An experiment in low-power springer tuning: Diana 240 .177 soft tune

[happymecanic]

I bought this 240 T05 rifle new a couple years ago, and have always been pleased with it, but also found it to be a little harsh on the shot cycle. You could easily feel both recoils, especially with lightweigth pellets. Accuracy was always good to very good though, depending on the pellets used (and caffeine level in my blood!). After shooting it for a while I decided the rifle could be improved.

I started with the transfer port, as I felt it was way too big at 4 mm (0.158’’), considering the small ’’swept volume’’. I re-drilled and threaded the TP to M5x0.8 and made a threaded sleeve in brass, drilled to 2.5 mm to start. Velocity went a bit too low to my taste, so I gradually increased the TP ID to 2.8 mm, which yielded high-450s fps velocity with JSB 7.33 grains domed pellets. It also greatly improved the shot cycle as now the ’’second’’ recoil is almost unnoticeable, and there’s much less noise from the action. But it also made the rifle have a light spring buzz with heavier 8.44 and 8.65 grains pellets. (While the rifle was apart I also cleaned and deburred the receiver tube, re-lubed the barrel pivot pin, cocking arm and ball detent with tungsten paste, and did some trigger tuning. As a finishing touch I made an MDS top hat with a steel thrust washer and a new well-fitted delrin spring guide.)

The trigger had the ’’second screw mod’’, this actually replaces the fixed front trigger ’’bump’’ with an adjustable screw, for a very nice two-stage feel. I also added a trigger stop screw, easy and worthy mod. Then the sear plates, screws contact points and pivot pins of the trigger were lubed with tungsten paste. Piston latch was also polished and tungsten lubed. To finish, the trigger spring was replaced with a slightly weaker one for a light 14 oz pull.

I put the rifle back together and shot it like that for a while, then went back into it as I felt the shot cycle could still be improved. I found out I had a spare main spring from a deceased Beeman Biathlon air rifle. This spring has the following specs : 37 coils, 6.570’’ free length, 0.590'' OD and 0.095’’ wire. The new spring Id is 0.400’’ and the D240 piston rod OD is 0.389’’, so the rod now is the inside spring guide. I made a nylon spring seat/sleeve to replace the OEM guide, ID is 0.625’’, OD 0.787’’ (to fit inside the piston), length is the same as the OEM guide. There’s also a new delrin spring seat/bushing in the piston to replace the top hat. The spring ends were polished, spring was coated with a very thin layer of synthetic grease, and a steel thrust washer is used in the rear guide. I originally intended to also use one in the front bushing, but I made a slight miscalculation and the spring got coil-bound just before the piston latched so I removed it.

The rifle was put back together again, and the astonishement began lol! First time I cocked the rifle it was so easy I immediately tought :’’ it’ll shoot 200 fps!’’. When I shot the rifle it felt weak, but then I looked at the chrony and saw 449 fps, with the same JSB RS Exact 7.33 grains pellets the rifle likes!!! I was truly stunned by the smoothness of the shot cycle. You can no longer feel a second recoil, you only hear a muffled ’’pfttt’’, no spring noise at all, even with the heavier pellets. Accuracy is easier to maintain when shooting off-hand, and rested it’s pellet on pellet, @ 10 meters anyways. I didn’t measure the cocking effort with the OEM spring, but with the new spring the cocking effort registers @ a ridiculous 5 lb on my fish scale, seriously!

So the rifle didn’t loose much velocity, but gained quite a lot regarding smoothness and shooting pleasure. Shot-to-shot consistency seems to be rather good with a spread of 10 fps. I’ll see with time how it fares, but the rifle seems to be breaking-in nicely, as I feel the shot cycle becoming more ’’regular’’. That tuning job was an experiment in low-power tuning, and a very enriching one at that. I believe things could be improved a bit more by using a lighter weight piston, that would reduce the first recoil even more.

Airgun tuning, a never ending story lol!

All the best, Francois

[DanD]

That's interesting. What was the pre-mod velocity?

[happymecanic]

That's interesting. What was the pre-mod velocity?

Thanks! Velocity was about 10 fps faster with JSB 7.33s (458 vs 449), but now the rifle shoots the RWS Hobby wadcutters @ 405 fps (471 before) as they're a tight fit in this barrel. Pellet choice becomes more critical at this power level, fortunately she still loves the 7.33s (most accurate).

[DanD]

Hi Francois, I had a 240 a few years back that was shooting 550 with midweights and 580+ with lightweights. I wonder if yours might have sealing issues that exacerbated the recoil/ harshness.

[happymecanic]

Hi Dan, if my rifle was a full power one as sold in the U.S. I would totally agree with your diagnostic. Actually, checking my seals was the first thing I did when I started messing with the gun. However this D240 is a Canadian-detuned airgun, so right ''as sold'' it's supposed to shoot under 500 fps, so I'm guessing it doesn't have the same main spring as the U.S. D240.

Sleeving the TP definitely made a huge difference in smoothness, so I believe the original 4 mm size (0.1575'') was overkill for this power level. I know rifles with bigger compression tubes and making 10+ fpe in .177 use 3 mm (0.118'') transfer ports so why such a big port on this little gun?  Dunno but a smaller port works better IMHO . I seem to remember seeing on another forum there's an ''ideal'' ratio between compression chamber volume and TP size, that can vary depending on  spring preload and strength (and some other factors). I'm not able to calculate it but at least I can experiment.

Cheers!

[Jim-in-UK]

Before his untimely passing in 2015, a friend and I conducted a good deal of research into transfer ports, him largely from the physics standpoint (him being a Professor of engineering), me from experimental engineering.

Before the adoption of synthetic piston seals, most airguns from German manufacturers sold in the UK had 4mm diameter transfer ports, and they all had leather piston seals bearing lubricant that supplied a ready source of fuel for dieseling, which they relied on to make their muzzle energy. When they switched from leather to synthetic piston seals, the dieseling stopped, and they had to reduce TP diameter to nearer 3mm. So why?

Dieseling creates a pressure/temperature spike that gets the pellet moving earlier in the stroke (about 74% into the stroke with a typically 400psi start pressure 1980s pellet), and the 4mm port delayed the point at which sonic choking occurred in the barrel, which would otherwise occur while the piston still had some way to travel. Without dieseling, the pellet would not start until nearer 86% into the compression stroke, so sonic choking had to be advanced to closer coincide with the piston coming to the end of the stroke, which compensated for the loss of diesel energy with improved energy efficiency.