Dukemeister wrote:
McRobert wrote:
tagged as 80% nitrogen, which means it is just filled with air.
LOL, like it like it!
The temperature drop from inside to outside (20 C to -20C) is 40 degrees C which is also 40K, and the pressure ratio will the same as the absolute temperature ratio, Inside T=293K, outside T=253K so the ratio is 253/293=86%. If the gar ram is filled at room temp to 2500 psi, it will be 86% of that value at -20C, or 2150 psi. the force will drop about the same ratio. Note gas springs have a different force-displacement curve than wire spring, the force output is relatively constant over the compression stroke, the initial force to get it moving is high. See the graph attached. That said, there is very little benefit to shimming a gas ram, the increase in stored energy is negligible. For a wire spring you get some appreciable increase in the stored energy even with a small shim. (yellow line)
If you are finding the gas spring is losing power on each stroke, it's a faulty ram - likely internal leak.
Thank you, that's some great info!
Thanks to you as well, McRobert!
I did lots of experimenting, all indoors. I was advised that *some* pre-load is going to give you more FPS. I got 22-25 FPS average improvement with a 3mm shim at the base of the gas ram. I tried 4mm, 5mm, 6mm, none of which improved on the 3- 4mm shim. The placement of the shim didn't seem to matter - front of the gas ram, back by the rear chamber cap, both alone and split between both sides. Nor did any shim over 4mm. Through these tests, I was able to determine (at least in this model), what the effective pre-load is.
My tests below will also show this marked decrease in velocity as the shots wear on. I'd say about 8-10 FPS decrease after each shot. Important to note that the decrease is certain, but there are exceptions. Sometimes I'll see an increase in about 15 FPS for one shot, then subsequent shots decrease. After ten shots, we'll see about 20 fewer FPS. I take into account that no pellet is created equal. Rifle is 2 months old. There are no bleed holes in the piston. Using the stock 'BT5M22-00-1A' gas ram & the piston is properly oiled with Crosman break-barrel oil & I diesel'd it out a bit before my tests after the last lube. Gas ram is dry. The rifle sits on a workbench near a wall heater & a window, but temps aren't that warm - maybe 20C. I wonder if letting it sit allows for the piston to settle somehow - or if I should put more break-barrel oil on the piston. Do any of you add that oil in the barrel itself?
I took at least 7 consecutive shots to derive each average in these tests:
STOCK:Avg FPS with 17gr pellet: 390 (118.775 m/s) - Max: 395 fps
Avg FPS with 14.3gr pellet: 441 (134.38 m/s) - Max: 452 fps
3mm Shim:Placed between the Spring end of the gas ram and the anterior wall of the piston (X marks the shim): <- ( {X} [[[[[[[[[[--------- )
Avg FPS with 17gr pellet: 416 (126.86 m/s) - Max: 424 fps
Avg FPS with 14.3gr pellet: 465 (141.64 m/s) - Max: 474 fps
5mm Shim:Placed between the Spring end of the gas ram and the rear chamber cap (X marks the shim): <- [ ---------))))))))) {X} ]
Avg FPS with 17gr pellet: 408 (124.33 m/s) - Max: 412 fps
Avg FPS with 14.3gr pellet: 461 (140.63 m/s) - Max: 472 fps
6mm Shim:Placed between the Spring end of the gas ram and the anterior wall of the piston (X marks the shim): <- ( {X} [[[[[[[[[[--------- )
Avg FPS with 17gr pellet: 404 (123.26 m/s) - Max: 412 fps
Avg FPS with 14.3gr pellet: 460 (140.12 m/s) - Max: 472 fps
REVERTED TO 3mm Shim:Placed between the Spring end of the gas ram and the anterior wall of the piston (X marks the shim): <- ( {X} [[[[[[[[[[--------- )
This is obviously the sweet spot for shimming this rifle. Maybe even a lower shim size would help? I never tested lower than 3mm yet.Avg FPS with 17gr pellet: 412 (125.5 m/s) - Max: 418 fps
Avg FPS with 14.3gr pellet: 468 (142.6 m/s) - Max: 468 fps
I just got my BT9M22 replacement piston today & have been testing with it. The EBay Australian contact was a good one! Diesel'in like a WW2 tank, my god the smoke! That should subside soon anyway. A bit louder, but not bad. Kicks a *bit* harder but I can hold onto it just fine.
I'll do a full-on test of the rifle with this new piston soon, I need a day or two to work it in. Some shots are awfully slow, some are too fast to mention.
Rather than return the Gamo 9.9 gr pellets, I've decided to cherry pick the ones that fit. Still pretty pissed that > 80% of them are unuseable.
I left the 3mm shim intact while I test with the variety of pellets (17gr, 14.3gr, 9.9gr).
It's still very much in a 'break-in' mode, diesel'in like crazy, some pellets barely moving.
Max velocities so far, and remember, the first shot of the day is always the fastest:
9.9gr: *94.8 FPS (there are about 5 reasons I can't divulge that).
14.3gr: 489.5 FPS
17gr: 453 FPS
So basically, after taking a crap on Gamo, I'm still ordering more of their pellets, but a different style & even lighter. Nobody seems to sell light pellets other than Gamo. I know 9.7gr is too small for a .22 cal with any power, but I want to see how much I can get out of this sucker. Ultimately, I want to get a reliable 550 FPS out of this so when I'm PAL'd, the thing will be useful.
I still don't know why leaving the rifle to settle for a few hours gives it such a boost in velocity. I was hoping it's 100% nitrogen, otherwise why get nitro piston at all...has anyone tried a nitro piston AND a spring together?
Maybe I'll try the new gas ram without a shim - it's just so erratic - and I'm running low on ammo!