RWS 48/52 Tune

by Scot Heath


Following is documentation of the tune of my new .25 cal RWS model 48 with TO5 trigger group. The objective of the tune was:

  • Improve (smooth) the cocking stroke
  • Reduce the spring noise and felt vibration
  • Reduce the trigger pull

    I’ll cover the “main” gun work first and then the trigger group work. To begin the tune, I assemble the necessary tools, etc:

    Some necessities are:

  • Spring compressor
  • Digital calipers
  • Cyanoacrylate (super glue), medium flow
  • #2 Philips screw driver
  • 2.5 mm Allen wrench
  • Small screwdriver, dulled edges
  • 1/8” drift punch
  • Various honing stones
  • Small flat file
  • Small round or rat tail file
  • ¾” round Delrin bar stock
  • ¼” round Delrin bar stock
  • Moly paste
  • Spring tar
  • Clear tar

    Disassemble the Gun

    The two screws holding the gun in the stock are the forearm screw and the forward trigger guard screw:

    The screws shown are Allen head (button head cap for the forearm and socked head cap for the trigger guard) while the factory screws are #2 Philips (probably actually Posi but Phillips will do). I remove the trigger guard screw first to reduce the possibility of breaking the stock in this area. Here’s a photo of the original screws and their replacements:

    The button head cap is a straight replacement but the socket head must be modified with a 60 deg. bevel on the bottom of the head. I did it on a lathe but you could do it with a drill and file. It’s best to get a socket head cap with the same length unthreaded portion as the original to prevent the stock hole from being damaged by threads. These screws are much nicer for repeated work than the Philips (note the "chewed up" head on the trigger guard screw, it's actually one from an old 52 I had, I grabbed the wrong one from the drawer when taking the photo).

    Once the action is removed from the stock, I immediately put the action in the spring compressor, cocking lever up. Enough clearance for the trigger group retaining pins must be provided under the rear of the action to allow them to be driven out after the spring is compressed. The 48 trigger group has a plastic cover/extension that is a half sphere so you’ll need to build an adapter to keep from damaging this piece. I used a piece of 1” diameter AL bar stock, drilled a hole to fit over the rod on my compressor and cut the forward portion into a half cylinder:

    Put the gun in the compressor and tighten enough to take the free play out of the compressing mechanism:

    Next, remove the e-clip from the bottom of the cocking lever pivot pin:

    Here’s where that “dull” small screwdriver comes in handy, sharp edges/corners can easily scratch the finish of the cocking arm. I stoned off the sharp corners on all surfaces of a couple of give-away screwdrivers and keep them for just this (and seal replacement) purpose.

    Once the clip is off, push the pivot pin out. The lever must be released from its home position to relieve the pressure from the pivot pin:

    Note the position of the washer, seems like it should go on the clip side of the pin but mine was on the head side. Now lift the cocking arm from the bracket, taking care not to loose the 2 thin thrust washers:

    Here’s the stackup of the cocking arm pivot pin and washers, the two free thrust washers go over the pivot sleeve on each side of the cocking arm. This sleeve is free to slide and must be positioned so that it protrudes the same amount from each side of the cocking lever, just enough to retain the washers when they’re installed with a little moly paste during re-assembly.

    Now, tighten the compressor until the trigger group retaining pins come out with light tapping:

    You may even be able to push them out without the use of a hammer but in any case, light tapping is the maximum it should take. If they seem stuck, adjust the tension on your spring compressor until you find the correct setting. With the pins out, release the compressor. Nearly the entire trigger group will be pushed out the back of the action due to spring pre-load:

    Notice that the trigger blade has been removed. No need to do this now, I do just to keep it out of harms way (it’s plastic).

    On to the bigger parts, through the loading port, remove the cocking post retaining screw from the front of the compression tube using a 2.5mm Allen wrench:

    Pull the post out of the chamber and slide the chamber/piston/spring out the back of the action. Also remove the anti-bear-trap mechanism:

    The photo shows the correct orientation of the spring and pawl. The two ends of the spring hook into the small notch shown on the bottom of the pawl.

