Sounds like an extreme case of retained air, but nice to have if it suits your shooting needs. I worked on a friend's vintage Benjamin pumper and found I could pump it 6 times, take a shot and then just 4 more pumps got me a shot with the almost same velocity. The actual trend was a slight increase over a few shots, but hitting a "steady state behavior" of about 730 FPS after about three or four shots. This rifle retained air on shots taken with four or more pump strokes. With six pumps, the shot would be about 700 FPS and a second shot on the retained air alone was about 500 FPS.Some guns retain more air than others, but as you have found, you can influence the behavior. Makes me think of what a person can do to tune an unregulated PCP.Without a chronograph you just won't know anything exactly. You could improvise a penetration test by shooting some medium. "Ballistic" gelatin probably isn't easy to come by, but others use regular food gelatin or even clear soap base. Maybe even multiple layers of cardboard? Anything that will let you at least compare each of the shots will be a start. I even used a laptop and a noisy target to do an acoustic chronograph. It required I zoom in on the actual time history of the audio of a shot and impact, but it was certainly useable data. Once you get a way to estimate velocity, I wonder if you might find it better to look for a "steady state" behavior like what I found. Maybe you can use that retained air such that just a couple pump strokes per shot is all you need on a bunch of consecutive shots. Here's the data I was getting before the piston seal I had made broke...Each string starts from dead empty. I forget the pellet weight, but they were probably average .177 pellets. The last two columns started with 6 strokes for the first shot, then 4 and 3 pump strokes respectively for consecutive shots. consecutive pumping retained+pumps shot no 1 stroke 2 stroke 3 stroke 4 stroke 6 (s+4) 6 (s+3) 1 324 481 549 652 716 693 2 352 498 586 648 719 675 3 371 500 597 675 734 661 4 348 502 610 676 746 651 5 350 505 619 692 734 649 6 347 507 608 695 738 651 7 356 504 620 705 732 648 8 363 496 606 717 735 654