It does limit your movement which is why you need special training to even move around in it.
T-45d maybe, not T-51b, no reduction in agility at all.
Edgar Hardin:
"Not even close. Power armor isn't something you wear so much as use. It's a weapon in its own right, and takes skill to wield. If you don't know what you're doing, you won't even be able to walk with it on. So how about a bit of quick instruction? ...There, I think you've got the hang of it now. You should be able to use any type of power armor you come across using what you just learned."
Think less 'slows you down if you wear it' and more 'no workie unless you know what you're doing, at which point you have full mobility'
Like I stated before, it's because as everyone says in the game they outnumbered the Brotherhood 15-20 to one. No one referenced using hunting rifles to plink them into oblivion. Furthermore I never made the statement that "power armor is so invulnerable to even large AP rifle rounds." That's just yet another Strawman. One of the two you made in two sentences, which is impressive.
And how'd they kill them even if they outnumbered them 100-1 if they were so invulnerable? 5.56 is even below the 2500 joules figure lmao. The NCR does have some heavy weapons but they have a lot more .50's than they have missiles and energy weapons.
The intro cutscene itself shows them using them! XD
The Gauss Rifle is still significantly more powerful than the Bozar, faster to fire in using less AP (obviously the Bozar has burst fire) and longer ranged. In game and according to the lore the Gauss Rifle is meant to be a far superior weapon meant specifically to penetrate armor. Creating a super advanced, expensive weapon with ultra rare ammunition using fancy electromagnetic engineering and coils propelling rounds to hypervelocities just for "More dakka" instead of an actual purpose like armor penetration when as you argue, its almost equivalent to a .50BMG round and barely better than a common hunting rifle just seems like suspect logic and not terribly convincing. Are you going to invoke the early games 'Bloody Mess' perks next as a good faith argument since you are bringing up BB guns now?
You get gibs in the classic games without bloody mess jej. You HAVE played them right?
Gauss Rifles seem a lot lighter and less recoil than the .50 rifles, so its not like they're inferior to the .50. Not to mention ammo weight, you can carry a LOT more 2mm EC than .50.
The existence of the T-51 Power Armor was decisive in shifting the Great War on every front. But there's no way it could've been decisive if every machine gun, hunting rifle and sniper rifle could reliably kill them even if "its not the best way to take out power armor." We've seen that the Gauss Rifle is clearly superior to other comparable weapons not just in damage output, but crit chance, speed, and even in range almost fairly consistently across several games. We've seen that that Power Armor is nigh invulnerable to small arms in general, not just everything until the magical .308 rifle from "bubba's hot loads" shows up. In the first two Fallout games we see Paladin's taking on literal swarms of raiders armed with all sorts of small arms. Power Armor got nerfed in subsequent in game mechanics sadly but the lore has repeatedly illustrated their resiliency. If Bubba with his hot loads "WORKS" as you state, then there is literally no reason for Gauss Rifles to exist and very limited reason for use of Power Armor in combat, at least on the front line since it has armor comparable to a lightly armored Humvee, only taller.
Just because .308 AP rounds can penetrate power armor with favorable conditions wouldn't make them THAT vulnerable. There aren't even any canonical Chinese weapons above 5.56 aside from the Gauss rifle and questionably .308 variant of the sniper rifle.
Even assuming they did have a GPMG, AP rounds for such weapons aren't especially common even in real life.
Again even taking the minimal estimation of exactly 2500 joules you need to be within 500 yards with a 90 degree hit.
Add angles to the mix, and layers of armor beneath the composite plates and you quickly end up with .308 needing near point blank shots 90 degrees with exotic ammo.
None the less the plates canonically only handle 2500 joules. If we up the armor to handle .50BMG, where's the extra 16000 joules of protection coming from?