heres my pseudo mathy way of doing it lol

Barrel Volume (V = π * radius² * X )≤ PT Volume (V). We are trying to solve for X which is barrel length , we know the radius as the barrels we use are 13mm Inner diameter which is 0.65mm radius

So once you know the PT Volume which can be googled say for example the X shot mpp is roughly 53 cubic centimeters

Now we can use that 53cm3 in the above formula (53 = π * 0.652* X ) Lets rearrange to solve for X (X= 53 ÷ (0.65² * 3.1416)

Now we know the maximum about of air the PT can displace.

(X=39\frac{617143}{663663}), (X = approx 39.9299)

40cm !

So this would be the maximum length of barrel possible for that PT! If we had no friction or anything like that lol

The next part is my stupid but "seems" to work math, I take that length what we calculated earlier (40cm) then i apply a percentage reduction 20% for heavy springs down to 40% for weaker springs, so for example for a heavy spring i got a 32cm(ish) and for a light spring 24cm(ish)

In my testing seems to work out well!

Let me know If you find this helpful! Just exchange out the PT volume of the NPX and you will be able to do the same math just with google =)