Tuning .25 Marauder performance

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ChrisS

Member
Jan 23, 2021
292
161
NV, United States
So, I'm curious, I have a Gen 1 Marauder, Cothran valve, Hill guage block and a bit thicker main spring. The best I can do is 1075fps using 20 grain pellets. What kinds of performance do others get? Ever tried more than 3Kpsi? I bumped up to 3500psi for one test (Cothran valve will not lock up) but saw very little change in velocity...perhaps another 25fps. Now, I'm not neccessarilly put off with this performance, the thing is dead accurate, but I had thought I'd get a bit more velocity than I have here.

What say you?

Chris
 
i say thats alittle weak .. mine was doing 1025 or something round that with 25 jsb's when i checked what they would do .. and thats with about 1/3rd left on the hammer spring lol ... but i had it set to 900 with the 33.9s and didnt want to mess with it ...but yeah, thats with a hill setup dont know zip about your parts .. id say off the top of my head you need to open fhe transfer port up a tad if you havent ..
 
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the actual hole size that leads to the barrel .. lets see hold on ... ok, what is supplied in my kit is a very sharp 3/16ths drill bit and a piece of HFPT plastic tube that replaces the stock transfer port and seals .. you drill your barrel transfer hole and debur it lightly on the inside with a small round burr bit you can chuck in a drill ... im sure Hill has the plastic tube if you need it pre ut to proper length . ..
 
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Cothran port:

C85F209E-4FFC-4B64-A094-C315BA0D66E4.jpeg

 
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The Cothran TP adapter is only about 0.14” id. At a muzzle velocity of 1075fps, port velocity is sonic. With a larger diameter transfer port, you would likely get those 20gr pellets to a higher velocity buts that is almost pointless. Try a heavier projectile. 25gr or 34gr will give higher fpe. And with a higher BC, likely a higher velocity at the target.
 
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I get that and I certainly agree that it is best to shoot the heaviest pellet manageable by the gun. But I was doing an apples to apples type comparison. With the stock setup, stock valve, stock guage block, etc, I could get the 20 grain pellet up to a tad under 950fps. Now I've added the Cothran valve, Hill guage block and a spring out of an old truck suspension and the gun is barely making a 125fps more than in stock form.

So, my question is: Anyone else make similar changes to their Marauder and what kind of increases did you see?
 
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That valve is designed for really easy opening and the stock hammer spring should suffice with minimal pre-tension. Too much and you can start slamming into the valve face, no bueno. The design changes the entire dynamic of the rifle and tuning because you are not fighting a valve spring nearly as much. In fact the valve spring on that valve is mainly needed for filling and lower pressures as the stem is closed via air. Sounds to me like the transfer port adapter and port are fine but the air path is being choked down at the barrel. Hill max power kits include the drill bit and deburr tool, but he includes a heavy hammer spring and matched valve spring. My suggestion would be increase the barrel port to match the transfer port and use a lighter hammer spring like in the Hill regulated kits.
 
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