It's just an opinion but by education I am a mechanical engineer, if it matters. I am also new to PCPs, I purchased my first one last year, a Prod (Benjamin Marauder Pistol). My second was purchased this year, a 25 caliber Avenger. I sent the Avenger in for a warranty repair for leaking but it is back and functioning well now.
I think the pressure difference in guns is a bit like asking why some cars have bigger gas tanks. It's a somewhat arbitrary choice of the designer. I suspect the Avenger designers wanted to have a reasonably high shot count on an air rifle that can shoot at 50+ fpe. Without a big tank. So they designed it to work at higher pressure. I would not assume that it is just how much risk Air Venturi is will to take versus Benjamin. I think it is better to assume there are real engineering reasons for the differences. There may be a difference in the factor of safety - the difference between the calculated or tested failure point and the working pressure - but that may also not even be different. No sane person would let you fill to 4000+ without a basis that the gun would not come apart until some much higher pressure - that is the factor of safety. A 3,000 gun should not fail until much above 3,000 too. But you don't want to just jack the fill pressure up and see what will happen. When it fails, there could be pieces of metal coming off at high velocity and it one comes in your direction it wouldn't be pretty. I fill my Avenger to 3500 psi not because I am worried it will come apart at 4350 but because I think the poppet may start leaking again and I would rather delay that. It isn't a poorly made or designed gun IMHO but it was made to a rather low price point, especially for the features. Mine shoots 1/4 inch groups at 33 yards with pellets it likes.
BAR versus psi is like metric versus imperial dimensions. They both work. But depending on where we live and what we grew up with we like one system instead of the other. If you remember that atmospheric pressure is a little over 14 psi, you can convert. 1 BAR is almost but not quite 1 times atmospheric pressure. So 300 BAR is almost 14x300.
Fill pressure on an unregulated gun is often best set lower than the working pressure. At the stock tune, filling my Prod to 3000 psi was asking for lower velocity. It needs the hammer spring pressure increased and possibly the transfer port diameter increased to use 3,000 psi. I shot a string to figure this out and then I knew to fill to 2500. It's just a matter of looking to see when the velocity gets up near maximum for the hammer spring setting and then filling to that point. With a regulator, you don't need to do this. The velocity is a function of the regulator setting and the hammer spring pressure. But mostly the regulator. The hammer spring is more of a fine tuning tool in this case. For example my Avenger is set to a regulator pressure of 2400 psi and I found that increasing the hammer spring tension 1.5 turns in did not increase my velocity at all versus 1 turn but it cut my group size about in half (from 1/2 inch or so at 33 yards to 1/4 inch or so). I only got about 5 fps more at 1 or 1.5 turns in than I did set to the minimum hammer spring setting. So mainly I set the regulator based on the velocity I want then vary the hammer spring to fine tune velocity but also looking at accuracy. If you just want maximum shot count and don't think it will hurt the poppet, there is no reason not to charge the Avenger to 300 BAR.
