Tuning the Benjamin Marauder

Forums PCP Airguns Tuning the Benjamin Marauder

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    ajshoots
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    I still read alot of posts on multiple forums about Marauder issues. Normally, accuracy issues. Lets start off with a few simple tips that apply to all air rifles not just the Marauder. Airguns shoot a pellet with a very low BC (ballistic coefficient) which hinders the pellets ability to cut through the wind and fight gravity. This can be the cause of “flyers” or pellets that don’t land within the majority when shooting a group. Most pcp air rifles also need to be “tuned” to achieve full potential of that particular gun. Specifically in the case of the Marauder, I have owned around a dozen marauders in all 3 calibers and the Marauder pistol as well. I can assure you that every Marauder I have owned and worked on needed tuned!! Shooting the factory “tune” can be another reason for accuracy issues. Alot of Marauders come from the factory shooting what is called a “descending” shot string. Meaning that when filled to say 3000psi, the first shot is the fastest and every subsequent shot gets slower and slower. This is not condusive to shooting accurate groups. The list can be rather large when trying to find the root cause of your Marauders accuracy issues. Causes can include: trigger poorly adjusted, barrel needs cleaned or maybe deburred “lapped”, bad breech seal, bad barrel, baffle clipping, barrel/shroud movement, and a host of other problems. WOW!! Sounds like alot could go wrong and cause you issues. It really isn’t that difficult to fix these issues and that is the point of this post. If you have a few basic tools and even a small amount of mechanical ability you can get your Marauder shooting good. If you are to timid or don’t have tools or the ability to try fixing things on your own, then I highly recommend sending your gun to a reputable tuner that will return your gun shooting to its full potential. For those that might want to attempt things on their own, read on!! Be aware that the ONLY way to tune any airgun is with a chronograph. There is no other way to tune an airgun but the use of a chronograph!! If you plan on staying in this hobby for any amount of time, do yourself a favor and buy a chronograph if you don’t already have one.

    DISCLAIMER: High pressure air is very dangerous!! Before attempting to open up the airtube, make sure that it is degassed of ALL air!!

    The Marauder is marketed and sold as a budget PCP. Something that the average Joe can afford to buy and get into the world of PCP air rifles. What does that mean?? Well, it means that many of the steps that the high end airgun makers would take are skipped. Lesser quality materials are used and the fit and finish are less than stellar. However, that doesn’t mean that a Marauder can’t shoot extremely well. Just means it probably will need some TLC to get it performing they way it was intended to.

    When I purchase a new Marauder, I strip it down completely. Degass the airtube and strip the entire gun down. Note: If you are not modifying the valve or installing an aftermarket valve or regulator, there is no need to degass and disassemble the front portion of the airtube. This only requires a few allen wrenchs, crescent wrench, and a dowel rod or pvc pipe to push the valve and gauge block out. Once the gun is stripped down, I like to start with the barrel. I always inspect the crown and bore for any damage or obvious signs that the barrel could be a problem. Assuming it looks good, I will lap the bore, leade, and crown with jb bore paste and kroil. Also check the air transfer port in the barrel for burrs and deburr if neccesary.
    I will then polish up the bolt and make sure the bolt slides very smooth in the breech and set the bolt tension via the flat head screw on top of the breech. I pull that flat head tensioner out and apply alittle blue loctite and then set the bolt tension.
    Install a new breech o-ring (which is caliber specific) and reassemble the barrel and breech. Use a magazine to properly set the barrel “head space”. This is an important step as you might not be able to insert a magazine if you don’t get the “head space” set correctly. At this point, I will clean all the baffles and the shroud tube. Then, I reinstall the shroud and baffles so that the top end is one complete assembly and ready to bolt back to the air tube.
    As noted, you only have to degass and completely disassemble the airtube if modifying the valve or adding aftermarket parts. It does make this next step much easier if the whole air tube is apart. I take a small diamond file and knock down any major burrs that exist inside the airtube where the hammer rides. Normally the slots for the hammer pin and sear have a slightly burred or raised edge that I will knockdown with a file. Then I go to a 1′ ball hone and proceed to hone the airtube so that the hammer has a super smooth surface to ride on. I also clean up the gauge block hole and breech screw holes. This will keep o-rings from being damaged during reassembly.
    Next is the hammer. Using a dremel with polishing compound and a felt cone, I polish the hammer to a mirror finish. At this point, you can tap the blank hole in the hammer for a set screw that will keep the striker from backing out. In stock form, strikers tend to back out which will affect your tune. Tapping the blank hole and installing a nylon tipped grub screw will eliminate this from happening. If tapping the hole is not an option, then using blue locktite or vibra tite is a good alternative. I have went as far as getting the gun tuned then pulling out the hammer and installing o-rings between the striker head and the face of the hammer to keep the striker from backing out. This is a major issue on almost all marauders and if nothing is done, The striker will back out. This is one reason I like the WAR hammer because it is tapped and has a nylon tipped screw to hold the striker firm enough that it can still be adjusted, but never back out or move unless you move it.
    Now it is time to reassemble the airtube. I use a silicone spray to lube the hammer, some you graphite and some use nothing. Lubing is a personal preference and I will leave that up to you. Once the airtube is reassembled, it can be “married” to the top end (breech/barrel assembly).
    At this point, I polish all the trigger group components and remove the “lawyer spring” then re assemble. Now the trigger can be adjusted to your liking.  Keep in mind that if you are like me and prefer a single stage trigger, the marauder trigger can be adjusted to a single stage trigger. Mine are all set single stage at 7oz as measured by a Lyman digital gauge. I also use alittle silicone spray on the trigger group before reassembly.
    Now you can finish assembling the gun. One of the weak links to the Marauder is the barrel band. The free floating idea is what leads to alot of POI changes and is easily knocked around when shooting and especially hunting. You can install o-rings to “unfloat” the shroud, drill and tap the stock band for set screws to securely hold the band, or buy one of the aftermarket barrel bands that are available. The stock barrel band is a major issue leading to POI shifts and other accuracy issues.

