Teathered fps versus filled fps

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Quad82

Quad82

Member
Jan 23, 2017
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Michigan, United States
I have a nice dual gauge regulator connectet to my fill station off a 60 min tank. Shooting my big bore at say 3500 psi I am getting a consistent 880 fps. But if I use the same setup anf just fill the rifle, I get a consistent 725 fps on each first shot. I have tried this at many fill leavels and the results are always the same. Continuous tank pressure yields higher fps at any given fill pressure. I like using the tank for sighting in or target shooting. But in a hunting situation, I want the same outcome. Any ideas?
 
If this is a _big_ big bore making mondo power, the reservoir is experiencing a major pressure drop during the shot. When tethered, the external tank maintains the pressure at a higher average level. That’s quite a difference though so there may be something else going on. Would need to know more details of energy, caliber, reservoir volume, etc.
 
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I was mistaken. Teathered, I am in the high 800s at 3200 psi not 3500. The rifle is a nonregulated .40 Badger shooting a 220 gn hp. I keep good records whenever I am testing. I agree that it’s probably a plenum issue. Now that I installed a gauge in the rifle, I can see I use between 500 and 600 psi per shot. I will give my last readings.

Teathered 3 shots

3000psi 699, 711, 713

3100psi 726, 726; 732

3200psi 818, 880, 884

3300psi 869, 888, 714?

3400psi 719, 721, 880?

3500psi 706, 717, 720

3600psi no readings

3700psi 686, 693, 615

untethered 3 shots

3200psi 706-2800psi, 640-2300psi

3300psi 714-2800, 659-2300

3400psi 723-3000, 685-2500

3500psi 733-3100 ,696-2500

3500psi 733-3000, 707-2600

3600psi 732-3100, 705-2600

3600psi 740-3300, 692-2800

My fastest and most consistent 2 shots untethered are at 3600 psi. This is where I have it set and sited in for hunting. I have several strings at 3550 and 3650 but there is not enough change to deal with. Teathered at 3200 and 3300 psi I am in the high 800s. Untethered at 3200 and 3300 I am only in the low 700s. That’s 150 fps difference with a 220 gn slug. Sure wish I could get the faster fps. The lower fill would be nice too!


 
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OK, so I have a theory why tethered produces higher energy for big bore:

The tethered tank has a much larger volume, so when you take a shot, it's a smaller percentage of the tethered tank rather than the possibly 1/3 of the volume of the air tube on the gun. This allows the tank to maintain a higher pressure through the shot cycle

As for a theory on the Marauder being the same FPS tethered or untethered, it could be that a .25 uses 1/16th of the air on each shot, so the cylinder no the gun is able to maintain roughly the same pressure during the shot cycle as an external tethered tank.

Just my theories, feel free to disagree, or add, as I'd like to see others with the same experiences chime in.
 
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The tethered tank has a much larger volume, so when you take a shot, it's a smaller percentage of the tethered tank rather than the possibly 1/3 of the volume of the air tube on the gun. This allows the tank to maintain a higher pressure through the shot cycle
Click to expand...

Precisely. The larger reservoir experiences less of a pressure drop. The effect is more pronounced because a big bore uses a lot of air per shot.
 
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