Blowback semi auto action on pcp

[David.Soliman]

For the geeks out there, a while back I saw Ed from Edgun playing with what seems to be a blowback type action kit for his Matador. The concept seems interesting, the more I think about it the more it makes sense, I mean you may think you'll be wasting air when the breech opens, but what if the breech spring and the mass of the breech were heavy enough to keep the breech closed untill the pellet leaves the muzzle?

So as the pellet is accelerating down the barrel, the bolt will be accelerating backwards but due to the larger mass it will be accelerating at a much slower speed.
sure changing the pellet grain can affect this but can it be done so that almost no extra air is wasted?

What's the speed of the air traveling down the barrel and driving an 18 gr pellet at 900 fps?
Other than the weight of the bolt, spring tension on the bolt, friction on the o-rings and the probe length what else can affect the time it takes for the breech to open?

[rsterne]

"Most" of the air is travelling about 900 fps, the same speed as the pellet.... However, since the average speed of the gas moelcules is ~1650 fps, some are travelling backwards, towards the valve.... noe of which matters....

You need to know the residual pressure of the air in the barrel, which is constantly declining as the pellet moves down the bore.... It is affected by the valve duration (dwell), porting, caliber, pellet weight, and a host of other factors, to determine the force on the bolt.... That force starts out very high and declines throughout the shot, the opposite of what you want.... I think the most logical system may be the one that steals residual gas pressure from the muzzle as the pellet exits and uses that to actuate the breech....

Bob

[Airsnipe]

For the geeks out there, a while back I saw Ed from Edgun playing with what seems to be a blowback type action kit for his Matador. The concept seems interesting, the more I think about it the more it makes sense, I mean you may think you'll be wasting air when the breech opens, but what if the breech spring and the mass of the breech were heavy enough to keep the breech closed untill the pellet leaves the muzzle?

So as the pellet is accelerating down the barrel, the bolt will be accelerating backwards but due to the larger mass it will be accelerating at a much slower speed.
sure changing the pellet grain can affect this but can it be done so that almost no extra air is wasted?

What's the speed of the air traveling down the barrel and driving an 18 gr pellet at 900 fps?
Other than the weight of the bolt, spring tension on the bolt, friction on the o-rings and the probe length what else can affect the time it takes for the breech to open?

I know Ted (Ted'sHoldOver was EdgunUSA) was playing with the FX Revolution. This is a semi-auto and has a blowback type action that uses the remaining pressure from the mussel to cycle the action. He has a couple videos using this gun. 

[Chako]

http://www.youtube.com/user/EdgunUSA/search?query=revolution
first 2 are the accuracy
 

[rkr]

Hmm, you could probably use the valve bounce for timing by devising some kind of locking mechanism that releases the bolt when the firing valve hits the hammer. That would eliminate the valve bounce and if there's enough air pressure in the barrel the rest would be quite straight forward.

[David.Soliman]

Hmm, you could probably use the valve bounce for timing by devising some kind of locking mechanism that releases the bolt when the firing valve hits the hammer. That would eliminate the valve bounce and if there's enough air pressure in the barrel the rest would be quite straight forward.

let me see if I understand this right, like a gear rotating at it's own momentum, locking the bolt for say a full revolution which is exactly how long it takes for the pellet to leave the barrel? Done mechanically without using an electronic switch.

Hahaaa, I can't see how that is a problem. LOL

[rkr]

Hmm, you could probably use the valve bounce for timing by devising some kind of locking mechanism that releases the bolt when the firing valve hits the hammer. That would eliminate the valve bounce and if there's enough air pressure in the barrel the rest would be quite straight forward.

let me see if I understand this right, like a gear rotating at it's own momentum, locking the bolt for say a full revolution which is exactly how long it takes for the pellet to leave the barrel? Done mechanically without using an electronic switch.