    Acton Tube Work

    At this point, I clean and inspect the action tube. There are many effective means of doing so, I like to use paper towels as “patches” and push then through from the loading port to the rear of the action with a 3/8” dowel (I actually use a piece of plastic rod). You can use solvent if you have some old dried lube, this was a new gun and came quite clean just using dry towels. Now, stone or file the cocking slot flats to get rid of the tool marks:

    If you prefer, you can continue to work these until they are uniformly shiny, I prefer to remove the minimum material to keep the slop down (although, this is a very sloppy fit already). The idea is to provide a smooth surface for the cocking post to bear against. Next, I break the edge on all action tube openings, especially those that contact sliding parts. Use the small files to do this, round file for holes and the rounded end of the cocking slot, flat file for the slots. No need to make it a big taper, just get rid of the sharp edge. Be sure to do this after you smooth the cocking slot flats or you’ll re-create the sharp edge on that slot.

    Once you are finished filing, you’ll want to check for any galling in the inside of the action tube. If present, use either a brake cylinder hone or emery around a close fitting round mandrel to smooth any sharp edges here. You don’t need to remove the scrateches completely as this is not a sealing surface. Again, with a new gun this is a non-issue.

    Be sure to repeat the cleaning process after all these abrasive/cutting operations are done. Get every trace of both abrasive and metal filing out of the action tube.

    Compression Chamber Work

    I like to polish the chamber, removing the ½ blue finish. I do it by spinning it in a lathe and using 400 grit wet/dry paper and a little oil. Not much critical here, just apply light pressure with a flat such as a sanding block to keep the surface from getting ridges. Be sure to break the edge at the ends of the tube. If you’re not going to button, this is it for external work.

    Internally, clean the tube with your favorite solvent. The paper towel patch method works here as well but it’s a little trickier since you can’t push it through. Spray cleaner such as brake cleaner works well here, I use WD40 to clean many gun parts, it’s a good solvent and the residue protects against corrosion should you happen to miss any. I DON’T use it in the compression tube, however; any traces will certainly combust upon firing. If the tube is not scored and still has traces of hone marks, you’re done. If not, you’ll at least need to hone it with that brake cylinder hone or emery around a mandrel. If it’s badly scored, throw it away and order a new one from Dynamit Nobel RWS. Once the inside it done, clean it again if you’ve used any abrasives. Inspect the chamber to barrel seal. If it’s cracked or damaged, replace it. One nice feature of these guns is the ability to adjust the compression of this seal which you’ll check/adjust during re-assembly. If you’re not going to button the chamber, you’re done so set the chamber aside. If you are going to button the chamber, do so now. I put 3 equally spaced buttons around the rear of the tube, two just below the cocking post hole on each side of the front of the tube and 1 at the top of the tube directly behind the loading port. I used ¼” Delrin turned to fit snugly in the recesses:

    I machined the recesses with a 3/16” carbide end mill, the carbide mill is necessary because the front of the chamber is HARD! Here’s a photo of the milling process on the chamber:

    I machined all recesses in the chamber .015” deep. After machining the recesses, I repeated the polish procedure with the 400 grit to eliminate any proud edges on the recesses. I then parted off small buttons .030” thick:

    Here’s a photo of a button and a recess prior to assembly:

    I filed one side of the button flat after parting by placing it on a small fine cut file and using my finger to run it along the files surface. This only takes a couple passes to get rid of tool marks/center spur. Clean the recess and button both with solvent, I use denatured alcohol, then place a small amount of cyanoacrylate (about ½ the recess diameter) in the center of the recess. Immediately install the button and apply pressure for the amount of time the glue manufacturer specifies. I like the medium thickness glue as it stays put but flows well and cures quickly. Here’s a photo of the button installed before turning it’s profile:

    After installing all the buttons in a plane (like the three at the back of the chamber), I recommend turning their profile to lessen the chance that you’ll accidentally dislodge one while turning the others. Center the piece in the lathe chuck until total indicated run-out is less than .002” (note the tape on the piece shown in the photo.:

    Turn the buttons until the assembly fits snugly into the action tube. This gun had a tube that was out-of round just ahead of the trigger group slot. I corrected this to within .002” buy clamping it on the major diameter and deforming it a little at a time until it got this result. Be sure you check the chamber button fit by installing it in the position It will eventually be used. The chamber on this gun is very snug going through the rear of the tube (since it’s out of round) but slides freely in it’s final position. The buttons on this chamber are .003” - .004” proud:

