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PostPosted: Wed Mar 05, 2008 5:14 pm 
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Joined: Tue Oct 25, 2005 11:03 pm
Posts: 5330
Location: home of the Marshville Festival, Ontario, Canada
Yesterday I finally got a chance to spend some time doing some experimenting with the FWB 300S that I use for FT. After a tear down and clean up, all the parts were inspected for any signs of fatigue or excess wear from the heavier mainspring, and everything looked fine. The JM Merlin spring had a very tiny bit of cant, and a few shiny spots were noticeable on the OD of a few coils, but other than that, it could likely go another 5000 rounds with no problem - other than slightly reduced FPE.
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I made up a urethane piston seal that installed in place of the buffer, as an experiment to see if I could bump the power up to at least 750 FPS. The thickness of this seal - about 0.165" proud of the front of the piston, results in a negligible loss of volume - or so I thought. First hiccup noticed was that the ratcheting rack did not disengage once the sear had engaged the piston - the seal was holding the compression chamber forward just enough that the ratchet could not disengage. I removed the rack, allowing the gun to be cocked. This did not defeat the anti-beartrap mechanism - the safety sear was still fully functional. Next hiccup - the velocities were much lower than I had anticipated. I played around with the fit of the urethane seal in the cylinder, and the best velocities that I achieved were right around high 500s and low 600s - and that was with the piston seal at size-for-size with the bore. Hard to fathom that such a minute loss of volume could have such a profound effect on the velocity.
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One positive note in regards to the urethane seal - while the velocities were a bit of a disappointment, the consistancy was amazing - only about a 2 to 3 FPS variation over a 10 shot string. I tried it a couple more times with the same result - the urethane seal was giving better shot to shot consistancy than the OEM iron ring, which usually ranges from around 7 to 15 FPS, depending on the pellet. I may just try putting this seal in one of my match FWBs and see how it works out over time.

I returned the piston back to it's stock configuration with the iron piston ring, and the stock buffer. Installed one of the new JM Merlin mainsprings, and buttoned it back up. Performance is about on par with the previous spring, when freshly installed, with velocities in the 700+ FPS region.


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PostPosted: Wed Mar 05, 2008 5:47 pm 
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Joined: Tue Sep 06, 2005 4:54 pm
Posts: 1702
Location: Mississauga, Ont.
Interesting. Is it possible that the piston was slowed down enough by the parachute seal, that the pressure needed to achieve the 700 fps mark could not be obtained? If the speed of the piston is much higher with the steel ring in place, that might be where the reduction in velocity comes from. It could be taking the peak pressure point and moving it a bit down the barrel (synthetic seal) with the pellet starting on its move, rather than it being still in the breach, making the seal, with the faster steel ring seal. Also, if the speed is decreased, it will also increase the friction on the piston as it starts to come forward after rebounding from the initial compression. Without sensors and an oscilloscope, it is really hard to tell what is going on, but that would be my guess. By the way, what would the new buffer take up, as a percentage, of the swept volume of the chamber?

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PostPosted: Wed Mar 05, 2008 7:15 pm 
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Joined: Tue Oct 25, 2005 11:03 pm
Posts: 5330
Location: home of the Marshville Festival, Ontario, Canada
Keyrigger wrote:
Interesting. Is it possible that the piston was slowed down enough by the parachute seal, that the pressure needed to achieve the 700 fps mark could not be obtained? If the speed of the piston is much higher with the steel ring in place, that might be where the reduction in velocity comes from. It could be taking the peak pressure point and moving it a bit down the barrel (synthetic seal) with the pellet starting on its move, rather than it being still in the breach, making the seal, with the faster steel ring seal. Also, if the speed is decreased, it will also increase the friction on the piston as it starts to come forward after rebounding from the initial compression. Without sensors and an oscilloscope, it is really hard to tell what is going on, but that would be my guess. By the way, what would the new buffer take up, as a percentage, of the swept volume of the chamber?

Amazing how a different perspective can set one's train of thought back on course. I was so focused on the loss of volume being the cause of the reduced velocity, and how to get around that, that I completely missed the fact that the parachute seal would, without doubt, expand as approaching peak compression, becoming very snug, and, potentially slowing the piston. A new direction to explore.
Thanks, Keyrigger !!!!


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