AirGunTech: Re Engineering Spring Powered Air Rifles for smoothness, consistency, and to better suit UK power levels.

HW77, the parts, tuning and upgrading, even on a brand new gun.

Edit, seem this was not loctite or glue but excessive bluing salts hardening on the threads between the trigger block and the action. I have been advised a good soak in dewattering oil/release agent should have helped. Regards how long to soak…i was told at least 1 full day.

After waiting months for a new 77k SE laminate to arrive I took delivery and started testing. I pushed around 30 pellets thru it before chrono tests started and was a little shocked. This rifle is a Hull Cartridge supplied rifle so is full UK spec, the issue is it was UK spec and some.

Initial tests were with the new Brocock pellets supplied by Crosman, they are infact just Crosman premiers with a stated head size of 4.52. I had pushed these thru the gun to settle it somewhat, there was no dieseling at all, i had guessed the piston was shod with one of the new type seals from HW which are usually pretty efficient. Now, I shot an additional 30 pellets after the settle is batch over the SKAN, I had not weighed the pellets and i was only after a rough idea what the gun was delivering. This is where it gets a little serious, it was pushing 12.8 to 13.1fpe with the Premiers, so I swapped to 4.52 Exact RS 7.3gr, now we had 13.4…so i dug out the last of my Hobby and hit 13.8fpe out the limited number I had. Now remember this is a brand new UK spec gun.

Normally I would just take it back, the fact I had waited 4 months for the rifle and the fact I tune rifles as a hobby I decided it would just need a coil or 2 off the spring and all wood be good…how WRONG I would be.

To start HW are now loctiting the trigger blocks on the rifles with what amounts to concrete, I had to apply so much heat to the trigger block it destroyed the spring guide…and i was worried that I would damage the bluing. Once the trigger block was finally off and the melted spring guide was removed from the spring I proceeded to continue the strip of the rifle. Loosening both the side and top crew off the rear open sight I was able to slide the sight off the dovetail rail, only to find the action scratched underneath from where the sight had been installed…now I was really not happy. Anyway, out comes the piston and comp tube so I can check lube etc.

The spring is 255mm long, the transfer port is around 3 to 3.1mm in dia, the spring guide is melted at one end so now scrap. I check inside the piston and additional preload has been applied in the form of a steel washer 5mm thick which also added to the weight of the piston. So…I decide I will fit one of my own custom springs and rebuild the guides, adding a tophat and slip washer.

The rear guide I installed is a little different to the stock guides, I like to machine the piston rod hole in the trigger block to 14mm dia and 12mm deep, I then machine a guide with a 21mm flange face for the spring but a rear section 14mm dia and 12mm deep that pushes into the trigger block with a good interference fit. The guide length is 20mm short of touching the bottom of the piston so I can install a long tophat and play with preload by machining washers that slide down the top hat and not sit beneath it, this saves on weight…all preload washers are Acetal also to keep weight down.The piston was fitted with a new style seal so I left this alone, it was cleaned however and a test lube installed (will report on this later). Any traces of the molten rear oem guide were cleaned away, the new rear guide is glued into the back trigger block and then the spring is greased with Millers red rubber grease. I don’t run tight rear guides, I run guides that are a glide fit down the spring, the twang is removed with the grease 😉 tight guides end up breaking…plus they sap power.

I just happen to have a spare 26mm comp tube, this is already modified with a 3.9mm transfer port, so this was installed. The spring is a custom spec 240mm long unit with 33 coils, these springs will be available to buy soon and I will have details for from where here on the blog. The rear guide applied 3mm preload off the face of the trigger block, the front tophat has a 2mm flange and a 1mm slip washer, and additional 2mm slip washer is installed on the rear guide only, so total additional preload from the guides setup is 8mm. Power is bang on 11.3fpe with 8.4gr Exacts, RS slightly lower and Premiers just under 11fpe. Hobby ran out but im thinking around 11.2 or so…the reason being I altered the peak pressure with the larger transfer port so they will not be as efficient.

With the gun now legal, although scarred, and the firing cycle much improved I noticed the underlever catch was ringing when operated and on the firing cycle, this is due to the spring on the ball catch vibrating in the tube. The cure is to drill out the dimple on the under lever (which is around 50mm from the end of the lever) with a 3mm drill bit, go in 3mm also. You can now grip the ball in a vice and drag the ball catch unit out the lever. I now disassemble the ball catch, shorten the spring by 1.5 coils, re round the ball using a lathe file and then polish before rebuilding the catch and packing the spring with good moly grease. next tap the hole in the under lever m4 and de burr the inside of the tube. Once the inner of the tube is clean slide in the ball catch, lining up the dimple you have drilled into the aluminium bush and lock in place with a short m4 grub screw and loctite…dead easy mod that does 2 things:

1 it stops the ringing noise

2 you weaken the spring a little so its easier to drop the lever off the catch and replace it to the catch..which also stops it wearing the catch as much 😉

The last job tackled was the trigger, I wanted to fit a set back blade, usually an easy job, however after removing the oem blade there was already a dimple worn into the bottom sear (the one the roll pins in the trigger blade act upon), all I can think is HW do not harden the bottom sear or if they do its not hard enough. Anyway, I decided a full trigger strip was in order so I can stone the sear to have a flat surface for the roll pins to rub against.

