After trying out the Sentinel for a bit, I must say, it feels a little... Underwhelming. I struggle to picture any situation where I'd want to pick it over any other sniper, given a choice.
TL;DR version:
The gun has a terrible fire rate, doesn't have the alpha damage to make up for it, and the gimmick is too clunky and costly for what little it does. Worst sniper in every single situation except one; Triple Take at long range without choke. Needs an overhaul.
The issues:
Very low rate of fire combined with only medium damage to show for it.
Only deals 65 (130 HS) damage when a Longbow, which fires nearly twice as fast, and deals 55 (118 HS). This means against legends with a normal healthpool and purple shield, both guns take 4 bodyshots to knock, since 3 Sentinel shots only deals 195 damage. In fact, the fire rate is so slow, it's roughly the same as a Triple Take firing off fully choked shots, which deals 69 (138 HS) damage, which actually DOES hit that 3-hit threshold that the Sentinel falls just short of. Without its gimmick, it falls short in both alpha damage and fire rate compared to all other snipers. Speaking of gimmick...
The gimmick is real weak for the cost of a whole battery.
For a shield battery, one of the most valuable items in the game to stock up on, you gain the ability to break any shield in a single shot! Sounds impressive? Well... Lets look into it a bit more. You get UP TO 6 shots, but since the charge is constantly draining and you're likely to take some time to actually aim the shots, you're more likely only going to get 3-5 charged shots from a battery. Provided they don't just hide completely after the first shot. And since the rifle already deals 65 damage, you generally only get 35 extra damage, assuming they have a full HP purple shield. Worst part? If you score a headshot with a charged shot, you deal ZERO extra damage. A headshot will deal 130 damage regardless if the shot is charged or not, effectively making the charge completely wasted. It also takes about 5 seconds to even charge it up to begin with!
So, you spend 5 seconds to use a very valuable consumable to get 1-6 shots that deals 35 extra damage on average. Only time it might actually useful is if you're repeatedly shooting a Gibraltar with zero cover in the legs.
Solutions..?
This thing needs to go back to the drawing board in terms of balance, unless it's meant to be the Mozambique of snipers. Give it at least 70 (140 HS) damage so it can 3-shot normal health legends, and nerf Gibraltar's gunshield to 70 HP to match. One shot should knock that gunshield out.
Make the charge mechanic use a shield CELL instead of a battery, giving you a single charged bullet that still break any level of armor in a single shot, and also increases the headshot multiplier to 2.5x for a total of 175 headshot damage. This is to give headshots SOME kind of bonus with charged shots, because again, right now they do NOTHING extra. The charge shouldn't last forever (even though it probably could with only one round), but it should still last for a decent while, 15-20 seconds perhaps? Enough time to line up that perfect shot after you've spent the 5 seconds or so charging it.
Keep in mind, helmets exist, so it wouldn't be busted against someone kitted out. In fact, with 70 damage and a 2.5x headshot multiplier for a charged shot, it'd deal a heavily reduced 111.5 against a Gibraltar/Caustic with a purple helmet, and the second uncharged shot with 70 dmg at the regular 2x multi would deal 89 damage, meaning the total damage would be 200-201 (depending on how rounding works in Apex). And if that's not perfect, I don't know what is.