I have just completed a lube-tune upgrade on a Gamo CFX which came to me with a broken spring.
I did the usual strip, clean, debur, hone, clean and lightly lubed the receiver with 70% Moly Paste.
The piston seal was undamaged and fitted well. The seals on either side of the rotary breech block were undamaged and in very good shape. I lubed the rotary seal contact area with a specially formulated silicone paste.
The rotary was put in place, the piston was lightly lubed with Moly Paste on all areas which would contact the receiver and was installed.
A gas ram pressurised to 700N was installed, followed by the trigger group upgraded with a GRTIII trigger.
The assembled gun was taken to the shooting range, shot in to bed everything down and the scope sighted in. The chrony was set up and then readings noted with different pellets.
At sea level this particular gas ram had been tested and had produced 890fps using JSB Exact 8.44gr 4.53mm pellets in a break-barrel Gamo Shadow. At 6000 feet altitude I expected the CFX to produce between 750 to 800fps due to air density loss.
Using H&N Field Target 8.49gr 4.50mm pellets I fired a 14 shot string which averaged 790.1fps - in the upper end of the expected muzzle velocity range. Very good!
I then fired a five shot string of JSB Exact 8.44gr 4.53 pellets. This string averaged 695.2fps.
Reverting back to the H&N pellets, the MV was again in the 790 bracket. Why was the MV low with the JSBs?On analysis, I concluded that it was a combination of the gas ram and pellet diameters of 4.50mm and 4.53mm.
Firstly, the gas ram at 700N pressure has an extremely quick lock time.
With the 4.50mm pellet, the pressured build-up in the compression chamber starts moving the pellet because of its relatively loose fit in the breech, and then gets it on its way quickly.
With the 4.53mm pellet, the barrel is effectively blocked while the pressure builds up in the compression chamber. The rear o-ring seal on the rotary poses less resistance than the pellet in the breech, so the pressure leaks past the 0-ring until the pellet gets on its way down the barrel. The small amount of pressure lost this way is enough to drop the MV by ±90fps when compared to the results achieved with the 4.50mm pellets. These effects are due to the design of the rotary breech block in the Gamo CFX.
The next day at the shooting range I conducted 3 tests using a break-barrel Hatsan Model 70 (Daisy/Winchester 1000).
Using the same pellets as on the day before and with 3 different gas rams, the same muzzle velocities (within a couple of fps) were achieved with the 4.50mm and 4.53mm pellets. The Hatsan has only the breech seal while the CFX has a breech block/barrel seal and a breech block/receiver o-ring seal: one seal versus 2 seals.
What do the acknowledged experts on the forum think of the forgoing?
