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PostPosted: Wed May 21, 2008 6:21 pm 
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Joined: Tue Oct 25, 2005 11:03 pm
Posts: 5330
Location: home of the Marshville Festival, Ontario, Canada
For those that had been following my experiments with tweaking an FWB 300S for use as a field target rifle - some further experiment results.

An internet acquaintance who had been following my experiments contacted me to ask if I would be interested in trying some synthetic seals, to see if that might help me to achieve the desired results, by virtue of elimination of any possible leakage at the metallic piston ring butt-joint. This very helpful person quite generously offered to supply me with the rings already machined to size - ready to install and test. Far too good an offer to pass up.

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The first material tried was UHMPE. The properties of this material were thought to be excellent for the application. Unfortunately, this proved not to be the case - all looked very promising at the outset, but soon ended in disappointment. After I fired a few heavier pellets to ensure that there would be no dieseling with the new seal, I switched over to the JSB Exact 8.44 gr. pellets. The chrony readings were showing promising velocities in the 739 to 767 FPS range. The rather broad variance in velocities was puzzling - I thought that it should be more consistant than the metallic ring. After around 130 shots with the 8.44 gr. pellets, it sounded almost as if the FWB 300S dieseled - a louder than normal crack, and an error reading on the chrony. After that, the velocities dropped right off to the high 300s/low 400s. The velocity kept dropping, so I disassembled the gun to see why. The detonation caused by the diesel had extruded the UHMPE right out past the ring groove, and a portion of the piston seal was now riding between the piston, and the compression chamber wall. I discussed the results with my distant friend, and he suggested we try a second, more durable material - a PTFE/bronze material.

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The results with the PTFE/Bronze piston ring were still short of what I was expecting. I am getting about the same velocities as what I got with the OEM metal piston ring - right around the 700 FPS mark. I am beginning to think that possibly that is all the energy that can be developed by the Merlin mainspring, and that a different mainspring may be what is needed next. (I am now thinking that the higher velocities that I saw when testing the UHMPE ring was the result of "mild" dieseling) The PTFE/bronze ring had been going strong, and consistantly for over 250 shots - unlike the UHMPE which blew out after only 130 shots, so I shot a 50 shot string using JSB Exact 4.51 pellets (8.44 grain) with these results:

Average velocity: 701.3 FPS

Average variation: 8.61 FPS

The average variation is the lowest that I have seen yet on any of the FWB 300S rifles I've worked on - they usually average around 10 FPS variation after a short break in period.

I have finally gotten close to 1000 pellets through the FWB 300S that is equipped with the PTFE/bronze piston ring, and it is still peforming well, no noticeable loss in velocity or consistancy. I spent spent some time setting up a 32X scope on it, and sighting in at targets that I had set up every 5 yards, from 10 yards, to 55 yards. The results were excellent, so, the experimental piston ring will see it's first FT match in about a month. Up until mounting the scope, I had just been shooting the rifle with the match sights, at 10 and 20 yards, just to put some pellets through it, both to break things in a bit, and to make sure that the PTFE/bronze seal would be trouble free. I think it is ready for competition.

I still have had no luck in finding a more powerful mainspring that will fit into the FWB 300S without the need to modify any parts, but I am still looking.

Stay tuned - LOL.


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PostPosted: Wed May 21, 2008 11:59 pm 
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Joined: Tue Nov 14, 2006 12:47 pm
Posts: 1726
Location: canada
In my best Artie Johnson voice, "Verrrrry eenterestink!" I'm sure you've checked but I find it hard to believe there's nothing in Maccari's inventory that would fit the bill?


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PostPosted: Thu May 22, 2008 4:41 pm 
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Joined: Tue Oct 25, 2005 11:03 pm
Posts: 5330
Location: home of the Marshville Festival, Ontario, Canada
windbag wrote:
In my best Artie Johnson voice, "Verrrrry eenterestink!" I'm sure you've checked but I find it hard to believe there's nothing in Maccari's inventory that would fit the bill?

The Maccari Merlin Custom mainspring is the closest spring that I could find that would fit without modification to any of the parts - and that is the spring that I have been using thus far. It seems that this spring, regardless of the piston ring or seal in use, maxes out at around 700 FPS. I had tried a couple other Maccari springs that had dimensions that looked close enough to try, but they didn't work out well in actual use, and ended up binding on either the ID or the OD.

A poster on another forum mentioned using a spring that he had gotten off eBay, designed to hot-rod the Slavia 631. He had to cut the spring back a bit, but states that he is getting 740 - 750 FPS with that spring, and the OEM piston ring. He had no other details on that particular spring. I would like to find something that would be more repeatable for results, rather than an eBay catch of the day.


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