Avenger bullpup
Air Venturi Avenger Bullpup.

Part 1
Part 2

This report covers:

  • Set the regulator
  • The fill
  • The test
  • Fastest and slowest shot
  • Pressure falls as shots increase
  • Reg pressure
  • Velocity declining
  • Discussion
  • Summary

Today we look at the velocity of the Air Venturi Avenger Bullpup with the regulator set as low as it will go. I learned a lot today, including the fact that we will need at least one more velocity test report. Let’s get started.

Set the regulator

To set the Avenger’s regulator the reservoir must be completely empty. The manual describes how to do it but it’s not clear, so I will try to clarify things. First off, the drawing in the manual doesn’t really show where the reservoir exhaust screw is located. It is on the bottom of the stock in back of the pistol grip, under a rubber plug that also covers the regulator adjustment screw.

Avenger bullpup depressurize
Depressurizing the reservoir takes a 3mm Allen wrench.

Secondly, the manual says the reservoir pressure release is a 2.5mm Allen screw, when it is really a 3mm. It’s a small detail, but the screw is deep inside the stock, so knowing what wrench to use is a big help.

Once the air is out of the reservoir, turn the slotted brass screw, located in the other hole, all the way clockwise until it stops. Then back it out counter-clockwise a quarter turn. That sets the regulator to the lowest pressure. That turned out to be 1800 psi on the regulator gauge.

Avenger bullpup reg gauge
When the reg was adjusted by the manual as low as it would go, this is where it ended up after the rifle was filled. Sorry for the bad picture but I shot this as I was recording the velocity and my plate was too full.

The manual states twice that the regulator adjustment screw should not be turned in (clockwise) when there is pressure in the gun. I would heed this warning.

The fill

My carbon fiber tank was full this time so I filled the rifle as full as possible.

Avenger bullpup pressure gauge
Where the pressure gauge reads when the rifle is filled.

The test

What I’m testing today is the velocity when the rifle is set on the lowest regulator setting. I’m also testing the shot count. We can compare today’s results with those from Part 2 where I tested the rifle on the factory settings, but I didn’t fill the rifle all the way in Part 2, so the shot count was not correct.

Also, it is possible to adjust the hammer spring. I have not done that today, which is why I said there needs to be at least one more velocity test.

I’m not looking for the absolute fastest velocity the rifle can give. I’m also not looking for the absolute highest shot count on one fill. What I’m doing is playing around with the adjustments to see what they do.

I am testing with the JSB King Heavy pellet. That’s the same pellet I tested with in Part 2. 

Fastest and slowest shot

The first shot went out at 769 f.p.s. Shot two went out at 721 f.p.s. For the first thirty-six shots on this fill those were the fastest and slowest shots. The average was somewhere around 732 f.p.s. Shot 37 went out at 719 f.p.s.

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Pressure falls as shots increase

Shot 10 went out at 727 f.p.s. After that shot there were 3600 psi remaining in the rifle. After shot 20 3,000 psi remained. By shot 30 the rifle was down to 2700 psi. At shot 40 the gauge read 2200 psi. On shot 50 the gauge read 1900 psi.

Reg pressure

I mentioned in Part 2 that the regulator pressure gauge fell as I shot and I showed you some pictures of that. That was with the regulator set from the factory. Their recommendation for .25 caliber rifles is to set the reg at 2900 psi.

the reg didn’t fall as much today but on shot number 30 I noticed that it had dropped by 200 psi. It remained there until the end of the test, when the reservoir pressure dropped lower than the reg setting.

Avenger bullpup regulator gauge
Here is the regulator gauge on shot 30. It has dropped by 200 psi from where it started.

Velocity declining

Before shot 30 the shots were mostly in the high 720s to the high 730s. Call the average 732 f.p.s. At that speed the 33.95-grain JSB Exact King Heavy pellets produce 40.4 foot-pounds of energy at the muzzle.

After shot 30, which went out at 729 f.p.s., the shots started a slow decline in velocity. I will show you shots 30 through 58.

Shot……Velocity
30………….729
31………….725
32………….722
33………….729
34………….724
35………….721
36………….721
37………….719
38………….725
39………….718
40………….724
41………….722
42………….721
43………….714
44………….719
45………….719
46………….722
47………….714
48………….716
49………….716
50………….719
51………….713
52………….710
53………….717
54………….714
55………….708
56………….704
57………….694
58………….685

The rifle “fell off the reg.” (where the pressure inside the reservoir is not equal to the pressure at which the regulator is set) around shot 55. After that the regulator remains open and the velocity declines steadily.

But the useful velocity, which depends on what you are doing, started dropping around shot 30. If you are shooting to long range, you want a tighter velocity spread, and, except for the first and second shots, you get that up to shot 30. If you are just plinking, there are at least 50 good shots at this setting.

Discussion

When the rifle was set by the factory (the Part 2 test) the velocity hovered around the 780s and 790s and gave at least 30 good shots on what was not a full fill. After looking at today’s results I would say that the hammer spring isn’t working well with the low regulator setting. If we can get over 30 shots at the higher velocity, there should be several more at the lower setting when things are balanced right. And that is what I want to test next time.

I want to set the regulator at about 2600 psi and I want to adjust the hammer screw in for greater striking force. I hope to see 30 good shots that are in the 800 f.p.s. range.

Summary

I said earlier that this Avenger bullpup is deceptively adjustable. Now we see that both the regulator and the hammer spring can be adjusted and it is my job to find a good setting for both. What I am looking for is a large number of shots with a consistent velocity. I’m pleased with anything in the high 700s with this pellet. We shall see.