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Old 12-03-2019, 01:19 AM   #1 (permalink)
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PGP "Shootdown" velocity issues

Been having some velocity issues with my cart-valve PGP (with RVA). I was having trouble getting the velocity up from around 150 fps with the stock springs, so I re-sprung it with springs from Palmers and now I'm getting some weird shootdown. On a fresh 12-gram, the first shot (after piercing) is around 280, then then next four or five drop down by 15 fps increments down to around 200, where it stays until the 12 gram is almost empty. This is using fresh Crossman 12-grams in around 45 degree weather, and using old paint that seems to match the barrel fairly well. I was also getting this shootdown with the stock springs but starting at 150 fps instead of 280. Any ideas?
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Old 12-03-2019, 03:28 AM   #2 (permalink)
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Originally Posted by bassninja View Post
This is using fresh Crossman 12-grams in around 45 degree weather...
This is probably your biggest issue.
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Old 12-03-2019, 10:16 AM   #3 (permalink)
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This is probably your biggest issue.
Yeah, it's hard to blame anything on the marker at that temp. You're dumping a lot of CO2 to get 280 if the 12g is anywhere near that, especially in that short barrel. Once it shoots that first shot, it just keeps getting colder and slower. It will likely be shooting hot at 70 degrees.

Even around here, when it gets down below 60F, we start putting the 12gs away for HPA. The mornings may be around 50F, and then the afternoon gets to 65-70F. Too much of a pain to keep up with when we are trying to snipe each other from across the field. In the summer, you hit the sweetspot on the valve and are generally pretty good across the spread.

I think you could tune (sweetspot) a marker to play 50-65F, but the efficiency would be about half of what you get at 75-90. It also would lock up if you tried to use it in the warm weather.
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Old 12-03-2019, 12:00 PM   #4 (permalink)
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Alright, thanks for the replies. I figured it was probably just too cold, but I also read a few threads that talked about using twelve grams in much colder weather, so I wasn't sure.
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Old 12-03-2019, 03:35 PM   #5 (permalink)
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The first few shots are liquid, so you get a little extra from the liquid mass and the phase change, but after that, your gas pressure is dropping rapidly from the temperature drop. I don't use any 12 gram equipment below 45 degrees F because the gas pressure is only 550 psi to start and as the temp drops from phasing to gas, so will the pressure.
At 75 degrees, you are at 900 psi for a comparison. Co2 is very temperature sensitive.
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Old 12-03-2019, 03:51 PM   #6 (permalink)
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Co2 is very temperature sensitive.
and self-chills. It's a slippery slope!
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