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Education / Training Spring gun tuning: Part 2 – Building a mainspring compressor

Spring gun tuning: Part 2 – Building a mainspring compressor

by B.B. Pelletier

Spring gun tuning: Part 1

This is the most important tool a spring gun tuner owns. It may not be used for every job, but working without one when you need it is like walking a tightrope without a net. I now use a B-Square compressor, but for many years I used a homemade rig that did everything I asked of it. The plans for my compressor came from Tom Gaylord’s Beeman R1 book. I’ve seen simpler compressors, but I’ve never seen one that was easier to make.

Mainsprings are under tension
To get maximum power from an airgun, the mainspring is usually under tension (compression). In modern spring guns, the trend is toward more compression than in the past. A few rifles such as the TX200 are under almost none – but they are the exception.

You can’t contain it!
Never think you can contain the force of a mainspring. Eventually, you’ll be able to do so with certain guns you have disassembled many times or even with certain gun models you may have learned very well; but the first time you work on a spring gun, you need to use a compressor.

Simple design
All a compressor does is restrain the rifle while relaxing (or decompressing) the mainspring with control. The task would be simple if all spring guns were built alike – but they aren’t. I will address several different methods of gun design in a later posting. For now, just take my word that the compressor has to be very adaptable.

This compressor is built on a 2×8 piece of wood. All the parts attach to a plank.

The headstock, the bridge and the tailstock
The headstock contains the moving ram that compresses the mainspring. You can make a rugged one from a bench vise. There’s really nothing to build! The vise is bolted to the plank and used in reverse. The tail of the vise puts tension on the end of the gun holding the mainspring.

The bridge is a tunnel through which the body of the gun passes. It keeps the body of the gun from moving sideways when the mainspring is under tension but not restrained by the gun.

The tailstock is a block of wood with the grain end exposed. The muzzle is pressed against it and the gun cannot move.

The bridge and tailstock are adjustable to accommodate different guns. They can also adjust for a gun that has the barrel on or off.

Here is the thousand-word picture. The bridge has a large hole running through it for the gun.
A great vise for a compressor. The tailstock has a long reach!
Tomorrow, I’ll finish this project and have some details and dimensions for you.

author avatar
Tom Gaylord (B.B. Pelletier)
Tom Gaylord, also known as B.B. Pelletier, provides expert insights to airgunners all over the world on behalf of Pyramyd AIR. He has earned the title The Godfather of Airguns™ for his contributions to the industry, spending many years with AirForce Airguns and starting magazines dedicated to the sport such as Airgun Illustrated.

30 thoughts on “Spring gun tuning: Part 2 – Building a mainspring compressor”

  1. I offer this user critique of the Walther CP99. By way of a back-story I had not previously considered buying this gun. While wandering around a local sports store I saw a silver sided CP99. I handled a few other similar sized pistols, but when I held the Walther I knew two things: it felt like a real arm and it was meticulously designed. I had to have it. I believe more people would buy Umarex and other adult air guns if they could actually experience them, hands on.

    The C02 cartridge is housed in the handle’s magazine. To release it press down on the rear of the trigger guard, which is really a hinged latch. The magazine pops out. Turn the magazine base clockwise and the cartridge is ready to come out. A large brass base screw holds the cartridge in place. Once firmly in place you simply turn the magazine base back, this action forces the cartridge up and in place causing the piercing of the cartridge. This works so well that I seldom hear any gas escape with the initial puncture. You pop the magazine back in and you are ready for business. This is by far the best system I have yet seen.

    The 8 shot magazine is the standard Walther unit. You should buy the auto loader immediately.
    You’re going to be shooting a lot. Press the right sided release lever and the spring loaded barrel shots forward. Drop in the magazine, slide the barrel back and you’re in business. When you reload you will have to point the gun straight up, or turn is to the side with a hand ready to catch the magazine because is will fall out.

    The gun shots single or double action. You simply push the rear slide back and the gun is cocked. I used Crossman Premiers for the testing. I used a pistol rest to assure accurate and repeatable aiming, as well. I did no testing using off hand shooting technique.

    The front sight is fixed, the rear is only adjustable for windage. The gun is said to be sighted in for 10 meters. However, after 200 shots I was unable to consistently hit any target. It shot between 6” and 1 ½ “low, and shots were all over the target. The rear sight had to be pushed to the extreme right to get close to the center of a target. I was heartbroken. I exchanged it for another unit. It was right on 10 meters, but like the first gun, it shot way off to the left (almost off the target). I was sighting using an 8 ½ x 11” paper with a simple cross drawn onto it. Adjusting the rear sight to the extreme right put shots very near center.

