I bought a AirForce TexanLSS 257 at the first of the year and performed what you are describing.
I purchased two cf bottles, I worried that the one I bought off ebay might not arrive, so I bought another one from Tony at Talontunes. Then I got a 25 lb hammer spring from Doug Noble. Doug informed me that to fill over 3800 risked breaking the valve stem.
i received both tanks, choose the Talontuhes 3600 psi tank as it was a few ounces lighter than the ebay 4500 psi tank, that and I dud not need 4500 psi. I removed the valve stem and valve return spring, reassembled the valve without the valve return spring and screwed the valve into the CF tank and filled to 3650 psi.
Then I decided to replace the hammer spring and ran into trouble. I had watched the African video and the Mr Hollowpointmolds video instructing how to replace that spring.
Both video’s warned about tipping the Texan, which was upside down with the trigger plate off, as it would cause parts of the trigger assembly to come out of alignment. Well I tipped it over and my wife and I worked for three days trying to get it assembled to no avail. I called AF and they would ONLY assist if I sent the rifle to them, they would not instruct me on the reassembly.
I put two help threads out on GTA and AGN, but only got one reply in the form of a picture.
Doug Noble, the god of all custom AF Rifles, read my thread and called me, chastused mr for not calling him as he is a old friend, or would that be a friend of a old guy? Anyway in 15 minutes with Doug on the phone, the wife and I got I it back together. I also converted it to a manual safety rather than the auto safety.
I also used the .25 inch Crayton bulbs to open the barrel lead to a tight “cork in the wine bottle fit” that I usually do with this sort of rifle to fully seat the bullet in the lead.
This next part will not help you much. I never shot the rifle prior to converting it. I get 1100 FPS with the PW maxed out and my cast 75 grain 257420’s. I only shoot three shots, they are very accurate and then I refill, I can get 2 or 3 more shots but I have not chronograph them.
Hope this helps,
Roachcreek
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