Adventures tuning a Marauder - Or How Reloading relates to Tuning?

All you guys that reload, check this for accuracy. I think it's pretty close to being right. Anyone that knows Marauders, have a look as well, it's my learning of the gun if you will!
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Tuning a Marauder, like reloading bulletsAs far as I know, there are three major elements to reloading for powder burners:
  1. Bullet Weight = Mass
  2. Amount of powder = potential energy
  3. Speed that the powder burns = speed energy is released
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    Tuning an airgun can be compared to reloading for powder burners like this
    1. Bullet weight = Pellet weight. Heavier pellets using the same amount of energy (Powder) move slower. Lighter pellets, using the same energy (Powder)move faster.
    2. Amount of powder = Hammer adjustments = the amount of air (Powder) used for each shot. So, the Hammer Spring Preload and the Hammer Stroke adjustments are the equivalent of changing the amount of powder used for each shot.
    3. Powder burn rate = transfer port or Valve Metering Screw adjustment. Opening the port to allow faster air transfer from the air supply = faster burning powder and slowing the air transfer by closing the adjustment is the equivalent of using slower burning powder.
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      Number of shots per fill are controlled by Hammer spring adjustments as the hammer spring adjustments determine how much air is used on each shot. Heavier hammer spring adjustments for more power uses air at a faster rate, and reduces the number of shots per fill. Lighter adjustments use less air and increase the number of shots per fill, but supply less energy per shot.

      The velocity is controlled by the Valve Metering Screw. Closed means slower velocities while open means higher velocities

      So, when tuning, decide what you want, higher velocities or higher shot count. By the way, my particular Marauder tops at about 35 shots per fill, no matter how it's adjusted. If you want max shot count, start at the Crosman recommended factory settings. In .22 it'll fire a 14.3 Premier Dome at around 830 fps. The chrony string I did shows 6 fps Standard Deviation, so it's an excellent setting to start with. The settings maximize shot count and are:

      Caliber Hammer Tension Hammer Stroke Transfer Port
      .177 1.5 5.5 2.125
      .22 1.25 4 2.5
      .25 5 4 4

      In .22, the Crosman Premier Dome 14.3 flies at around 830 fps. The JSB Exact Heavy at 18.3 grain flies at around 750 fps. My goal is to get the JSB Heavy's to about 830 fps.I ran some initial tests where I adjusted the Hammer Tension, and upped the Valve Metering Screw to 4. This did achieve the 830 fps I was after, but at the cost of more air. Remember the .25 Marauder is rated at 16 shots per fill, and I wanted to have closer to 30, so back to the drawing board.

      As a side note, I have used the Hammer Adjustments to get the JSB Heavy pellets to 830 FPS in the past, but only got 20 shots per fill, meaning I was wasting a lot of air. This whole experiment is to see if I could get the JSB Heavies to fly around 830 fps, and get more shots per fill.

      So, using the reloaders comparison, my powder load per shell is the most economical I can use with the hammer tension and Preload set to factory Crosman specifications. The only thing left to play with is the burn rate or the Valve Metering Screw.

      I set the Valve Metering Screw to 4, and the shot string of 30 averaged out at 842 fps, but with quite a variance is speeds. Standard Deviation was 21.83 with the high of 877 and the low of 801.

      There could be other elements in play as well. The O-Rings could be a bit worn and such, so I'll look at replacing the o-rings and ensuring everhtying is properly lubed.Work will need to wait a while now, as my air tanks need to be re-certified. That'll take a couple weeks.

      My next step may be a Huma Regulator. It's only $125 and should really even out the shot string and might give me a few extra shots per fill. It's all an experiment, so I'll keep you posted.
 
The one thing you have wrong is the transfer port screw, It effects flow as in the amount of air(volume) allowed to push the pellet. It has nothing to do with how fast the air moves. For the same reasons you have terms such as burn rate and charge(amount of powder), airguns use terms like (air)volume and pressure. They each apply to a specific physical action that relates to the type of system used. When trying to relate one to the other you will only find frustration.

I understand you're trying to make sense of the terminology used for airgun tuning. You are however doing yourself a disservice by lumping it in with pb tuning, even if only to try and learn it. You are better off studying what makes an airgun work and how airflow is managed in the process. You will find tuning any pcp easy and simple once done.
 
I do understand the transfer port screw, though the words may have been poorly chosen. It was late . . . By opening the transfer port, the air volume provided by the hammer hitting the valve transfer from the tank to the back of the pellet faster than if the transfer screw is closed. So, volume passing through the port is unchanged whether two turns or 4 turns. Simply that 4 turns creates a bigger hole for the air to transfer through, so it transfers faster, hitting the back of the pellet with more initial force, and probably wasting less air. 

While you may believe it's a disservice to compare to powder burners, I do believe it shows the similarities between the physics of the guns. Fundamentally, you are dealing with the same three elements of mass, potential energy and the rate at which that potential energy is applied to the mass to force it down the barrel. Whether a PB or Air rifle, it's all about balancing the forces involved.

And tuning may seem simple, the permutations available are actually quite high and it takes time to find the best compromise for a particular gun, pellet, and purpose. This was just the story of my initial steps to getting an efficient tune, meeting my needs.

At this point, the tune is efficient with 35 shots per fill, but not as consistent as I'd like. Couple weeks when I get my tanks filled again, I'll start trying different combinations.

And, if anyone has an efficient, consistent tune, I'd love to try it out.
 
My recommendation to achieve your goals on an otherwise stock .25 Marauder is buy a Huma .30 regulator with gauge port and fill to 3400 (to partially overcome the loss of air reservoir volume. Watch the videos of test where it takes over 9000 to a failure point https://www.youtube.com/watch?v=syT6yoT3TX8 ). Tune the regulator to get the plenum pressure you want then go on to tuning the hammer/spring.
Also, for excellent strings, use a lightweight hammer (Jefferson State MDS LW superhammer or Hill aluminum are both good) and either a SSG or TSS spring system which will give you power AND save air, increasing shot count.

One thing I have learned is that any reasonably handy person can take the huge amount of technical and DIY info on this and some of the other air rifle forums and rework their own valve. After you have tried my above suggestions you might be very happy... but 'we' seem to always want more and I believe you will be doing valves too. I started out openning up my stock Marauder valve and was happy to get about 20% more power. And some of the modern replacement valves are VERY good compared to the Benjamin factory valve and do not cost that much for the performance they give. Look up Travis Whitney's work on valves, he is a master! Or search for rsterne's posts and the trove of research n data he puts out. For over 20 years I paid someone else to work on my air guns but now I DIY and it feels D good.
 
The advise is excellent. My goal is to see what I can do with a stock Marauder, but I may spring for the regulator. First, I need to tear it down and clean up the rough edges. marauders are known for being a bit rough in the finish area, so I probably need to polish some of the parts up and clean up the rough edges and see if that helps. I suspect the hammer needs a bit of work as it binds once in a while.

First, though, I need another gun to shoot while the Marauder is torn down. I plan on buying something else after a couple family obligations are dealt with. Maybe mid-summer for a new gun, then I can tear it down and take my time getting it back together again.

 
For most people, the real adventure begins with aftermarket parts, and discovering 40 shots at 40 Foot Pounds of Energy (FPE) is quite achievable. Then, the itch starts to get just a little bit more...
​Right now my goal is maximum shots at 50 FPE (or maybe a bit more), shooting the JSB exact king heavy mark II pellets. Step one will be raising my regulated pressure up from the 1800 PSI it is currently set to (for 40FPE shots).