Nielsen 20.2 slugs losing velocity

[Maizen]

Hello there. So I was tuning my rifle (FX IMPACT MK2) for 21 grain H&N slugs. Right around 930 FPS…but I noticed something strange. When I go to use Nielsen 20.2 grains, the velocity drops like 15-20 FPS. Why is this? Slugs not sealing well in the barrel?

 

[Scotay42]

Check diameter, they come in a few sizes.

 

[Motorhead]

Yup ... different diameters will have differing "In bore drag profiles"

 

[Moses888]

I have 25 cal m3 700 mm. Barrel shooting jsb II. And shooting 34.9 Nielsen slugs awesome. Haven't shot pellets in some time love them slugs

 

[Moses888]

here’s a shot string

 

[Maizen]

here’s a shot string

View attachment 340699

What’s your reg at?

 

[Moses888]

130 I tuned.it myself. Also have a m3 30 cal shooting 47.2 slugs awesome and back reg at 135. When I got it it was at 160 shooting at 945-960 now shooting at 975-983. Your tune your rifle with one slug that’s what I do speed is not everything. Accuracy it’s a lot better.