Grom rifle

[NeoTheFox]

Hi guys, here is my first spud rifle. I made it using the remains from countryside home construction.
It fires well, but I have a problem - the cover of the blast chamber is always cracking after a few shoots - it happens because it is held by 2 bolts and metal frame, and bolts are tearing the cover apart. I want to enforce it somehow.







It is not painted yet, because of my chamber problems. And yes, there are calamps and other stuff under the tape.

It fires screws covered with tape.

What do you think?

[Gun Freak]

Cool fueling, but that hose connection is unspeakably hurting performance.

[jackssmirkingrevenge]

I think I prefer the original Grom

You don't need such a big chamber, and as Gun Freak pointed out, the way it's connected to the barrel is not the way to go for good performance.

Also, consider syringe fuelling

[youtube][/youtube]

[NeoTheFox]

Thanks for feedback, guys!
When you are saying hose connection, you mean pipes connected to each other, or the hose I used to connect combustion chamber and the pipe?
I can see, how having funnel at the end of combustion chamber would help, but I never thought it is SO important. I'll build a new one soon enough, using an old teapot as a chamber, so I want to do everything right this time.
And yes, about syringe fueling - I am using spray as a fuel, and there is no way I can use syringe

[jackssmirkingrevenge]

Thanks for feedback, guys!
When you are saying hose connection, you mean pipes connected to each other, or the hose I used to connect combustion chamber and the pipe?
I can see, how having funnel at the end of combustion chamber would help, but I never thought it is SO important. I'll build a new one soon enough, using an old teapot as a chamber, so I want to do everything right this time.

It's got to do with how the fuel burns.

[youtube][/youtube]

That skinny pipe means that most of the energy is probably being wasted.

You should aim for a chamber that is short and fat, and if possible directly connected to the barrel.

And yes, about syringe fueling - I am using spray as a fuel, and there is no way I can use syringe

Yes you can! If you can't buy one from a Pharmacy in Moscow, then you could always try eBay

Jimmy has an excellent page on how to use a syringe and cigarette lighter: http://www.inpharmix.com/jps/Fuel%20Met ... ghter.html

[Technician1002]

To get an idea of what the hose does to your performance, Exhale as quickly as possibly through the paper core from a roll of paper towels. Blow a marshmallow out of it.

Now repeat the above task but use a drinking straw between you and the paper towel core.

Your hose does the same thing to your flow from your chamber to your barrel. The same chamber pressure you get blowing through the straw is the same pressure your chamber feels being restricted by the small hose. The pressure can't get out, so it breaks things.

[NeoTheFox]

Hi again guys, I followed your advices and removed hose connections, now I have a solid pipe coming from the chamber to the barrel. I also tried to reinforce the cover with a metal plank, but now the pressure bent metal corners of it. I then tries syringe fuelling with no luck, my calculations tells me I need 3 cm^2 of the propane, but even with twice as much gas it would not ignite (neither it will with less gas). However I figured out how to mod a lighter to fill the chamber, and now it fires even better, but it eats lighters very fast. Any idea why Propane fuelling failed? And thanks for advices I already took!

[Blitz]

Did you ensure you have the right mixture? ~4.2%. Beyond that, the more you add, the less powerful it'll be - until it won't ignite due to the lack of air (Too rich)

Also, just curious - how thick is that plate?

[jackssmirkingrevenge]

I could be that the large mouth of your chamber is allowing the gas to escape. Inject slowly, and with the syringe nozzle as close to the "bottom" of the chamber as possible.

Also, 3cm³ sounds far too low for your chamber size. It looks like you have a volume greater than one litre from the photos, meaning you should be injecting at least 30cm³.

edit: units...

[NeoTheFox]

The chamber's volume is 31cm^2, and the pipe is 2cm inner diameter, and 2,2cm outer diameter. When I decided to use syringe fuelling I sealed the cover, and made a small hole for fuelling.

[dewey-1]

I could be that the large mouth of your chamber is allowing the gas to escape. Inject slowly, and with the syringe nozzle as close to the "bottom" of the chamber as possible.

Also, 3cm² sounds far too low for your chamber size. It looks like you have a volume greater than one litre from the photos, meaning you should be injecting at least 30cm².

Should it not be 30cm^3 (30cc) or 30mL?
Maybe I am missing something here!

[jackssmirkingrevenge]

Should it not be 30cm^3 (30cc) or 30mL?
Maybe I am missing something here!

I stand corrected

NeoTheFox, I'm pretty sure your chamber is much bigger than 31cm³... are you sure you calculated radius x radius x 3.142 x length?

http://www.online-calculators.co.uk/vol ... volume.php