I received the Suppressor piston from Kel-Tec on Saturday 8/6/16. It looks exactly like my original piston with one major exception. The shaft was 0.248” and my original piston had a shaft diameter of 0.219”
I attempted to put the new suppressor piston in my gun and found the shaft was too large and it would not fit. Everything was effective the same except the shaft.
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I polished the rust preventive off the suppressor piston rings and that reduced the diameter by *0.001” so they were exactly the same diameter as my original piston and it still would not fit. So with an abundance of caution and a piece of emery cloth I started to carefully reduce the shaft diameter by hand. I took off 0.001” of an inch at a time and tried it and then took off another 0.001” and tried it, until an hour later I had taken off 0.003” and reached a shaft diameter of 0.245”
At this point it would go in the gas chamber if I tapped it very lightly with a small plastic gunsmith hammer. I had to tap it very lightly to take it out. By the way, before I started the fitting process, I scrubbed the interior of the gas piston chamber with the correct size of wire bore brushes and carbon solvent so it was very clean.
It will be interesting to see if this tight fitting piston will let in enough gas to lock back with the cap at bottom +0 and operate the action without a suppressor. I suspect it won’t, which means it probably won’t be over gassed with the suppressor. If it does lock back at a few clicks from bottom I may have enough gas adjustment left to have it operate correctly and lock back with the suppressor and suppressor piston installed. Ok, I know that is wishful thinking, but it sure would be nice not having to change pistons.
I cannot test the new piston with my suppressor yet since I am waiting on the government, anti-gun, bureaucrats to issue my tax stamp.* It has only been 4 months so I have a long wait still ahead. Once I get the suppressor I may have to take an additional 0.001” off the shaft but I will wait and test the various combinations first since I can’t but metal back on if I remove too much.
It runs flawlessly unsuppressed with the original piston at +12 clicks from bottom. I am shooting my custom load with a Hornady 150 Grain SST bullet over 46 grains of 4166 which burns very clean and is very temperature stable. It chronographs at the muzzle at 2738 fps in 34 degree weather and at 2735 fps in 95 degree weather. All shells were loaded at the same time and were cold soaked along with the gun in the winter or heat soaked with the gun in the summer to test the loads. Same magnetospeed chronograph was used. I used once fired commercial brass FL sized and trimmed to 2.000” since the FAL magazine is tight and after much testing I found that my RFB seems to like the slightly shorter overall length shell. Yours may be different.
I intend to test everything with both my supersonic load and with a sub-sonic load I will develop. I am hopeful that the subsonic suppressed load will work with my original piston since I have a lot of room to vent more gas and I will be using a low back pressure suppressor. I am sure that the suppressor piston will be required with my supersonic load when shooting suppressed.
All the testing and load development will have to wait until next year when my tax stamp comes in.
