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3mm HPA protoype w/detent seal

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3mm HPA protoype w/detent seal

Unread postAuthor: jackssmirkingrevenge » Mon Oct 18, 2010 3:35 pm

I made a prototype to test the viability of a 3mm HPA launcher for my sentry gun project, basically a tube fed detent type launcher. Sadly the rate of fire is far higher than I expected, in spite of the tight detent, I guess it's a result of tube as opposed to cloud type feeding, which puts the BBs directly into the breech.

Similar behaviour can be observed in this design along the same lines though without a detent.

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Here it is in action at 420 fps, the first video is a 25 round burst which as you can see happens in very little time, followed by some single shots.

[youtube]http://www.youtube.com/watch?v=L_gCCC0c04g[/youtube]
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Unread postAuthor: jrrdw » Mon Oct 18, 2010 3:52 pm

Why not restrict (slow down) the action on the bolt to slow the ROF?
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 19, 2010 12:48 am

The tube feed allows a BB to get into the breech with very little flow, but the problem with this style of detent is that it stops the BB assymetrically which skews it under pressure and allows it to leak, so I can't restrict air flow too much because the air will start leaking past the BB at a higher rate that what is coming from the source and pop-off pressure will never be reached.

I suppose my best bet (without going through the complication of making a dual detent) is to have a washer/o-ring-in-a-compression-fitting style detent that will support the BB uniformly.

edit: in line with the above paragraph, plan B:
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Unread postAuthor: Gun Freak » Tue Oct 19, 2010 4:44 pm

Looks ok, when will testing begin? I am using the same principle on my detent bbmg. The problem is when I give it too little flow there is not enough air to seat a bb in the detent, but when I give it enough, the ROF is really high.
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 19, 2010 11:19 pm

Gun Freak wrote:The problem is when I give it too little flow there is not enough air to seat a bb in the detent, but when I give it enough, the ROF is really high.


The "tube fed" coniguration means the BBs don't have to be agitated as much a cloud design to feed into the detent so low flow isn't much of a problem.

Testing due later today, watch this space.

edit: ah, much better, you can see the individual projectile impacts at 420 frames per second:

[youtube]http://www.youtube.com/watch?v=AJbcJNUxQQc[/youtube]
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Unread postAuthor: Gun Freak » Wed Oct 20, 2010 4:16 pm

Nice, so it slowed the ROF down?
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 20, 2010 11:43 pm

Gun Freak wrote:Nice, so it slowed the ROF down?


Very nicely, you can see the delay between holes in the coffee can quite clearly.

The problem is that the elbow is causing some jams, I'm going to have to re-do it in the same configuration as the original with an inline fitting for the barrel.
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Unread postAuthor: JDP12 » Wed Oct 20, 2010 11:48 pm

Looks good, definitely slowing ROF down. Power's a little weak, do you have any idea what pressure it is approximately "popping" at?
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Unread postAuthor: jackssmirkingrevenge » Thu Oct 21, 2010 12:35 am

JDP12 wrote:Power's a little weak


How did you figure that, goes through both sides of the coffee can as if they weren't there ;)

do you have any idea what pressure it is approximately "popping" at?


Somewhere close to 800 psi, if I tighten the fitting further the BB just seals up and doesn't fire.
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Unread postAuthor: JDP12 » Thu Oct 21, 2010 12:57 am

My bad, looked like it was merely denting. Couldn't tell.

That's good. I may have to try this with my HPA BBMG, as an o-ring detent seems alot more air efficient for reasons you mentioned about regarding sealing.

Are you satisfied with the performance other than slight jamming issues?
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Unread postAuthor: jackssmirkingrevenge » Thu Oct 21, 2010 1:01 am

JDP12 wrote:My bad, looked like it was merely denting. Couldn't tell.


Actually you have a point, the first shot bounced off the can, but the rest were straight through penetrations. What seems to affect power with detent mechanisms is how quickly the next round follows the first into the breech, because effectively the delay between one projectile fiting and the next one sticking in the detent is the "valve dwell time".

Are you satisfied with the performance other than slight jamming issues?


Very much, I'm confident that remaking it with an inline fitting will solve those issues. If I can make it a tad quieter it's going on the sentry turret ;)
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Unread postAuthor: JDP12 » Thu Oct 21, 2010 1:05 am

Nice. how far away was the muzzle from the cans? Have you tried any distance shooting-- say 10 yards or so?
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Unread postAuthor: jackssmirkingrevenge » Thu Oct 21, 2010 1:15 am

JDP12 wrote:how far away was the muzzle from the cans? Have you tried any distance shooting-- say 10 yards or so?


A couple of feet from the cans, haven't tried it further away but over 10 yards there shouldn't be too much energy lost, and I don't intend to use the sentry much further than that. I'll do a couple of longer range tests once I have it working reliably.

Edit: version 3 curing, testing due tomorrow:
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Unread postAuthor: lozz08 » Thu Oct 21, 2010 11:47 pm

for my 6mm mag fed the tee jammed until I inserted an angled object into the tee where the bbs come up that doesn't allow the bbs to jam against the top of the tee and sort of guides them forward instead.
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