Canadian Airgun Forum

Crosman 'F11' Nitro-Piston (495 FPS max CDN version)
Gas Ram: Crosman [BT5M22-00-1A] - OEM for CDN model (a.k.a. 'sissypiston')
Chrono: Franc Moody Ballistic Precision Chronograph (Muzzle approx 6" from entrance of chrono)
Tests performed indoors. Avgs based on at least 6 consecutive shots per pellet type.
CDN Limit of 495 FPS was not exceeded in any of these tests! Admittedly, no care was taken to honor this limit, I was trying for max possible FPS - but no laws were broken.
I'm an amateur scientist with no scientific training or knowledge (or eye protection for that matter) who's in the process of obtaining his PAL.


No Mods
Gas spring ('fat') end located toward front: <- [ (((((((((-------- ]

[ Ruger 'Superpoint' (Lead, Pointed), 17gr ]
- Avg FPS: 390
- Max FPS: 395

[ Crosman 'Premier' (Lead, Hollow Point), 14.3gr: ]

- Avg FPS: 441
- Max FPS: 452

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Gas spring ('fat') end located toward front: <- [ (((((((((-------- ]
Pre-compression of 4mm (3 dimes), placed between the Spring end of the gas ram and the anterior wall of the piston (X marks the shim): <- ( {X} [[[[[[[[[[--------- )

[ Ruger 'Superpoint' (Lead, Pointed), 17gr ]
- Avg FPS: 416
- Max FPS: 424

[ Crosman 'Premier' (Lead, Hollow Point), 14.3gr: ]

- Avg FPS: 465
- Max FPS: 474

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Rather than post my counts/details of my other shimming efforts, just a few notes on those experiments:

Tried 4mm, 5mm and 6mm of shimming in all the following combinations:

Spring ('fat') end located toward rear: <- [ ---------))))))))) ]
Spring ('fat') end located toward front: <- [ (((((((((-------- ]

Shim placed between the Spring end of the gas ram and the anterior wall of the piston (in 4mm, 5mm and 6mm shims): <- ( {X} [[[[[[[[[[--------- )
Shim placed between the Spring end of the gas ram and the rear chamber cap (in 4mm shim): <- [ ---------))))))))) {X} ]
Shim placed between the Spring end of the gas ram and the anterior wall of the piston (3mm) AS WELL AS between the piston end of the gas ram and the rear chamber cap (3mm): <- ( {X} [[[[[[[[[[--------- {X})

Neither increasing the shim beyond 4mm nor changing the location of the shim or reversing the position of the gas ram within the piston, served to improve the FPS with either pellet type/weight tested.


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