I have seen questions to this topic asked quite frequently and there seems to be a great deal of confusion about it so I will explain as best I can so you can make an informed decision as to what size and pressure of bottle is most suitable to re-charge your PCP airgun.

For those new to PCP shooting, please be aware that bottles are available in different pressure ratings and different volume sizes. Generally, the common pressure ratings are 209bar, 232bar, and 300bar. Volumes of bottles are most commonly 3L, 7L, 9L, and 12L.

As a general rule of thumb, the higher the pressure and the larger the volume a bottle has, the more re-fills it will provide but that is not absolute, as a smaller, higher pressure bottle may give more re-fills than a low pressure, high volume bottle as we are about to find out.

Firstly, I would like to state that there are advertisements in the shooting press etc which claim that a 3L bottle will give X amount of fills, and that a 7L will give XX amounts of fills, and a 12L will give XXX and so on. This is not strictly true (and in my mind, a bit mis-leading) as the amount of fills a bottle will actually provide, depends on the fill pressure of your particular gun.

So here is the formula for working out how much usable air a bottle contains:

If your gun has a 190bar fill pressure and you are using a 300bar 3L bottle, then that bottle holds 110bar of usable pressure (300bar, minus the 190bar your gun requires = 110bar).

Multiply this by the size of the bottle, in this case 3L, so......110 x 3 = 330.

Lets look at another example:

A 232bar 12L. 232bar minus 190bar gun re-fill pressure, 232 - 190 = 42bar. Multiply that 42bar by the size of the bottle (12L) 42 x 12 = 504.

A 300bar 7L. 300bar - 190bar = 110. So multiply 110 x 7 = 770.

Lastly, a 300bar 12L. 300bar minus 190bar leaves 110 bar, multiply that by the capacity of the bottle (12L) gives us a figure of 1320.

In short, the higher the end figure, the more usable air the bottle will contain.

So by looking at the figures you can see a 7L 300bar bottle contains more usable air than a 12L 232bar bottle, so in certain instances, smaller sometimes is better.

Obviously these figures will change if your particular rifle has a different fill pressure (160bar, or 200bar for example) as it is all about the amount of usable air needed to fully charge Your rifle.

You could keep on re-filling your gun below it's fill pressure, but that would only give a partial charge, and hence a reduced number of SPC.

So by using the formula above you can work out which bottle is going to be most suitable for your particular needs regarding usable air.

I hope I have explained it so it is easily understandable, and of help to you.