On the Shooting-the-Breeze.com and also on the Pimpmyairgun.com forums I got the advise to check the LG400 for point of impact (POI) shift due to temperature changes. Someone was convinced that the way I had done the 7,5 Joule to 16 Joule conversion would certainly lead to POI changes when the ambient temperature changed.
I do not think this is a real danger because the LG300 pressure regulator that I use is not temperature sensitive. Proven so and used by plenty shooters from all over the world on their field target rifle for over many years. And Walther solved the problem with the temperature sensitive firing valve over a decade ago in the LG300 Dominator. The LG400 has a flat firing valve and a hard black polymer valve seat. Both features reduce the chance of temperature sensitivity.
The adapter that I made to fit the LG300 regulator on the LG400 rifle is a short aluminum bushing that sits between two o-rings. It is floating between the o-rings and does not touch the breechblock directly. To me it seemed highly unlikely that this could cause temperature sensitivity.
To test the temperature sensitivity of the LG400 in the 16 Joule conversion I decided to put it in the freezer and to shoot it to see what would happen.
A temperature sensor was attached to the breechblock and the temperature of the rifle was 18.2 Celsius / 65 Fahrenheit (bottom number on the display of the thermometer). I put a paper target at 50 meters / 55 yards and shot a 10 shot group and recorded the velocity. It was a rainy evening and the wind was playing around from left and right so I had some horizontal spread in the group. But in the vertical plane the spreading was not to bad. The average velocity over 10 shots at 18.2 Celsius was 236.3 m/s with a extreme spread of 2.9m/s.
I put the whole rifle with scope and all in the freezer at -25 Celcius / -13 Fahrenheit and left it there until the temperature of the rifle had reached 1.5 Celsius / 35 Fahrenheit.
I took the rifle to the shooting range and tried to shoot it at the 50 meter paper target again. Unfortunately this did not work out. Because of heavy condensation on the lenses of the scope I was not able to see the target at all. I tried to wipe the lenses but within seconds the condensation was back.
So I just did 10 shots over the chrono while keeping an eye on the thermometer.
|Shot||Temp (C)||Temp (F)||Velocity (m/s)||Velocity (Fps)|
The temperature had already risen from 1.5 out of the freezer to 5.4 Celsius when I started shooting over the chronograph. During the shooting temperature increased to 7.3 Celsius at the tenth shot. There is no trending relation between temperature and velocity in this data.
Average velocity with the cold rifle was 235 m/s compared to 236 m/s with the rifle at an ambient 18 Celsius / 65 Fahrenheit.
After about 10 minutes the temperature of the scope had increased somewhat and I was able to see a very hazy image of the target for 20 seconds or so when I dried the front and rear lenses with a cloth. So I decided to try to shoot at the 50 meter target and clean the lenses before every shot. The rifle had a temperature of 7.8 Celsius at the first shot and 13.3 Celsius at the tenth shot.
The picture shows the 10 shot groups with the ambient temperature and cold rifle. Top left group is at ambient temp and bottom right is at a temp between 7.8 and 13.3 degrees Celsius. Both groups show quite some horizontal spread caused by the wind. The cold group is also not a nice one because I needed to clean both lenses of the scope before every shot and then quickly take a shot before the condensation was back.
But what is important for this temperature test is that the average center of both groups in the vertical and horizontal plane are almost the same.
Average velocity of Walther LG400 rifle at 16 Joule differs by -1.2 m/s (3.9 fps) when the temperature of the rifle drops from 18.2 degrees Celsius to 5.4 degrees Celsius (65 to 42 Fahrenheit). This 1.2 m/s difference is smaller than the normal velocity spread observed in field target rifles at a constant temperature.
A temperature drop from 18.2 to 7.8 degrees Celsius has no effect on the point of impact at 50 meters / 55 yards.
I think it is safe to say that the Walther LG400 in the 16 Joule conversion is not temperature sensitive for point of impact in the temperature range between 5.4 and 18.2 Celsius (42 and 65 Fahrenheit).