At the local FT shoot last Saturday, at around the mid point of the match, my FWB 300S began exibiting the most peculiar behavior. After having just cleaned a lane with a 6/6 score, I was completely without explanation for why I couldn't hit a thing on the next lane - 0/6. My mind was racing - out by a rotation on the elevation? No. What did I do wrong? I approached the next lane a bit unsure of what to expect, and shot another 6/6. Hmmm. Now I am really scratching my head. On to the next lane, and another 0/6 !!!!! This lane was out in the open a bit and I was able to see more clearly where a couple of the shots landed - the last of which was at least 6" low !!!! On the very last lane, I only managed 1/6, and noted that the last shot was 4" low at 27 yards. I assumed that the experimental PTFE/bronze piston seal had failed.
Upon arriving home, I ran a few shots over the chrony, and all shots read almost a full 100 FPS lower than they should have. Subsequent chrony tests revealed something even stranger. the velocity was actually intermittantly changing by just over 100 FPS every few shots. No rhyme nor reason to it - a few shots at around 700 FPS, then a few shots at 595 - 610 FPS. Then a really low one at 435 FPS, then back up to 700 FPS. Now I was really confused. Time to tear it down to see what I could find.
What I found upon opening it up was something that I'd have never expected - the buffer had blown out of the end of the piston !!! It seems that the lubricant that I had used in conjunction with my experiment with the PTFE/bronze piston ring had thinned out, and seeped in behind and around the buffer, and was blowing it out of it's seat - and depending upon the point at which it blew out, was the distance by which the stroke of the piston was shortened on that shot cycle. It appears that at least once, the buffer actually flipped front to back in the bore, as can be seen by the impression of the transfer port on the rear of the buffer!!!(pic 1) The face of the buffer shows the evidence of the buffer having rotated through more than 90 degrees from the transfer port impressions. (pic 2) The retaining ridge of the buffer has the leading edge torn and ragged from popping out, and then being forced back in, from shot to shot. (pic 3).
The lubricant used was a product called Superlube. It is advertised as ideal for synthetic to steel lubrication, and is good for high temperature use. It also contains PTFE. I thought it would be the perfect lubricant to use in conjunction with the experimental piston ring. I suspect that the Superlube thinned out with the heat of compression, and seeped behind the buffer, and due to it's properties, allowed the buffer to easily blow out as chamber pressure built up.
A good cleaning in varsol, a new buffer, and relubing with my usual moly grease/oil concoction, and the old 300 is back up and shooting like a champ.