    Piston Work

    I polish both the edge of the rear flare of the piston and, if necessary, the bearing surface just behind the piston seal. Use the same process as for the chamber OD. If the seal is damaged, replace it. Pry it off with your “dull” screwdrivers and install the new one like a tire on a rim using the same “dull” tools. Using a stone, break any sharp edge on the piston rod that could gouge the ID of the spring guide. Do this to the absolute minimum so as not to alter any piston catch bearing surface. If you’re not going to button the piston, you’re done. If you are going to button, machine recesses exactly as in the compression chamber, 3 equally spaced on the flare and 3 just behind the bearing ring. Since the buttons are going to be taller, I made the recesses .025” deep at both the front and rear of the piston and the buttons .050" thick. Install and profile as in the case of the chamber taking note to indicate off the bearing ring just behind the seal for the front set of buttons. Check the fit of the front set of buttons all the way to the end of travel of the piston in the chamber. Check the fit of the rear set in the actual operation position of the piston in the action tube. Finished piston looks like this:

    Notice that the rear buttons are installed on the flared portion of the piston skirt but profiled parallel to the centerline of the piston.

    Spring/Guide Work

    In my opinion, the heart of a tune is the custom spring guide. You can either purchase a kit with fitted guide and spring or you can make your own guide to fit the spring you choose to install. I used the factory spring in this gun. Thoroughly clean any lube off the spring. Next, stone/file the flats at the both ends of the spring until smooth. To do this, I insert one of my “dull” screwdrivers into the gap between coils where it begins to get narrow at the end of the spring, then force it into the closed portion so that the end of the spring is separated from it’s adjacent coil slightly. This prevents the filing/stoning from damaging the complete coil next to the end. Be sure to round the inside corner of the spring flat so that it doesn’t damage the spring guide. Next, if you’re making your own guide, it’s time. I use ¾” Delrin and a separate grade 8 flat washer to replace the one piece factory guide. If you’d like, you can turn the guide out of 1-1/4” bar stock and duplicate the factory guide rear profile exactly. I first drill a slightly undersized hole in the bar stock ¼” or so deeper than the target length of the guide. Next, using a live center, I turn the OD of the guide until it is a very snug push fit into the spring. I like to be able to remove the guide from the spring by hand with just a slight amount of twist to loosen the coils. I’ve done them tighter with no apparent negative effect on velocity but they’re harder to work with and don’t seem to dampen any better. Once I get the OD correct, I ream the center hole to be slightly oversize (.004” or so). Finally, I part the guide off the bar such that the bottom flange is the same thickness as the recess in the front of the trigger group is deep. This minimizes free play during firing while keeping the stress from the spring on the housing of the trigger group. To make the washer used to replace the original flange, I use a ½” grade 8 flat washer, turn the OD to be a very close fit in the action tube then bore the ID to be a close fit on the guide body. I re-install the original hard washer over the grade 8 for the spring to bear against. Here’s a photo of the original and new, fitted guides:


    One note on re-assembly. Unless specifically lubed for friction prevention, I put a small amount of oil on my fingers when I’m handling anything steel for final assembly. I purposefully coat the pieces with just the oil that’s on my fingers as a rust preventative measure. Again, it’s a tradeoff with particle retention but especially in humid climates, acid from your hands can certainly promote corrosion.

    Lube the piston seal and front buttons with moly:

    Take care to remove any lube from the leading edge and face of the seal so as to prevent dieseling. Assemble the piston into the chamber, lube the chamber buttons (I actually lube the OD of the entire chamber) with clear tar and install the chamber/piston in the action tube. I like to leave this such that the piston rod is about flush with the rear of the tube, it makes installing the spring and guide easier.

    While I believe it would be just fine to leave the guide dry, I typically “burnish” the surface with moly paste. I’m looking to deposit a very small amount in the tool marks on the guide and rub as much off as I can using my bare fingers in the direction of the major dimension of the guide, it’s a very small amount. Deposit a small amount of moly paste on the flat washer where it will contact the spring end. I then install the guide in the spring and put a minimal coating of tar on the spring OD, again using my fingers rubbing along the major direction of the spring:

    Put a small amount of moly on the free end of the spring and slide the spring/guide assembly over the piston rod, pushing the piston and chamber into the action until the cocking post can be re-installed through the loading port. Re-install the cocking post and retention screw.

