Generally without high quality paint-to-barrel match or first strike rounds, Magnifying scopes are near useless. A red dot or reflexive sight is preferred, but a low power 2X scope would be the best if you really wanted a cross-hair scope.
Maximum range in paintball is largely considered 150ft or 50yards, nothing that you would need a magnifying scope for are you can easily see that far with your eyes alone. Beyond that, paintballs are not the most accurate things either so even with really good paint and a really good barrel, a crosshair scope will only be telling you where the paint "might" go. The only way I would recomend a crosshair scope or magnification is if you are using first strike rounds as they will be more accurate and give you more range
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You want something with a thicker shell than tournament grade paint, as all mechanical guns can be a bit rough with paintballs. Any local sporting goods store brands (besides monsterball) will work fine.
A regulator or expansion chamber firstly. Then a quality ($60) non flatline/Apex barrel is a good addition. You can go for a 20 inch, but only if you actually use a bi-pod and don't try to run and gun. Then a red dot or reflex sight helps to line up shots quickly. A butt-stock and remote line also helps make shouldering from prone easier. A magnifying Scope will only give you tunnel vision, not to mention being too accurate for paintballs, much less paintballs from an A5.
All said for, The A5 is not a great platform for a sniper rifle. Besides being a hammer valve with a physical disruption velocity adjuster, The cyclone feed takes a few shots sometimes to get a round in, which can be fatal for anyone looking for a one shot kill.