SpudFiles

Hi! My name is Rabbit from Atlanta Georgia. I build and sell a lot of war machine replicas and I recently started in on a 1/2 scale german wwII 88mm AA gun. It has a 1 1/2" sprinkler valve, an 8' long barrel (2"), and a 36" chamber (3"). It has 360 degree traverse, -5 to +65 degree elevation.
The few shots I got off in testing were great. At only 40lbs I got a 250 meter shot ( 2" nerf) at about 15 degrees elevation. But I got too crazy with lubing the barrel with wd40, it got into the valve, and the seal blew out while I was charging it.
I thought that the firing blast (s) would keep out the lube, but somehow it got sucked deep into the valve and beyond down into the chamber.
Ive cleaned it best I can, and repaired the seal.
I feel I get much better performance with wd40 added. Is there a way to stop the suck back, or Im just using too much...or shouldnt be using it at all?
Thanks!

Use a dry mold release on the Nerf Vortex like this:

http://www.northwoodstm.com/store/pc/co ... roduct=190

Thanks! Ill try it.

Ah, the 88

Nice work!

Have you thought of using a cartridge system to up the realism?

Here's some work I had done with combustion cartridges: http://www.spudfiles.com/forums/cartrid ... 20150.html

Thanks! I will surely look into those cartridges.
I also want to learn how to do breach loading cannons for the tanks I build.

I'm working on a very similar project (Breechloading). Very nice, sir!

Rabbit, can you post a picture of the rear of your cannon? I believe I came up with a breech-loading solution, but I don't know if it'd work on your cannon.

I would like to see your breechloading system.
Im going to sell this prototype 88 as a muzzle loader, but the next one i very much want to try for breech loading.
I made a fake breech on this one and the box in front covers the valve. The fake breech covers the 2 elbows to the chamber. See pics.

On a side note, im having bad problems still with the valve. I can get 1-2 shots before it begans to leak badly. Fires while im charging it....I can disassemble, everything is clean and looks fine, and then reassemble...get 1 more shot, then bad leak. Im only putting 30-40lbs in. I dont know what to do. Its been sitting since saturday

That's some pretty nice handiwork there sir, me gusta.

That tube with the tee at the rear -- is that a sight? Pretty cool, I can see how you'd adjust it.

So okay, with your breech -- I don't know how the elbows in the rear of your cannon interact between the barrel and the chamber, but if your valve is between those elbows and your air chamber, then you could hack into it and use the breech system I was thinking about. Having the structure around the rear like that will make the breech system easier if that box is durable material (though you could reinforce it with metal strips or something if that's wood), though it wouldn't be what was accurate to the gun at the time (horizontal sliding block)... actually, you may be able to. Is it easy to get in that box to the elbows and valve?

Alright, your homework assignment is to answer the following questions for me.

- Naturally, your valve is between the barrel and chamber. Is it between the rear elbows and the barrel, or between the rear elbows and the chamber?
- What's the pipe diameter between the valve and barrel?
- Tell me more about the boxes your valve and rear elbows are in. How thick is it, what material are they and how difficult would it be to access the plumbing inside?

I'll say it again, cartridges! Insanely simple, powerful enough for your purposes, and couldn't possibly be more realistic!

I reckon there's enough power there to power a semi-automatic breech just like the real '88. The only pisser is that some fields might class it as a "pyrotechnic". Still, pneumatic cartridges are also possible.

Well sure it could be more realistic, they could use smokeless powder.

That was part of my idea, honestly, but optional. Realistic use of cartridges in this design would still require a properly functioning breech.

Blitz wrote:Realistic use of cartridges in this design would still require a properly functioning breech.

A sufficiently powerful cartridge would allow you to use a close variation of the original mechanism, saving design effort and adding realism

That'd be sweet. How so?

Plenty of nice footage of the 88 in action here, you can see (for example around 2:27) how the recoil force is used to open the breech and eject the spent cartridge, ready for the next round to be inserted.

[youtube][/youtube]

There's a lot of power in a combustion cartridge, believe me:

[youtube][/youtube]

ATL Rabbit wrote:Fires while im charging it...

Sounds like you either have a leak in the pilot area, or you're not filling it fast enough.

Technically, the recoil is not directly responsible for the actuation of semi-automatic breech mechanisms, but does initiate it. I get what you mean (Minutes before I made my previous post, I watched that exact video. Probably the best footage of FlaK 18 action IMO!), just wanted to make sure everyone's aware that the recoil isn't what forces the breech open, but rather some awesome cam action and a breech actuating spring.

Here's a nice guide on operation explaining this in more detail here - I truly believe this type of semi-automatic breech actuating can still be accomplished without self-contained cartridges. But you're right, that's a level of bad-assery that shouldn't be overlooked.