Airgunning

     S400 Tuning    

Although the S400 doesn’t need a regulator, at least that was what I thought based on the chrono results of the first S400 I ever had (68 shots within 3 m/s), I wanted to know how a S400 with a regulator would perform. I was also intrigued by the Audrius regulator so having already the S400, I ordered the regulator from Audrius in Lithuania.

Before installing the regulator, I shot the S400 in its standard configuration (this is an early S400) over the chrono. I decided to take flatheads to do the test. Probably not the best decision looking back, but we’ll have to do with it. I first set the S400 to 246 m/s with JSB Express (about 11.5 fpe), my preferred pellet and speed and with this configuration I shot the flatheads.

The S400 turned out to be nowhere near as consistent as that first S400 I had (and never should have sold), so it was clear this S400 could clearly benefit from a regulator.


Mean V0 (m/s): 253.1
Energy (fpe): 10.7
Range (m/s): 9.8
Minimum (m/s): 248.3
Maximum (m/s): 258.1
# shots: 84
 

The Audrius reg comes at a working pressure of 115 bar, bu since I wanted as many shots as possible, I set it to 100 bar.

Installing the reg is pretty straightforward. After the installation I got the following results.

Mean V0 (m/s): 250.4
Energy (fpe): 10.5
Range (m/s): 2.9
Minimum (m/s): 248.9
Maximum (m/s): 251.8
# shots: 50

The consistency had improved quite spectacular but the number of shots was far less than I hoped for. I repeated the chrono session, also with JSB Express but the result stayed the same: good consistency but only about 50 shots.

Obviously too much air was used, which you could also hear by the muzzle report. The valve stayed open too long, waisting air even when the pellet was already gone. So I needed to find a way to keep the valve from opening too far and too long.

My first attempt was to increase the pressure on the valve return spring, but the result stayed the same, about 50 shots.

According to Robert (thanks Robert for the interesting discussions we had, and still have, on this subject ;-)) and Audrius, I needed to use a weaker hammer spring. Since I didn’t want to shorten my original hammer spring, and I wasn’t too keen about the side effect of a weaker hammer spring, being a longer lock time, I decided to go for a different approach, i.e. short strokening the hammer. So I started the lathe, and made a messing hammer which was 3 mm longer than the original one. After installing that hammer, it was time for another chrono session. I switched to JSB Express.


Mean V0 (m/s): 246.2
Energy (fpe): 11.4
Range (m/s): 5.4
Minimum (m/s): 243.4
Maximum (m/s): 248.8
# shots: 81

The number of shots increased spectacular, but the consistency was not really what I wanted. The shot-to-shot consistency however was excellent, but the velocity was increasing by the number of shots so I concluded this might be caused by the messing hammer breaking in and going smoother shot after shot. Indeed, the replacement hammer didn’t feel as smooth running a the original one when installing it.

So I decided to adapt the original hammer. I made a messing sleeve for it to make the hammer longer. I first made the hammer about 5 mm longer but this gave me only 220 m/s. So I took off about 1 mm from the sleeve which gave me 240 m/s. I took off a little bit more ending with a modified hammer that was 3.4 mm longer than original. With this I got the following chrono results.


Mean V0 (m/s): 243.7
Energy (fpe): 11.2
Range (m/s): 3.4
Minimum (m/s): 241.7
Maximum (m/s): 245.1
# shots: 98

Now we are talking ! 98 shots with a spread of 3.4 m/s, 95 of these shots were within 2.5 m/s and even 86 of the shots were within 2 m/s. This confirmed my impression that the messing hammer had been breaking in causing the velocity to increase from shot to shot.

I am happy with this result, the consistency is as good as I hope, and I get more shots than I hoped for. I took a few 0.1 mm of the sleeve, leaving about 3 mm extra length. The velocity went from 243-244 m/s to 248-249 m/s. I now reduced the velocity with the restrictor screw to 246-247 m/s giving me a little margin to up the speed in cold weather (velocity goes a few m/s down in winter with most guns I had, including S400 and EV2). I haven't tested the number of shots yet, but it seems the gun is perfect now.

Picture of the hammers:

 

This gun is pretty special to me.  It's a "old" secondhand S400 I bought years and years ago from BAR for little money for a friend, Roger  The guns had some cosmetic imperfections, mainly being the blueing having worn off and a little bit of rust, but that was no problem for Roger, certainly not since the gun turned out to be accurate as hell. So when Roger gave up shooting, I was more than happy to buy the gun from him.  Unlike Roger, I like to have my guns in perfect condition, even cosmetically ;-).  So I started with polishing the air tube to remove the rust and after installing the regulator, I also installed a barrel sleeve.  The gun is a real looker now, and I am considering giving it a custom stock.

October 21, 2010 by Mario Severi