Sizing Pellets Affect Velocity?

[Guest]

A few days ago, I installed a Tony Leach 21 mm sleeve kit in my TX200. 100 shots later, I decided it was time to check velocity. I used 8.44 grain 4.53 JSB's that were run through my 4.52 sizer. Shot 10 shot string through my Shooting Chrony F1. Shots averaged 690. According to Tony, setup should have been around 750-770 fps. I decided to try and shoot the JSB's straight from the tin. 10 shots averaged 770 fps. The pellets I ran through the sizer had slightly bent skirts. Any idea what may be going on with the sized pellets? I also tried using the 4.54 die with the same results as the 4.52 die.

 

[dizzums]

no idea .. pellets size themselves to a barrel when theyre pushed in .. if your undersizing them i could see maybe some of the air charge escaping past them .. 

 

[OldSpook]

A few days ago, I installed a Tony Leach 21 mm sleeve kit in my TX200. 100 shots later, I decided it was time to check velocity. I used 8.44 grain 4.53 JSB's that were run through my 4.52 sizer. Shot 10 shot string through my Shooting Chrony F1. Shots averaged 690. According to Tony, setup should have been around 750-770 fps. I decided to try and shoot the JSB's straight from the tin. 10 shots averaged 770 fps. The pellets I ran through the sizer had slightly bent skirts. Any idea what may be going on with the sized pellets? I also tried using the 4.54 die with the same results as the 4.52 die.

The firing cycle on most spring rifles compresses the air substantially before the pellet begins to move. Before the pellet can move it must be sized to the bore. This requires energy AND time. The time part of that equation allows further compression and more importantly HEATING of the air between the pellet and the piston head. That heating causes a spike in pressure. If you size a pellet smaller it will take less energy to force it into the bore AND less time. That reduces the instantaneous pressure at the moment the pellet begins to move down the bore. That will result in lower muzzle velocities.

Well, that's the theory anyway.

 

[nervoustrig]

If you are feeding the pellets completely through a sizer, you are compressing the skirts. From the tin, the skirts are oversized by a few thousandths to help ensure a proper seal with the bore. Making them smaller may permit air to escape by before the skit obturates (expands) and seals to the bore, whereby reducing velocity.

 

[AirShot]

If you are feeding the pellets completely through a sizer, you are compressing the skirts. From the tin, the skirts are oversized by a few thousandths to help ensure a proper seal with the bore. Making them smaller may permit air to escape by before the skit obturates (expands) and seals to the bore, whereby reducing velocity.

Yep....sounds about right!!