I keep hearing about 300 bar bottles for the M3. What's the deal? All CF bottles from FX have been rated for 310 bar, even the ones on the M3 that I can see. It's it just a different valve? I know there is a difference between MKII and M3 bottles, but that's a simple part upgrade.
You have to look at the bottle labels carefully, PW (pressure working) is the actual max working (fill) pressure of the bottle, usually rated at 15 degrees celsius (59F)
PSmax is the pressure the bottle will safely handle if it was filled to the max working pressure (PW) at 15C and then heated to the maximum rated service temp (usually 60-65C) since the air will expand and pressure will rise. This is the safety margin built into the bottle to handle temperature transients within the service temperature range ("TS" on the label.)
The 250 bar bottles may say PSmax 310 bar, but the PW250 line at the top means "pressure working" 250 bar.
The 300 bar bottles are rated for PSmax 382 bar and PW300 for 300 bar working pressure.
250 bar bottle label:
300 bar bottle label:
The 300 bar valves are also fully threaded, while the 250 bar valves are only threaded for about 1/2 their length.
Do not simply put a 300 bar fully threaded valve on a 250 bar (PW250) bottle and start filling it to 300 bar. While the 250 bar bottles will
technically hold 300 bar and be safe as they're rated to 310bar PSmax at the upper end of the 60-65C max service temp, doing so doesn't leave any safety margin if you fill the bottle to 300 bar in the house at room temp and then take it to a hotter location, say going outside and leaving it in the sun or in a hot parked car. The temperature and pressure will climb and quickly exceed the maximum rated pressure. Granted there's probably quite a bit of safety margin in that PSmax rating, but I wouldn't recommend tempting fate by repeatedly filling beyond the rated working pressure / PW. A failed pressure vessel at these kinds of working pressures isn't something you want to experience...