I tried to reduce the regulator output pressure needed for 16 Joule / 12 ft/lb. The idea was to reduce the shot reaction by reducing the muzzle flip caused by excess air leaving the barrel. The LG400 already shows little shot reaction, is very dead to shoot. I find it markedly better than my LG300, LG110, P70 or EV2 but I thought there could be room for more improvement here.
I kindly got information from Sighter who made an extension chamber for the LG300 regulator to increase the muzzle energy for benchrest competitions under USA rules. This extension screws in between the both halves of the LG300 regulator. My friend Sidney did a lot of work to produce the extension.
The extension increases the air volume of the regulator by 9.3 cc (9.3 cm3). This extension more than doubles the volume of the Walther 16 J regulator which has a volume of 8.25 cc. The total volume of air inside the regulator can be changed in various when the extension is combined with the 7.5 Joule or the 16 Joule regulator.
7.5J regulator = 3.95 cc
16J regulator = 8.25 cc
7.5J regulator + Extension = 13.25 cc
16J regulator + Extension = 17.55 cc
Ideal for experiments whit regulator volume.
The Extension in combination with the 7.5J regulator on the LG400 made it possible to reduce the regulator output pressure by 5 Bar (from 85 to 80 bar) while maintaining 16 Joule muzzle energy. I found this somewhat disappointing. I had hoped to be able to reduce reg pressure by 10 Bar to a setting of 75.
I noticed no difference in shot reaction/recoil or shot cycle. But I did not really expect that with a reduction of only 5 Bar, and knowing the LG400 is already very quiet to shoot.
The next thing I thought of was that a large part of the air the volume in the regulator has to travel through a hole of 4mm diameter inside the regulator. This port restricts the flow and could easily be the reason for the low response to the increased volume.
So the next thing was to increase the size of this 4mm hole. I milled it to a slot of 4 x 12 mm.
I was of course very curious about the effect this would have. I thought it would do a lot to lower the output pressure. I mounted the new setup (80 bar) and shot it over the chronometer : 30 meter per second !!!
NO typo there, not 300, only 30 m/s. Seems the increased airflow through the large port hits the valve very hard and pushes it shut very quickly.
To overcome this low velocity I mounted a weaker valve return spring. This worked. Some fiddling on the hammer spring preload and the velocity was back up to 240 m/s. Good. But reducing the reg pressure did not work. Still needed 80 bar.
The other thing is: When testing the weak valve return spring in combination with the 16 J reg of 8.25cc it was also possible to get 240 m/s at 80 bar pressure. And the hammerspring preload could be reduced by 1mm.
My conclusion was that the reg extension that doubles the regulator volume did not do anything to reduce the reg output pressure but that the weaker valve return spring made a bigger difference.
The big deception came when testing this setup for accuracy at 50 meters: the accuracy was nothing like I was used to from the LG400. 5 shot groups at 50 meter of about 30mm. With the old setup (16J, 8.25cc, 85 bar reg and original Walther valve return spring) I got 20mm all the time, with best groups around 15mm (all in the sitting FT position).
I think that in the LG300 and in the Dominator the whole story may be different. These rifles have a whole different setup of hammer, hammer spring and valve. Maybe the Extension will do a better job there.
So I quickly mounted the original 85 bar 16 Joule regulator and original Walther valve spring and the accuracy returned to what I was used to. No more experiments with reg volume for the LG400 for me. I think I have found the ideal setup a long time ago.