turn a barrel with fire form chamber for doing the fire forming.