Under barrel air tank?

[Tom]

On the a5 you won't get the balance issue

I couldn't find it online in a quick search but there used to be a reverse asa mount for a5s by spec ops paintball
This still had the air mounted from the base of the grip but facing forwards

 

[Ricwin]

Spotted this handy looking device on Punishers, However it does look very bulky and may not be compatible with a shroud or magazine.

 

[Tom]

Bulk ought not to be an issue with that
As its for a different gun (but very similar to tippmanns) then you may need a little modding for the body and then again you may need modding / tweaking for your shroud

Costwise it may or may not be worth it for you as you get the benefit of a pre built item but can get air fittings cheaply and then work on mounting yourself
(Note - remember anything imported from the U.S. could suddenly go up in price with postage and any import charges - ask about other postage options, and if you add more items to justify postage then the package value goes up and you get hit by 20% import vat plus £8 to £20 handling fees

 

[Tom]

The reverse mount can be seen here:

You won't find one now, but this can be replicated with drop forward and reversing the asa

 

[Ricwin]

The pre made item to mount the tank below the barrel from Punishers looks like it might be too big to fit a magazine on as well. Theres an on/off switch to the side of it will will interfere with the mag well.
Costs can be a problem, however a few buddies of mien are all looking at ordering parts from Punishers for various markers. Postage and import duty/costs would be less if split between three or four of us.