    Now you are ready to tune. There are alot of online resources that can help with tuning and I will not go to far indepth as it will just take to long to explain. Your goal is to get a certain number of shots from a certain fill point to all stay within a velocity that is a maximum of 4% from the highest velocity. Some also measure this 4% from the average velocity. Regardless, you want an ES(extreme spread) that is 4% or less. The further you plan to shoot, the lower your ES needs to be if you want accuracy. The Marauder has 3 adjustments. The hammer spring tension, striker length, and VMS screw that can all be used to tune the gun how you want within the limits of the stock parts.

    Part of tuning is finding the pellet that your barrel shoots best then tuning to get the most accuracy out of that pellet/barrel combination. So, lets assume that the gun is tuned perfect to a certain pellet, your scope holds zero and is working properly, but your groups aren’t all that great. What could be wrong? You already “blueprinted” the gun and removed most of the variables that can lead to problems. First check the baffles and shroud cap for any signs that pellets are “clipping”. If that is not the problem, then what could be wrong? My reasoning for “blueprinting” a stock Marauder is simply to remove the possibilities that its the guns fault. I further remove another variable by finding the best pellet and tuning the gun for that pellet. I made sure there are no clipping issues and I know for sure that my scope and mounts are good. Now, lets go one step further and eliminate the human error factor. If you can garuantee that neither the wind or your shooting is causing the issues than maybe you have a sub par barrel??

    There are so many variables that can lead us to unhappiness with our Marauders. Learning the steps to remove these variables as possible causes of accuracy issues or other problems is the key. Crosman does have a history of barrel issues. But, I have had a number of barrels over the years that were deemed “bad” by others, that I was able to make shoot well and in a couple of cases, ended up with a really great barrel.

    This is just a brief overview of the steps to fixing or even avoiding problems with your Marauder. I would have loved to go into much greater detail, but what I have above took me much longer than I had planned. I will not claim to be an expert airgunsmith, but I have been very succesful at making Marauders shoot really good and hope that I can help others on their quest for Marauder performance and accuracy. I do not tune guns or sell products. I am just a guy that loves the Marauder platform and hoping that what I have learned might be beneficial to others.

    There are some great resources when it comes to the Marauder. The marauder air rifle forums has alot of great info that can be very helpful and is a must visit for marauder owners.
    https://docs.google.com/document/d/1Jn0RFfnH9YnFaEbT5ynizTnIyJ9DG6u5htJ_65R9P5g/mobilebasic?pli=1 this is a great resource that might be of some value to marauder owners.
    http://www.network54.com/Forum/79537/thread/1302911557/Marauder+.177+and+.22+O-ring+list this list has been very helpful for o-ring sizes.

