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rsterne · Moderator

I finally got my 2560 assembled today, running on a 17 CI tank (it was advertised at 18, but I actually measured it at 280 cc = 17 CI) regulated at 1600 psi.... I used 10 shot strings of three different pellets at each hammer spring preload and recorded the total psi drop, which allowed me to calculate the volume of HPA used per shot.... I then graphed the velocity at each setting vs. the volume of air released by the valve as follows:

There is a big jump in air consumption at the final hammer setting, as those shots resulted in the valve still being open when the pellet left the muzzle.... The increase in muzzle blast was significant.... You will note that the plateaus which indicate very little additional velocity gain is taking place start at about 10 cc for the Lasers, 12 cc for the Kings, and 14 cc for the EJ Points.... That is consistent with other data I have from many guns that indicates that heavier pellets can stand a bit more preload at a given pressure before "hitting the wall".... Since the volume of the barrel (23.5" long from the transfer port to the muzzle) is 18.9 cc, I was also able to plot the velocity vs. the position of valve closure as follows:

This places the point at which very little velocity is gained at about 53% of the distance from the breech to the muzzle for the Lasers (at 1080 fps), 63% for the Kings (at 965 fps), and 74% for the EunJins (at 815 fps).... I'm sure that those values will provide for an interesting discussion.... but they are not inconsistent with data I have from numerous other guns....

Bob

radar · Silver Status Member

So the volume of air used at the 53%-74% marks...is that at STP, or based on 1600 psi?
If at standard temp and pressure, it is interesting that the maximum velocity occurs so early in the bore having dropped to (if it has) atmospheric pressure. That would indicate that the momentum of the column of air carries the pellet the rest of the way at its peak velocity, no?
Rob

rsterne · Moderator

The volume of air is the volume of air released by the valve at 1600 psi.... arrived at by the pressure drop in the tank for 10 shots....

That air continues to expand after the valve closes, adding velocity and energy to the pellet until it leaves the muzzle....

Bob

radar · Silver Status Member

Still amazing that air can keep accelerating the pellet as it expands and as its pressure keeps dropping.

rsterne · Moderator

Dropping yes, but it may still be 500-1000 psi at the muzzle, and that's a lot of force....

Bob

radar · Silver Status Member

That makes sense. The rate of acceleration may go down as the pressure drops, but the force on the pellet still would far exceed barrel friction.
You make me think about physics! Good for the brain... Very Happy

Rob