New, more efficient bolt idea...

[Cardinal_Biggles]

It occured to me while messing with my mag on a nearly empty tank that the bolt stem releases the air from the dump chamber almost at the beginning of its movement forward. This could be the main cause of blow-back. I thought to myself that if the bolt stem were extended into the dump chamber and measured so that it released the air at the foremost position of the bolt instead of at the beginning of the cycle, it would stop blow-back AND significantly increase gas efficiency. Since this has most likely already been explored by AGD, I just want to know why it doesn't work. If it hasn't been explored, maybe someone can try it.

[Bonx0007]

I think that the valve when properly gassed up releases the air close to what you are explaining. I thought that most of the blow back occured when the bolt returned to its original position. I could be wrong. Good post topic though.

[BlackVCG]

No, it's designed to release the air at the end of the stroke. AGD is well aware of this and there's a delicate balance between the bolt stem length and PT tip design.

[steveg]

No, it's designed to release the air at the end of the stroke

Black what you describe is simply not possible.
The instant the bolt stem clears the o-ring the air starts
to escape. If you compare the size of the spacer and
powertube tip to the stem you see a gap, and there is a gap
between the outside diameter of the powertube and the inside of the bolt.
The air WILL escape through these gaps, most will go
out the front of the bolt, as planned, but some is going
to escape thought these gaps.

An interesting experiment(that I haven't tried, yet):
take some talcum power or chalk line chalk, put a bit into
the powertube so that it goes into the dump chamber,
assemble the marker , fire once, disassemble and look were
the chalk goes.

[BlackVCG]

Do this: Take the bolt and look on the plunger rod and you'll see a raised surface at the end of the rod. Slide on the PT tip so that the threaded end is over the raised surface on the bolt. Look and observe the gap. There shouldn't be much of one. About a .002" gap to keep the surfaces from dragging, but still control the air blast. Once that raised edge clears the smallest I.D. of the PT tip enough, it releases the dump chamber air in about 1.5ms. Yes, there are gaps that can let out air once the plunger clears the o-ring, but you have to consider how fast everything is moving. What I meant by there being a delicate balance between the bolt stem length and the PT tip is that if you drill out a PT tip so that it's even .050" oversize, the pressure will release too soon and the gun will shoot like crap.

[DC Nelson]

Well, we were messing with it on about +50 psi of air in a 110 cui tank...
Not enough pressure to push the bolt forward all the way.
However, it released the air just after the bolt started moving.
The air, without a doubt, releases at the beginning of a normal/full bolt action (in this case, the bolt action went a mm or 2 at most... yeah, i stuck my finger in there. hehe).
The main thing i was thinking about... is it releasing it enough through the duration the bolt is going forward, that there is air going up the feed tube?
If so, how much is going up the feed tube, and how much is making it down the barrel?

So, if the stem was lengthened, it would have 2 main effects.
1) The air would release from the bolt at the END of the bolt cycle (when its furthest forward), instead of the beginning, causing it to be released when the bolt is already past the feed. Getting rid of the blow back up the feed.
2) Again, since the air would be released at the END of the cycle, There wouldnt be air going out the bolt the entire time the bolt was moving. Would you have to use the same ammount of air to fire the ball? Instead of waisting all that air up the feed, it goes down the barrel, increasing velocity.

And thats what the Spanish Inquisition has to say on that.

DC

[Butterfingers]

Air is NOT released to any significant degree until the taper in the PT tip at the end of the cycle. Air will leak obviously but the amount of air that gets released in the 10MS duration the bolt takes to blow forward will be insignificant.

Lengthening the stem will do nothing except impede flow and reduce efficency.

AGD has spent ridiculous amounts of money designing the system. There is a delicate balance between the stem and the PT tip design. If somthing as easy as lengthening the stem would imporove performance they would have done it.

[DC Nelson]

OK, so the air going up the feed is basically backing out the barrel after the bolt retracts back past the feed?

I know that there has been a ton of time put into this... Thats why we were wondering what we were not seeing.

Why cant we be friends.
DC

[athomas]

The gas escaping out the front of the bolt while on its forward motion is miniscule because of the forward velocity of the bolt itself. The escaping gas is essentially traveling at the same velocity as the bolt itself.

"Blowback" occurs because the bolt returns to its opened position before the ball exits the end of the barrel. The pressure remaining in the barrel expands in two directions, out the barrel pushing the ball, plus up the feed tube. The amount of residual pressure in the barrel directly affects the amount of blowback.

I wonder if barrel venting helps with the amount of blowback since there is greater release area for escaping gas.

Hope this helps

[jwyke]

Just a thought, does anyone know exactly how far down the barrel the paintball is when the bolt returns to its open position? Since the paintball is not going to be accelerating past this point, could you put a large amount of venting in the barrel right where the paintball would be when the bolt is open without affecting speed or accuracy?

[Butterfingers]

it takes about 6-7ms for the ball to reach the end of a 12 inch barrel.

The bolt is mostly closed for that time period. If you hava a longnose bolt blowback for the most part has been eliminated.

[Cardinal_Biggles]

Ok...

I see how the speed of the bolt moving forward would be about the same as the gas. That makes more sense combined with BlackVCG's and butterfingers' posts. Thanks guys, I think this was an interesting topic. I find the immaculate timing of the mag to be nothing short of miraculous .

[soilent green]

yeh extending the stem would not only restrict proper flow but would cause friction and bolt stick