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    NMshooter
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    Great write up AJ and very helpful. I have also been praising the Marauder. When I first bought the gun, the barrel was filthy. It took 2 hours to get all the crud out. I didn’t know what it was called at the time, but I had semi-lapped the barrel. When sighting the gun in, it was fortunate that Van Jacobi helped me (well he did it) adjust the barrel band and get the trigger adjusted. The barrel was being pushed to the left by the band. I later refined the trigger even more. Although I have as of yet to take the gun apart, just doing these two things has obviously helped the gun to shoot more accurate. I will confess that when my son and I just started shooting PC’s he was out shooting me with the Marauder while I was shooting the Cricket Carbine. Go figure. If I do what  you have suggested, I am wonder just how good this rifle will shoot!
    It was worth the effort AJ!

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    ajshoots
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    There is alot of info and I do apologize for not covering things more indepth, but I will offer that anyone needing further info or help, feel free to pm me.

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    billydjann
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    I have 3 marauders that seem to need help…. my .22 cal. needs the most.
    I am sure you will be hearing from me … once I decide to tackle the guns issues… (soon)

    Thanks for all the great info

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    NoMan
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    I have never touched my gen1 .25 Marauder and it shoots right with the best of any other rifle I own for two full magazines.  The gun is a freak.

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    ztirffritz
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    My Gen2 .25 Marauder shoots as well as I could want out of the box.  I haven’t turned a single screw and haven’t felt a need to.  I’m putting groups into an area about the size of a 50¢ piece at 50 yards.  I know others are doing better, but I’m happy as a clam with that.  I don’t have any local expertise to turn to if I were to screw up my gun, so I’m breaking with the age old engineer tradition of fixing it ’till it’s good and broke and instead leaving well enough alone.  I’ve read that the barrel on the .25 is made by Green Mountain rather than Crosman, so maybe that makes things different somehow.

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    ajshoots
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    The .25’s are gm barrels and all tend to be accurate, but can shoot even better with minimal effort. I have shot 50 cent piece size groups at 100 yards with both .22’s and .25’s. But I also have alot of time and money in all of my marauders.

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    JD
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    Thanks for all the good info AJ. I have been considering buying a marauder in .25 cal for awhile now. How many shots at what velocity do you get out of your .25 cal marauder?
    Jon

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    ztirffritz
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    I don’t know about after AJ’s tuning, but out of the box I get 16 shots taking the pressure from 3000psi down to around 1800-1900psi. I could probably get another couple shots but why bother. 

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    CptMoonlight
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    Nice write up and great info.

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    spysir
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    AJ has good points (thanks) but should your marauder not respond do remember they are all slightly different. Some need the stock attachment bolt rather loose, my .22 needs the barrel band loose or removed to group best. The worst shooting .22 “I” ever saw was due to , 1 needing the barrel lapped, 2 needing the barrel installed correctly, factory hadn’t gotten it far enough into the breech which cause huge problem’s.
     The marauders are a value, likely the best $ for $ rig for anyone on a budget to compete with, or just plink.
    For those that , haven’t touch a screw, but, do like a good trigger, do read here:
    https://avveduti.wordpress.com/2010/04/30/marauder-project-lightening-the-trigger/
     Anyone who’s looked at both a marauder and an MFR know’s what the trigger can be, just a few years ago just a trigger unit that good would have cost close to $300.00, the marauder is a value.

    John

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    ajshoots
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    “JD”Thanks for all the good info AJ. I have been considering buying a marauder in .25 cal for awhile now. How many shots at what velocity do you get out of your .25 cal marauder?
    Jon

    With stock parts, I got 16 shots at around 850-880fps. Some guys tune for lower fps and get 20-24 shots. Lots of tuning info on the marauder forum. With aftermarket parts and mods, I get 16 shots at 3% ES and 20 shots at 4% for 964fps avg with JSB Kings. My other .25 that is also modded and loaded with aftermarket parts get 8 shots at 3% ES and 12 shots at 4%  for 840fps avg with EunJin 43gr pellets.

     

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    ajshoots
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     “Should your marauder not respond do remember they are all slightly different.” Great point John!! The variations from gun to gun and part to part definitely can cause unique problems or specific fixes that work for one gun and not the other.

