auto-piston v4.0 - the bullpup

[SPG]

Yes it's yet another auto BB theory which will

• 1. Probably not get made
  2. Probably have a fault even if it does get made

But in the spirit of sharing and the fact I know JSR won't be able to resist, here goes.

I think it's fairly self explanatory.

• 1. HP gas enters the chamber
  2. Pressure builds inside
  3. Pressure overcomes the spring
  4. Barrel assembly moves forwards opening bb port
  5. Whoosh
  6. Pressure drops in chamber
  7. Barrel assembly movers rearwards closing bb port
  8. New bb drops into port out of magazine
  9. Repeat

Three things would have to be adjusted to get it to fire correctly I think:

• 1. Spring tension
  2. Gas flow
  3. Chamber size

1. & 2. could be adjustable in the design easily, a screw in end cap at the front of the chamber to adjust spring tension and a simple ball valve for gas flow. Chamber size would be a question of working it out in advance with GGDT based on your optimum gas pressure.

I can see this working at BB scales with something as simple as a 12 gram CO2 tyre inflator, like this



which even gives you a nice looking little trigger to squeeze.

[Skywalker]

Not bad, not bad! I just wonder, though, whether this would suffer the same fate as other similar guns that 'hang,' so that they act like a regulator and not a gun.

[jackssmirkingrevenge]

Ingenious, but it's basically a different configuration of the blow forward bolt:



The problem - basically what Skywalker said - is guaranteeing sufficient pressure drop on firing to allow the bolt to return far enough to pick up a new round, it will be the constant battle between spring tension and incoming airflow that probably won't be resolved.

[SPG]

Yes it is a blow forward design but greatly simplified, I've had it hanging round in the back of my brain for ages, but always resisted it because I don't like the idea of bofors barrels.

The tension/flow battle is the killer though isn't it? I'm thinking very very low inlet flow and high spring tension, that's why I was looking at CO2 tyre inflators, they have truly tiny ports and most are regulated to about 110-120 psi. It takes my CO2 inflator a few seconds to inflate a 700C tyre up to 110 psi, so I reckon that it might just give enough time for the action to cycle before the chamber pressure is too high.

But that'd be down to fine adjustments and at least with this I think built-in adjustability is possible.

Actually looking at this I can see it'd work fine for my AutoBB Hybrid as well, fill chamber with propane/air, close fill valve, trigger spark, bang, slide, whoosh, oww, repeat.

[jackssmirkingrevenge]

Low inlet flow will push the barrel forward enough for the air to leak through, without cycling the action - this happened consistently with my blow forward designs, in spite of all the adjustments. For these systems to work adequately it's clear to me that you need the air to be pulsed, hence my current R&D.

Agreed re the Bofors barrels, I suppose it is a cool effect, expecially on a stubby large calibre weapon like the Mk.19 AGL, but I'd rather see a reciprocating bolt than barrel.

[SPG]

Low inlet flow will push the barrel forward enough for the air to leak through, without cycling the action - this happened consistently with my blow forward designs, in spite of all the adjustments.

I have a theory it's down to the springs but my understanding of Hooke's law in relation to this kind of application (actually any application) is nill.

HINT, HINT

[SPG]

Ok then, how about this. Same action as above but with the built in slide valve to pulse the gas supply.

I don't even think you'd need to worry too much about the valve sealing. As long as the flow is almost cut off it should cycle, and you've got a second valve on your gas supply to act as the trigger valve.

[jackssmirkingrevenge]

That's more like it - you could apply the same logic to the blow forward breech too.

[Skywalker]

What's to make the piston keep moving forward once the air can start leaking to the barrel? That's where the problem is with these designs. Once the air starts leaking, you can't make the piston go any further forward. But you need it to, so that the chamber vents out the barrel and the pressure drops so that the bolt will return and thus complete the cycle. There's no reason it should work!

[SPG]

The air can't start leaking to the barrel, because there's a BB in the way. As long as you've got a decent fit barrel, and not just something the BB can rattle down, then the BB is keeping the system closed until it leaves the barrel.

Which obviously means you don't want a 1:1 chamber:barrel ratio.

I actually think I might try building this one (for a change) I think though I'll make it so that I have two alternate gas entry ports, so I can replicate both versions of this, just to see which one works in reality.

I've got a theory on combination epoxy and plastic tubing for complicated gas flow systems like this too.

[jackssmirkingrevenge]

With the blocked off flow I would say it has a chance of working, bring it on!

[Skywalker]

I still say it won't, but you may as well build it because I can't seem to explain myself very well, and I may be wrong anyway!

[SPG]

The thing is the air can't go anywhere but out the barrel, and there's a BB in the way stopping it going out, so until the BB's got out the way, the system is still effectively sealed. As soon as the BB's left the barrel it doesn't matter what happens. In all likelyhood the system will seal leaving the chamber partially pressurised, but that doesn't matter because the next shot needs less new gas to fire.

[Hotwired]

With the blocked off flow I would say it has a chance of working, bring it on!

JSR will further encourage you by implementing the same dual piston sliding seal system in his auto piston

[SPG]

*wonders if he wants JSR's influence* I mean it's not like he's had a lot of success.