Alright, had a chance to also test an HPR with Bipod/Railscope
It's garbage.
RoF is too low, accuracy is too low; damage in turn is too low. You can scare off a single xeno assuming you've got a lane like a 2-tile hallway, but you'll never kill it. Felt like I should've just kept my M41 with AP out instead. Couldn't even shoot in a straight line on single shot while scoped in, bipod deployed - bullets veered off by about the 7-10th tile of travel. Burst fire it was even worse. Needs slightly higher RoF, significantly better accuracy on single shot. 3/10
To re-iterate my experiences with other setups:
HPR + QFA/AG: Fires juuust barely fast enough to be usable. This setup is basically like an M41 + RDS/AG/BC. Kept it on single shot unless I needed to PB burst something. Accuracy wasn't amazing, but it's good enough for the distance you shoot at. Not an amazing setup, it's simply passable, and the only setup I've found usable while letting you move like a regular marine. Agrip COULD be replaced with Vgrip, but I found quicker wield time better than accuracy. Needs slightly higher RoF to be good. 6/10
HPR + RDS/VG/Suppressor: Fires too slow, lacks that oomf sound that rallies marines around you, but lets you FF more easily. Burst fire cooldown is too long to be worth using except PB (a problem in general with the HPR). Burst fire itself is obscenely inaccurate, so there's little reason to use it except PB. Unless it's HvH I wouldn't bother with a suppressor. Needs a tighter burst fire spread and higher RoF. Easily the worst setup I've tried. 2/10
HPR + QFA/Bipod: The only setup that's actually good, instead of passable or straight up awful. You can actually lock down a hallway (until you burn through your ammo), and if xenos overextend themselves, you'll actually kill them. This is honestly what the M56D should've been like. It's difficult to use on-the-move, requires some setup time, and gets hard-countered by gas, so it's really only useful defensively. Also it has some sort of weird spinup time, I'm not sure if it's just a byond thing or not, but you can't fire quickly singleshot for the first few bullets, so it takes time to actually get up to full speed. Worth mentioning that the amount of bullets put downrange by this setup is a massive morale booster. I fucking love it. 9/10 setup in its niche defensive uses, 5/10 otherwise.
HPR: With no attachments at all? Garbage. RoF is too low, full stop. 1/10
General impression I get from the HPR is that by default its RoF is too low. Most attachment options would be viable if the HPR just had slightly faster RoF to begin with.
The only exception is the Bipod/QFA setup. This doesn't need any faster RoF than it already has.