Low pressure inquiry

[Manuel_FZR]

Originally posted by steveg

(...)
the mag bolt is still under ~18 lb of cocking pressure, ouch!

[steveg]

the bolt pin is about .240" diameter, the operating pressure is about 375psi
so that equals about 17lbs on the bolt being held back by the sear

[RaV3n_Pa1ntba||]

now i dont believe that the mag is that low. How is this possible.... I have my bushy set at 280 fps and i am getting +/- 2 over the chrony...plus getting about 1100 shots outta a 45/4500. And my mag same but i got +/- 6 over the chrony and was sadly getting only about 500 - 600 shots per fill. =[ Dont get it...

[AGD]

The bolt pressure at 17 lbs is not quite accurate. You have to subtract the preload of the bolt spring and then add in more preload as the spring starts to collapse.

The cocker style systems use a 1/4" ram too with about 80-100 psi cocking pressure. This equals about 4.5 lbs force for comparison.

We are right now making changes to the bolt spring which will favorably reduce the bolt pressure.

Efficiency can be greatly affected by barrel length and diameter.

AGD

[hitech]

Originally posted by AGD
We are right now making changes to the bolt spring which will favorably reduce the bolt pressure.

Would that be a variable rate spring?

[KamikazePenguin]

would such a spring retrofit existing valves or would the reduced pressure require a greater volume in the chamber? Or is there something I'm missing here...?

From what i understand a lighter spring would allow the bolt to have less pressure behind it (lower opp pressure) and still force the bolt forward, but to do this you'd need more volume to achieve 280FPS. Am i reading you wrong Tom?

[steveg]

The bolt pressure at 17 lbs is not quite accurate. You have to subtract the preload of the bolt spring and then add in more preload as the spring starts to collapse.

Darn caught again! I did know about the spring, actually
mentioned it and did the pressure calculations in another post a while ago,
Including fishing for the spring rate.
Oh well if you want something done etc. etc.

Calculated spring rate = 22lb/in
measured (on digital bathroom scale that is distressingly
accurate) =18lb/in

so lets choose 20lb/in

the spring is compressed about .5in

.5in X 20 lb/in = 10lb

so 17lb behind the bolt and 10lb spring preload in front
of the bolt leaves 7lbs to mangle the ball.
apparently 10lb to break the ball was a bit optimistic

It takes about 18 lb to almost completely collapse the spring.

Sooo if you are changing the spring to reduce pressure
on the ball you would be increasing the spring rate(a stronger spring)

[Ityl]

but even with a softer spring, there's still that ~375 psi pushing the bolt. When the ball half feeds it's going to chop. Autococker and matrix (with trinity) are more like 90 psi, and will usually more pinch the ball.

And is the chamber pressure of the mag so low because of gas escaping up the feed tube and is the cocker so high because of the closed bolt design?

[gmag]

I don't understand. If 375 psi is exerted when you pull the trigger then how come only 60 hits the ball? Where does all the rest go?

[steveg]

That 375psi is trapped inside a fixed volume, as soon as
you pull the trigger the air is let out of that volume,and
starts expanding, the more it expands the lower the pressure is.
Also the flow from the fixed volume is restricted a bit
by the bolt piston, slowing the flow so the whole amount
of air doesn't empty at once.

In contrast a cocker, angel etc are more like a garden hose,
as long as the valve is open, the air flows

[gmag]

Thanks for clearing that up steveg.
~Brett