Sharp as a Marble - I have wondered about this technology for some time now

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I have wondered about this technology for some time now

Guns. Lots of Guns.

Plastic cases for ammunition. Shotguns use them albeit with a brass base. Seems like a strong enough plastic could be a heck of a lot lighter, cheaper and easier to make.

Now, heat transfer is a bonus when it comes to brass cases. A lot of the heat gets dumped with the casing as it exits the firearm. I don’t know how much that affects the firearm or if it’s significant enough how you would counteract the heat buildup while using plastic. If I recall correctly, that was one of the big issues with caseless ammunition – that the chances of cookoff were higher because there was no brass casing to pull some of the heat out of the chamber.

Lord knows I’d love to be able to pick up 1000 cases for $10 rather than $100.

Hat Tip David Hardy

posted @ 12/8/2010 9:19:28 AM

Comments

Gunnutmegger says:

on 12/8/2010 10:01:47 AM

Some magazine tested polymer-cased .223 a while back. Might be worth looking up.

My concern would be separation of base & casing, during extraction. You are FUBAR if that happens.

Robb Allen says:

on 12/8/2010 10:11:58 AM

Gunnutmegger, I'm pretty sure that's easily solved. And I don't see why if the entire case can be made polymer why they'd really have to be two different pieces.

LC Scotty says:

on 12/8/2010 10:28:31 AM

I don't think heat transfer would be an issue-the plastic case will transfer even less heat to the chamber than a brass one will, meaning it will take even more heat away with it. Also, it will not transfer as much heat from chamber to propellant making cook-off even more unlikely.

LC Scotty says:

on 12/8/2010 10:29:01 AM

Are they reloadable?

Robb Allen says:

on 12/8/2010 10:31:56 AM

LC Scotty, that depends on their cost. I'm sure they ARE reloadable, but is it worth it? I mean, theoretically you can reuse your primers by reshaping the anvil, smoothing out the cup bottom, and filling it full of primer material, but at $30 for 1,000, why bother?

DaddyBear says:

on 12/8/2010 10:51:00 AM

What about the heat from the chamber causing contact melting of the case and either causing a jam or a "sticky bolt" type problem like we see with lacquer coated steel cases?

Robb Allen says:

on 12/8/2010 11:01:57 AM

Well, one would think this is solved by the appropriate polymer. One that doesn't melt at those temps.

mac says:

on 12/8/2010 11:41:30 AM

Near the end, it shows case cost reduction of 0-8% for the 5.56-.50bmg round. Where are you getting $30/1000?

andrew Sarchus says:

on 12/8/2010 11:48:31 AM

I think most of the chamber's heat gets there through the brass case (some must come from the barrel which is heated by friction and the burning powder charge). Therefore, a less conductive plastic case should lead to a cooler chamber.

I wonder if the (presumably) thicker case walls would limit load options in some cartridges.

Robb Allen says:

on 12/8/2010 11:51:17 AM

Near the end, it shows case cost reduction of 0-8% for the 5.56-.50bmg round. Where are you getting $30/1000?

Mostly my nether-regions, but the $30/1000 was in reference to primers. I was hoping more for $10/1000 on plastic cases ;)

Kurt P says:

on 12/8/2010 11:43:56 PM

I'm no expert, but I think the big difference between shotgun shells and rifled ammo is that the majority of shotgun shells use ---shot.

The (probably necessary) thickened cartridge won't matter so much when the cup and shot expands to fill the barrel.
The solid bullet needs to pretty much fill the barrel before it's shot, and the plastic neck would probably be too thin.

Gunnutmegger says:

on 12/9/2010 7:40:07 AM

Well, the reason you see the polymer cased ammo with brass bases is because there isn't a polymer that is tough enough to withstand the force of an extractor pulling it from the chamber.

Any place that you join 2 parts is a potential weakness. And the difference in material properties compounds the weakness.

Factor in the small extractor on an AR15, and you have a lot of force being applied to a small area on one side of the case rim. AR15s are already prone to ripping the bases off cartridges, hence the need for a Broken Shell Extractor. That problem will probably get worse with polymer-cased ammo.

TJP says:

on 12/9/2010 2:53:32 PM

Hmm, I thought polymer case 5.56 was already out there for sale. That's what I get for thinking, I guess.

Shotshells endure significantly lower chamber pressures than a .223, but the hulls hold up for three reloads. The damage is usually from crimping. There's flow evident in the hull, and I doubt a thermoplastic exists that would not get melty from burning smokeless at extremely high pressures.

I would think a polymer hull would be less likely to stick in a chamber. It's also damn near impossible to tear the hull off of a case head because the polymer is so elastic, especially when hot.

I doubt resizing is feasible. I mean it works for shotshells, but those have a rim and it's expected that the shell will become shorter as the mouth is trimmed for reloading, so precise dimensions aren't a concern.

Skip says:

on 12/10/2010 1:26:28 AM

So how do you resize case mouth on a plastic shell?
No setback on recoil?

Cybrludite says:

on 12/10/2010 8:20:06 AM

5-6 years ago, a buddy of mine tried some of the plastic cased .223 which he had found at a gun show. Jammed almost every shot. The neck was tearing off of almost every round in his Mini-14. Might work in a bolt-action, but nowhere near ready for primetime for a self-loader.

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