    The short barrel length of the gun makes aiming somewhat difficult, so I added a Walther laser. Once sighted this combo allows 1 ½” groups at 10 meters. More about the laser in the future.

    I notice that rapid shooting causes a pellet drop on 1 to 3” depending on speed of shooting, temperature, etc. Bottom line: A really fun gun with a high pride of ownership factor. Good for plinking, and action shooting, but past 30 feet accuracy falls off, although I could get 3” groups a 50’ using the rest. I averaged about 48 good shots per cartridge. Once the pressure gets too low, the accuracy really tanks. I just saw a poster for the upcoming James Bond movie and he seems to be posing with a P99!

  2. D.B.
    I’m a big fan of the walther CP99 and enjoyed reading your critique of it. Actually, I own 2 CP99s and 1 walther nighthawk. A hint about the gun though; try using lighter pellets. B.B. likes to use gamo match pellets. They seem to be really good for Umarex pistols. For me, I like the RWS diabolo basics, those work just fine, and a tiny bit cheaper than gamo match pellets. I like the RWS basics because they seem to have better consistency in pellet shape compared to gamo. Seem to find more defected pellets in gamos, but try both brands and figure out which one is more suitable. You said the pellets went really low when you tried shooting your pistol. My guess is, maybe the crosman premiers are a bit on the heavier side compared to RWS basics or gamo match and might have an effect on accuracy. I usually shoot my gun at about 7-8 yards, and they are pretty consistent on the target. Red dot sight or lasers definitely help a lot (I use a red dot). It’s not a gun with a lot of power since it’s got a pretty short barrel, so I think it’s pretty difficult to obtain really good groupings at 50′ with heavier pellets. Try lighter ones, maybe you might be able to get good groupings. I’ve never shot my CP99 at that long of a distance before, so it would be interesting to hear what the accuracy potentials are for that distance with lighter pellets. From what I hear, crosman premiers seems to work best on springers and maybe pump pneumatics, but for something like the walther CP99, another pellet brand might be more suitable.

    -W.P99 Fan

  3. I just wanted to second what was written above conerning pellet weight and brand uniformity. I go through about 2-3 boxes of Premeirs a year, but they never see my pistols, I get Beeman Lasers for them as the Gamo seem to be good for only 2/3 the distance and retain desirable accuracy. I think it is the process used to form the skirt. I use Gamo for cans only, they seem to have a rather high defect rate, and any savings is lost in missed shots in any pistol I’ve shot using them. I still have two unopened cans I bought two years ago. They are not really horrible pellets, just not something to use for target shooting.

  4. I have 2 RWS 34’s (.177 & .22 )
    Because of the way the back end where the safty is . I would guess that I have to route out a space for the safty bar ?

    Also can you tell me is that cap / safty area ( plastic ?? ) and I am guessing AGAIN that it has to come off to get to the spring ??

    Thank you and or any coment’s about this .


  5. Jerry,

    The safety bar on a Diana 34 is a flat metal piece. It doesn’t take up much room. It reaches in and around the retaining pins, as I recall.

    The plastic decorative end cap does need to come off, but your gun has two crosspins that restrain the mainspring and hold the trigger unit in place.


  6. Jerry,

    On the Diana 34 you will need to make a small slot in your spring compressor to allow for the safety bar or you will squish it. As B.B. indicates, it is small so this will not be a major issue.

    As B.B. also states, the 34 has two pins holding the trigger assembly in place. They are perhaps one half inch apart directly above the trigger. The rear pin can be drifted out without compressing the spring, but I would compress the spring prior to removing either pin. The front pin holds the mainspring in place and additionally holds the front of the trigger assembly.

    I have a model 36 in .177 which I plan to tune as part of B.B.’s series on tuning a spring gun. This gun is a bit harsh and I hope to improve it and learn about tuning at the same time.

    If you, or anyone else, comes across a good tuning kit for a 34/36 please let me know.


  7. Hi, this is a bit ot, but i have to ask ,BB(or someone else), my friend just gave me an weird(to me atleast),looking air rifle, and i cant find anything on the internet of it,please someone help!

