if you are shooting say .30 caliber, i think even just tethered to a 3 L 300 BAR CF bottle, to deplete that bottle in one outing, you will have to carry so much lead, that you do NOT want to fall into a river or lake, CUZ you will be going strait to the bottom with all that lead in the pockets.
You dont need a inline regulator if your rifle are a lower pressure, but then you can of course not fill your big botle to a higher pressure CUZ the rifle will not take that.
I have a few times shot my old 200 BAR FX r ifle on my 300 BAR bottle, but not before i have shot its pressure down to 200 or lower using some of my 300 BAR rifles.
In general as i just shoot with a pressure of 100 BAR on the rifles regulator, well i shoot my big bottle down to 160-170 BAR or so before i fill it back up to 300 on my own Compressor.
Fair warning the carbon fiber bottles, many dive shops will NOT fill those if they have a little of the clear coat chipped, so you should probably not go full Rambo mode with a bottle like that on the belt / back.
And you do need a proper big boy compressor to fill the big bottles, at least from a fairly low pressure, my 4 cylinder compressor handle a 9 or 12 L bottle just fine, but one of the cheap 1 cylinder compressors will take a long long time and need many brakes meanwhile. but you can use those to top yup a big bottle if you only shot it down 50 - 100 BAR i think,,,,,, still slower than my 4 CYL compressor of course.