Hm, then some descriptions probably need to be revised (I need to check. The Sentinel should definitely warn against this gotcha).
A sniper rifle is considered a "heavy" weapon, which should mean a suit capable of only using heavy weapons should be able to use one (while it still makes sense that clumsy gauntlets wouldn't be suitable for precision weapons in the real world). If it means standard armor for the sniper, it's a good thing I haven't produced that many Sentinels before I got to use them (as well as before I tried to attack a base).