If I can take down scouts with the SMG on my AV scout fit, you're terrible to not kill them with an HMG

HMGs have extremely high RoF- even if they're moving, you're still going to kill them if you fire in their general direction.

As far as sneaking around, use common sense, scouts have to.

I go up against the same scouts you do, dumbshit.

If you end up seeing them, it's because luck wasn't on their side that time- there aren't that many places to hide and people can turn around at any moment.
If you can turn around after they land a blow, it's because your retardly-high eHP trumps their weapon

I've been scouting for nearly 2 years, and I can scout with the best of them. Scouts are by no means heavy hunters. They're dumbass hunters.

If you get killed by scouts often, it's because you like trying to run solo into areas with lots of dark corners and ledges.

A sentinel is a sentinel; shield tanked or not, they have significantly more eHP than any other infantry in the game.
It's also your fault that you chose a tanking type that isn't suited to you.

My main point is that scouts are VERY niche-based fighters. You can only expect them to counter a heavy if said heavy is in their niche, which isn't common enough to rely on scouts as heavy killers.

First of all, you stated that you're using a basic HMG- being able to kill standard relatively easily (also from unspecified opposition) is hardly any basic for saying heavies can be countered.
Also considering that so many scouts killed you in a single battle (and with so much variety) that probably means you were up against an organized squad, and plodding your sentinel around the wrong neighborhood to boot.

So, you're saying that on a map where the majority of the fighting was in the very niche of your so-called "counter".
With this (you worded it terribly, so I'm going to have to guess here), they only managed to kill you on occasion.

Shotguns are not a counter to sentinels because sentinels work well in areas where shotguns are worthless.
Ever see a sentinel going around in an LAV? Shotgun won't do **** against it, and they'll just jump out if AV starts firing.
Any remotely-open area as well doesn't work with shotguns.

[-Many counters to shield-based sentinels
This isn't about you in particular; most sentinels armor tank. Or at least the HMGers
-Sentinels ineffective on open maps
Sentinel open is different from scout open. Spine Crescent as an example. The immediate areas about A, E, and D are sentinel friendly, but too open for scouts. Occurrences in maps like this are pretty common.
-Scrambler rifles
Completely ineffective against your average armor-tanked sentinel, and the heat makes them unreliable against high eHP targets in general.
-Assault rifles
You said yourself it's difficult but possible. Difficult but possible isn't a counter. It's difficult but possible to destroy an HAV with a plasma cannon, but that sure as hell doesn't make a plasma cannon a good AV weapon.
-Combat rifles and rail rifles
HMGs can out-DPS combat rifles. Rail rifles are only really dependable when used as range, but it's very easy for a heavy just to step back into cover.

You're also talking to the wrong guy about "one-trick ponies". I'm one of Dust's few true jack-of-all-trades players, if there are even any besides me left after the legion announcement.
I have absolutely every weapon at advanced or higher
I have every scout at advanced or higher
I have every sentinel and commando aside from amarr
I have caldari and minmatar assaults and logis
I can use every type of equipment
I can pilot any vehicle with any type of turret
I've been playing for almost 2 years, so I've had plenty of time to get lots of experience with everything.

I believe you're misinterpreting the meaning of this thread.

I don't give 2 ***** about the balance system and what can be done about it- this is a thread on how to actively fight the current sentinels, not a nerf thread.
I'm trying to come up with the best possible "sentinel killer" fitting, and I believe I have it pretty close.
In skirmishes, I'm going with that swarm fit, but I'm not yet sure if I should use REs or proxies- I'm thinking REs, but proxies can also be manually detonated if I shoot them, as well.
In doms, I'm using a gallente commando with a breach AR and plasma cannon (the cannon, in the right hands, is a super-weapon in urban combat).