    Slide the chamber the rest of the way to home position, insert the trigger group behind the spring and compress using the spring compressor. Insert the retention pins. The sear cocking piece rides up somewhat when there are no pins installed and I’ve found it easier to install the pins if I insert my punch in the “other” hole when installing the first pin. Hold the cocking piece “down” toward the trigger group and insert the first pin. Very light taping is all it takes to do this operation and often, the pins can be inserted by pressing them in with your thumb. If they seem solid, something’s not lining up, don’t force them. NOTE: be sure the ends of the pins a flush with the side of the action tube opposite the cocking lever bracket. If they protrude, they’ll interfere with the fit of the tube into the stock and often damage the stock (experience talking here). After the pins are in place, pressure from the spring compressor may be removed. IMPORTANT TIP: after installing both pins and with the gun un-cocked, pull the trigger. This is necessary to re-set the sear, otherwise, the piston rod won’t latch (the gun won’t cock). Install the cocking lever, pivot pin and e-clip making sure the proper order of parts is maintained. Lube the thrust washers and pin with a little moly paste. Check and adjust if necessary the load on the breech seal. The tip of the cocking arm should be 2-3” from the body of the action when the breech seal contacts the end of the barrel:

    Slight pressure should be required to force the cocking arm over-center to it’s locked home position against the body of the action. If this is not the case, remove the e-clip and linkage pin from the body of the cocking lever. The loosen the jam nut from the clevis on the end of the link and turn the clevis in or out in ½ turn increments until the desired seal pressure is obtained:

    Tighten the jam nut against the clevis, taking care that the clevis is square with the cocking arm recess:

    Re-install the linkage retaining pin and e-clip, lubing the pin with a little moly.

    Prior to installing the action in the stock, I test fire. Take care when cocking not to damage the trigger blade and above all, be safe! If you’re not comfortable doing this, install the action in the stock first. When test firing, I’m looking for:

  • Proper function
  • Any rough feeling in the cocking stroke
  • Evidence of excessive dieseling (I typically have no noticeable dieseling to a very few (10 or so) shots.
  • Muzzle velocity (are the seals doing their job?)
  • Trigger feel

    When you’re happy, install the action in the stock. I start both screws, snug up the forearm screw to just past the point of stock/action contact then tighten the trigger guard screw. Finally, tighten the forearm screw. If you’ve replaced the factory screws with Allen heads, resist the temptation to draw them right through the wood, don’t over-tighten.

    With the addition of a Simmons 4-12x40 ProAir scope in BKL 260 mounts, adjustable curved butt pad, and a custom brass muzzle brake, here's the finished product:

    Click here for full resolution image.

    Click here for full resolution image.

    Go enjoy your twang free, smooth cocking gun! I hear the shot cycle of tuned guns often describes as a thunk. This gun is a little more definitive than a thunk, more of a thwack. It is very torque free and, in my casual observation, much easier to shoot accurately than the stock configuration. Part of that is the improved trigger so if you’re inclined, you can improve the trigger in the gun as well.

    Trigger Work

    Disassemble the trigger group:

    All pins in the group retain parts that are spring loaded. The trigger blade spring and the coil sear spring may be carefully removed prior to removing the pins which hold their respective parts in place. I recommend doing so. One method with I employ when working on “tricky” spring loaded assemblies is to put them along with my hands inside a large clear plastic storage bag. If anything happens to decide to take flight, this reduces the probability of it getting away. The photo shows the correct relative position of all the components. The two plates at the top are actually the piston latch (right) and a load bearing plate (left). It’s important to note the orientation of these pieces when re-assembling. The load bearing plate has square, sharp edges on one side and rounded edges on the other. The rounded edges face rearward, against the piston catch plate. The piston catch plate has tapered edges on the hole in the face that is installed facing rearward. These plates are hard and are dry from the factory. I put a little moly paste between them when re-assembling, it’s a tradeoff between lube and dirt attraction.

    To lighten the trigger pull, I first stoned the sear surfaces smooth and flat. In this photo, the active surfaces have the small “yellowish” light reflection. Keep them flat and take off just the surface roughness. The piece on the right is especially prone to rounding or angling when stoning. This could lead to an un-safe trigger so take care, better to leave it un-stoned than do it incorrectly:

    I also cut the two coil springs shown at the bottom of the “exploded” photo. I cut 1 turn from the large diameter light trigger return spring and I cut 1-½ turns from the sear spring. A better solution would be to replace the sear spring with a spring that has smaller wire diameter. It moves quite a distance during the first stage of the trigger pull and the large diameter stock spring wire results in a bit higher spring constant and resultant force at the end of travel than I’d like. As is, the sear stoning and spring trimming along with careful adjustment of the 2nd stage screw got me a crisp, creep free, 1lb, 14oz trigger. Here’s a photo of the re-assembled trigger group showing the proper placement of the safety, cocking piece, cocking piece spring, load bearing plate and piston catch:

    Questions? Comments? Please, drop me a line.

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