The 2mm dia pin that holds the blade and it rocks on was also bent, 100 or so shots should not do this, however I now use hardened ground pins 15mm long and grip washers on either side of the trigger cassette so the pin was discarded (this does mean I alter the inletting on the stock) The sear was stoned and polished, the grease removed and fresh lube applied and the trigger rebuilt, first stage travel set to 2mm and the second stage let off set as fine as is safe, it breaks like glass now 😉 Then the stock inletting is altered to take the wider blade pin and grip washers and the action dropped back into the stock…this is when i noticed the inletting for the breech area was incorrect and was rubbing on the breech so much is was splaying the front of the stock wider…putting strain on the for end of the stock.

So, out comes the dremel again, the inletting was again altered so the action just slipped in with no resistance from the stock…this is where i noticed another fault and something you need to look for on all mk3 77 and 97 rifles. What you need to check is the stock lug (Part# 1332) finishes flush with the underside of the inletting for the trigger guard, on my gun here it was 2.5mm below being flush. The fix is to add a washer or washers to pack up the stock lug till its flush.

Now, you may be asking why is this important? The answer is the 2 screws that hold the trigger guard can now pull the trigger cassette out of true and induce wear and irregular performance from the trigger. Once you get that stock lug true with the inletting for the trigger guard you can actually do away with the small m4 screw at the rear and just use the larger locating screw at the front (Its what I do here) and the trigger cassette is allowed to sit true and deliver the performance you expect from it.

I will detail these modifications with pictures so you can see exactly what is needed, the 2 alterations to the trigger (the longer ground 2mm hardened pin with grip washers and the stock lug) really do make a long term difference to the feel of the trigger. This build has stock 81mm stroke and no bearings to the piston, I want to test some special lubricants to see how they effect a stock set up, the rear guide will be helping the piston fly truer anyway and is reflected in the felt shot cycle.

Last moan about the gun is the stock has sustained some damage to the left side during manufacture, this was then lacquered over, while it does not effect how the gun shoots it does mean the stock is not 100% on the outside, which upsets me. I waited a long time for this rifle and really expected better from HW, usually I rebuild old rifles that have seen better days, this was my first new 77k out of a previous 10 that I have owned and im being honest I wish I had bought second hand and just tried to pick up this new laminated stock separately.

To finish, any RFD’s or tuners reading this, you are going to have serious issues servicing HW rifles with screw in trigger blocks (new ones from around 6 months ago) going forward. You will need to heat up the trigger blocks to break the loctite and in doing so you will destroy the spring guide and potentially risk damaging the bluing on the rifle.  This obviously applies to the following rifles:









  • Great info Tony, esp HW locking the screwed end.


    Steve ( ;-{)>

  • First of all i would like to thank you for this site , the information you share with us is priceless..
    I currently have a 2006 77K which i reduced the stroke to 74mm with a buttoned alu piston nose extention, drilled tp to 3.5 mm ,added a rear piston bearing plus and a fixed bronze spring guide.. this few years ago when i could still use use my old pre ww2 excel l3n lathe 🙁 .. the gun is quite nice to shoot but in my opinion can be made better ..
    regarding your 77 tune I have 2 Questions.
    What wire thickness is the spring used here ? mine is a shortened maccari 14mm id 3.25 mm wire spring ..also did you observe a slight piston slam at 3.9mm tp ? i am tempted to open mine a little maybe 3.7mm with a reamer this time if i could find a reasonably priced one of that size..

    Best regards

  • How will that effect the stl models? Is that finish more prone to heat damage?

  • Mark, im using a spring with 3.25mm wire thickness, no slam at 3.9mm BUT i keep power around 11fpe tops and shoot most pellets in the 10.5 to 10.9fpe power range.

    Nick, I have no idea, it did not damage the bluing BUT i was worried i was close on doing so.

  • Thanks for the reply.. mine is pushing out around 12.5 fpe with jsb exacts no power restricting laws here in Malta but all airguns like firearms carry a licence.. i wouldn’t mine lowering the power to sweeten up the shot cycle.. also my piston is around 10g heavier than standard due to the modifications mentioned..

  • Enjoying the read- fascinating stuff, and for us springgunners its an exciting time. Thanks for sharing your pioneering work with us. Jonathan

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