Hahaaa, I can't see how that is a problem. LOL

Well, something like that. When the hammer hits the valve it engages the locking device. When the valve bounces back and pushes hammer back from the valve block the locking is released and the pressure pushes the bolt back drawing the hammer back as well locking it to cocked position. In normal non-fire situation the valve pushes the hammer back enough to prevent the locking from engaging so you can pull the bolt back to cock the gun. Of course you need a return spring for the bolt but that's normal for all blowback systems.

[Bill G]

Bolt and hammer mass is what is considered for rim fire rifles.  I have found that 90 to 115 gram makes for good cycling. Barrel length to bolt/hammer mass is where I find reliability of cycle. Too light and cycle is too quick causing losses in velocity and dirty breech. Also results in heavy residue that makes your eyes burn.  Too heavy and you get failure to extract or feed.  The problem I imagine would be with the ever declining psi. Perhaps it would function more reliably in a regulated rifle.

[45Bravo]

There is a aeg(automatic electric gun) PCP out there, it uses a rechargeable battery to cycle the action.
Evanix conquest,  they USED TO make a full auto version, but discontinued it in favor of the semi only.

[David.Soliman]

Bolt and hammer mass is what is considered for rim fire rifles.  I have found that 90 to 115 gram makes for good cycling. Barrel length to bolt/hammer mass is where I find reliability of cycle. Too light and cycle is too quick causing losses in velocity and dirty breech. Also results in heavy residue that makes your eyes burn.  Too heavy and you get failure to extract or feed.  The problem I imagine would be with the ever declining psi. Perhaps it would function more reliably in a regulated rifle.

That is exactly what I had in mind. How do you calculate the air pressure, and pellet acceleration?? I know there are some equations out there but I'm not sure how to use them.

[Bill G]

Lloyd, Bob and myself have various calculators that we have made to do that tedious math.  The bigger question would be where to put the gas port in the barrel?   It looks like the fx revolution used the air that is captured from the muzzle blast. Looks like it is fed into a gas piston system from the "LDC" type muzzle attachment.  That seems logical since the projectile is going realitively slow compared to powder burners, although the pressures are much much lower in and airgun.  I would imagine the design would have to be FPS specific.  Lighter pellet =faster and that will = less time for reaction to take place.  I would think that it will boil down to volume over time.  Volume will be dictated by the stroke requirement to recock the rifle and the area of the piston needed to generate the required force to do so.  Port size will dictate flow rate and obviously, velocity will dictate duration that the flow is working. once the pellet has left the bore, no more back pressure to do work.  The fx rifle seems like it would have the most efficient system.  Porting the barrle and feeding the auto recocking system any earlier than the muzzle would result in lost energy and efficiency, I would think.  Not a huge deal when you have 36,000 to 60,000 copper units of preasure, buy really big deal when you only have about 400-600psi at the muzzel after the charge has expanded from it"s original 3000psi. 

Bill G

[dougroundup]

Would some sort of open bolt design help with efficiency? Perhaps the bolt could be designed to function as the hammer too. If one were to get even more exotic perhaps concentric  tubular air passages  of some sort to port exhaust air and return waste air? I am not even sure what I am trying to describe... but surely an open bolt might be thought....
Just throwing things out there for topicality sake.

[David.Soliman]

What made me think that this is feasible is this video here:



I saw this a very long time ago and to this date I haven't heard of the kit being sold or produced. The video shows how the mechanism works, my Russian is non existent but it is very obvious that this is a blow back mechanism.
The bolt is fixed by a lower slot at two positions, and the probe runs inside the bolt. During firing the probe gets pushed back and cocks the hammer and cycles the clip. Looks like a very simple design but how efficient is it, I have no idea.

Would that thing work???

[David.Soliman]

Кит крыс-новая версия:

This just came up and my Russian is non existent, but I think that he is using air to index the mag and hammer bounce to cock the gun??
No idea what he did to the trigger but that looks like a modified 1377 or one of its relatives.

The shots are not very far appart in terms of speed.

Can you even cock the gun using the hammer bounce? A light hammer spring, a heavy hammer and a heavy valve spring is the only way I think it is possible, right?
Would love to hear Bob's input on this.