    The following is engraved? on the gun: PREDOM-LUCZNIK kl.188 kal.4.5mm Made in Poland

    -Anything/all info is welcome!

    I think it was made ~1970

    Is it rare, complete sh*t or what.

    Good job so far BB!


  8. Thanks bb,so i was about right on the manufacturing year…Still i cant find any info of the gun, so maby it is rare.

    I still would appreciate help, how-to dissassemble ? this gun, i have many airguns, all of them i have done this , but this1 will not go apart without violence.(all bolts exc. have been removed,but the mainspring, and everythin else in there wont come out!


    the endcap/mainspring http://koti.mbnet.fi/pate/7756/1285.jpg

    The gun(trigger removed)


  9. Well others seem to use this for asking general information so here goes:

    What kind of safety backstop do I need with an adult airgun? I am planning to buy a spring powered gun in the 1000 fps range. I will have about a 20yd range in my back yard. My concern is for the posibility that one misses the main pellet trap (with taget).

    I don’t mind the stop taking on some damage, but do mind the pellet going on through. Will 3/4 inch plywood stop the pellet? Or is there something better (but not expensive? Will carpet o canvas do the trick?


  10. Kirk,

    Good question and, no, no amount of wood is good enough. Many of these adult guns will shoot through a 2 by 4 on the first shot!

    What you need is a bullet trap rated to stop a .22 long rifle bullet. Pyramyd AIR carries such a trap. It’s called the Heavy Duty Metal Trap. This is where it is located:


    I have cracked cinderblocks in house foundations with adult airguns, so please be careful.


  11. BB, I have a HW35 Export. I naively let a friend borrow it to shoot some rats on his farm. Since its homecoming it’s now unsafe. When cocked the trigger is frozen even after releasing the safety, yet inexplicably it trips when a little pressure is applied to the safety catch button? Could it be the safety catch spring, if so do I have to build a mainspring compressor?

  12. What I think has happened is a tiny bit of the end of the safety spring has gotten stuck in the channel in which the safety button rides. You might be able to free it without disassembly by cocking the gun repeatedly while pressing in on the safety button. Sometimes that’s all it takes.

    Hold onto the muzzle at all times while you do this. Just keep pulling the gun barrel all the way down and pressing on the safety button until it pops free.

    If that doesn’t work, disassembly will be required, but not of the powertplant. You can drop the trigger by drifting out the two pins and the safety is accessible.

    Read this report:



  13. Cheers BB I’ll give it a go, I really appreciate your kind help! Is there any way to tell how old my HW35 E really is, my father bought it many years ago and sadly he’s no longer with us.

  14. Cheers BB for your first-class advice!

    I’ve got to the point now were I’m in two minds whether to give this beautiful heirloom the restoration is truly deserves. When I fire it makes a right din and smells?

    The question is which products to use to tune it… restore the walnut stock and re-blue it?

  15. Cheers to the HW 35E chap,

    B.B. has been answering your questions since he is only one of handfull of people that will check back on these comments that were started in 2006. Most active airgunners are asking and ansering each others questions in the "comments" section under the most recent article that B.B. has written (B.B. writes a new article every day, Monday-Friday). You may want to ask your future questions there. You can always access the most recent article by copying and pasting this link:


    Secondly, please give some thought to the cost involved for buying all the supplies and equipment to tune an airgun. If you intend to tune many guns it will be worth it. If you just intend to tune one or two guns the cost is a consideration and although B.B.'s 13 part series on tuning is an excellent tutorial please realize that many advanced tuning techniques are not covered. An experienced tuner has a complete workshop including a metal lathe. A tune with trigger work by a very experienced and reputable tuner will cost $150.00-$250.00 US but the material & equipment will cost you more and the tuner throws in thousands of dollars in experience for free. Just a thought.


  16. Fair one Kevin,

    I get your drift; I’ve got the bug too just like you when you stripped your first springer except here in Switzerland it’s hard to find the right kit let alone find a English speaking tuner. Happily I found BB’s article on spring gun tuning which kind of answered my questions.

    Many thanks – Carabine d’air

  17. I still own the copyright of my books and that is why the link has been broken; it’s something to do with ethics…

    Although the books are long out of print they turn up on ebay from time to time and that is where I suggest you look.

    And, if you’ve made money out of the copies I suggest you send it on to me, I sweated blood writing them.


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