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    ajshoots
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    “My Gen2 .25 Marauder shoots as well as I could want out of the box.  I haven’t turned a single screw and haven’t felt a need to.  I’m putting groups into an area about the size of a 50¢ piece at 50 yards.  I know others are doing better, but I’m happy as a clam with that.  I don’t have any local expertise to turn to if I were to screw up my gun, so I’m breaking with the age old engineer tradition of fixing it ’till it’s good and broke and instead leaving well enough alone.” Glad you shared your thoughts and results ztirffritz!!

    I believe that your experience with the Marauder is exactly what Crosman hoped for. I have read of a number of other Marauder owners that have had good luck straight out of the box. Glad to here experiences like yours, but unfortunately there are as many or more owners that have guns that don’t perform very well. I will say that most .25’s I have bought shot respectable out of the box. I definitely can’t say the same for the .177’s and .22’s I have had.

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    BareBones
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    AJ, I really appreciated the time and effort you put into that write up. Just this month I bought a new .22 MROD2 and after 500+ pellets gave up and sent it back for a replacement with the vendor. I was getting 1.25-1.5″ groups at 20 yards and opening up to 3″ @ 33 yards. 14.3gr @ 889.1 AU, 4.12 SD, 16.3 ES in a 10 round group. 

    Air gauge on the rifle was 200 psi off from the hand pumps gauge, Crosman said to go by the indicated pressure on the rifle. 

    I tried two two different scopes, cleaned the barrel, inspected the breach seal with a loupe, checked for any air leaks with a tissue, didn’t see any clipping of the baffles, tried 6 different pellets with the JSB 18.13 shooting the best, but I’m not looking for an Olympic shooter, I just needed something good out to 50 yards preferably with 14.3 CPHP’s. 

    The replacement is on its way now and I am keeping my fingers crossed that I get a good shooter this time around. I planned to tinker and tune this rifle later on down the road mostly for a flat shot string with the highest FPE I can get in 40 shots with a 3000psi fill  down to around 2000 psi.

    I never figured on having such accuracy issues right out of the box, I had hoped all these accuracy problems would have been addressed by Crosman with the release of the Gen2 rifles, but apparently nothing has really changed in that respect. 

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    ajshoots
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    Wish you better luck on the new gun. Post your results and if you need help, we are here to help!!

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    BareBones
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    Thank you very much AJ for the offer to help.

    Idealy I would like to get as close to 28-32 FPE as I can with a 14.3gr pellet delivering 40 shots on a single fill, I realize that is probably stretching things beyond physical limits, but what do I know?  My hopeful benchmark would be to come as close to a rimfire CCI .22 CB CAPs FPE (29 gr bullet, 710 FPS at the muzzle, 32 FPE), but shooting a much flatter trajectory. 

    The numbers don’t suggest a .22, 14.3 pellet delivering 32 FPE as being very stable in flight at the speed needed to achieve that FPE. It would require around 1010 FPS to achieve that amount of energy, plus I want to stay sub sonic and that’s starting to creep up on the sound barrier. 

    So I see a more realistic FPS of around 900-950 FPS which would yield about 25-28 FPE, only real question is how close can I get to that desired 40 shot string? I guess what I’m really trying to achieve like I previously posted is the highest FPE I can get with a 14.3 gr pellet getting 40 consistent shots on one fill and maintaining accuracy. No point in pushing the pellet so fast down range that looses stability in flight and throws off the accuracy. 

    Some mods I had planned right off the top where Bstaley mod, drill transfer port, drill air stripper, unfloat barrel with o-rings. I don’t know if I will need a heavier hammer spring or if the stock spring could get me in that upper desired FPE range. I know the rifle I sent back, stock right out of the box was getting 25 FPE and 40 shots with factory settings, so I really don’t know how much farther I can push those limits. 

    The things I’m least likely to compromise on are using anything other then a 14.3 gr pellet and I expect a minimum of 40 accurate shots on a 3000 psi fill, the rest is negotiable. 

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    ajshoots
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    On a 3000psi fill with stock parts and a little tuning, I got 30 shots at 25fpe. I am sure that there was room for improvement, but I went with a WAR valve and hammer as I do in all my Marauders. I now get 48fpe out of my .22 for 24 shots.

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    BareBones
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    48 FPE is darn respectable, but I’m sure you are slinging a much heavier pellet then 14.3 gr?

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    ajshoots
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    Yes, I am shooting 25gr JSB Monsters at around 925fps.

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