Airgun Accuracy

Only accurate airguns are interesting!

Barrel Length and Velocity

I found an interesting post on an Polish forum about the effect of barrel length on pellet velocity. The original is here: Polish Forum

This guy Gruby experimented with barrels of three different lengths on his 16 Joule (12 fpe) Walther LG300 Hunter rifle. The Hunter has an 565 mm (22,2 inch)  barrel. With this barrel the rifle shot 239 m/s (784 fps). His Walther LG300 Hunter has a 16 Joule MK2 pressure regulator.

The Walther LG300 Hunter rifle

The Walther LG300 Hunter rifle (shown here with MK1 type regulator)

He then changed to a 425mm (16,7″) length barrel of a LG300 7,5 Joule rifle. The rifle then produced 229 m/s (751 fps). He upped the velocity by increasing the hammer spring preload with one turn to 240 m/s (787 fps).

He then installed the LG300 Dominator barrel that is 495mm (19,5″) and the rifle shot 251 m/s (824 fps). He decreased the hammer spring preload with 3/4 turn to get to 240 m/s (787 fps) again.

Then he installed the 565mm Hunter barrel again and got 247 m/s (810 fps). Turned the hammer spring back 1/4 turn and was back at the start with 240 m/s (784fps).

He also shot groups with the different barrels and shows pictures of them. But unfortunately only at 20 meters distance. I could not see any significant effect on accuracy from these different barrels.

This experiment shows that a 140mm shorter barrel (565 mm Hunter to 425 mm LG300) lost 10 m/s (32,8 fps) in velocity at the same setting of the preload on the hammerspring. Going to a 70mm longer barrel  (425 mm LG300 to 495 mm Dominator) increased the velocity by 11 m/s at a hammerspring preload that was 1 turn up. Going to an extra 70 mm (495 mm Dominator to 565 mm Hunter) increased the velocity by 7 m/s at the 1/4 turn up hammerspring setting.

For the process of converting a 7,5 Joule LG300 10 meter rifle to a 16 joule (12 fpe) field target rifle this means with that with its standard 425mm barrel it is no problem to get 16 Joule. It just means you have to increase the hammerspring preload by 1 turn clockwise.

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2 thoughts on “Barrel Length and Velocity

  1. This is an equally interesting read: http://www.airgun.co.uk/Airgun_Accurracy.html

    So wondering if your Walther LG400 10m barrel has a different twist rate, compromising the accuracy due to higher velocity ?

    • Some people have tried to measure the twist rate in airgun barrels and it seemed to be difficult to get good measurements. From the same barrel the measured twist rates differed as much as 2 inch. I do not know the actual twist rate of the LG400 barrel compared to the LG300 barrel. But Gert from South Africa has excellent accuracy at 50 meters from his two LG400 rifles with the original barrels. Mick Tromans also uses the original LG400 barrel as far as I know. So I doubt if the twist rate is the problem here. I would say for my rifle currently the high pressure of 112 bar behind the pellet might be the most likely cause. Gert is shooting his at around 85 Bar. So I will try the LG300 MK2 regulator on the LG400 en test for accuracy again.

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