I'm not coming from weapons firing experience, I'm coming from gaming and game design experience. Weapons that have a metallic sound in the games I've played sound heavier to me, as if the mechanism to move the ammo can be heard over the explosion it makes the gun seem just as powerful as the shot it fires and the added bass allows the player to feel the shot as much as hear it. The other way to go is the way of sounding like close by thunder for more energy based weapons.
As far as stun lock, the trick would be to blend movement with the hit animation (based on where the shot lands of course) then follow up with AI for the unit to find cover using a damaged state movement (like a desperate dash or a lifeless dive for cover). Its important that the timing between shots and the movement for a character to evade is well spaced to ensure pacing. If the weapon is meant to feel strong, then characters and environment should react appropriately to the weapons power. The numbers offered above seem useful if those are the animations you want to use. I've always hated the flinch animation most games use when characters get shot. When I unload a light automatic into an enemy and it does two or three flinch animations then a death animation, it drives me bonkers. I like to see characters(who still have an intact brain) think about the damage I just did to them and move appropriately to the pain and suffering I've cause by using such a sloppy weapon. Seems like every enemy in every game out there is morphed up and tweaking on adrenaline, its boring. I want to see the effects of my weapons and the only time I don't want to see it is when I've hit the mark. If I've done my job, the enemy crumples.
With particle work, all that matters is that the player thinks it looks right. This will all depend on the weapons design. A couple games that really sell the "feel" of powerful weapons are Gears of War(obviously) and Warhammer Space Marine IMO. As for heat warble, smoke and the likes, each gun should be just as much a character in the game as the NPCs and enemies the player faces. Think about each gun and its history, give it meaningful interesting and useful quirks, even with the most standard weapons, it'll be worth it.
As for knockback it depends on what the weapon's ammo(type) is doing against the type of surface its hitting. Obviously penetration is only going to create knockback if the character becomes mindful of the shot, possibly falling back after realizing they are beaten. But a few successive rifle shots against an armored target should push them back (especially if the shots are high). Even grazes and shots that connect with the shoulders should cause characters to reel back to A)create a smaller silhouette to land a second shot on and B)to inform the player of where the shot landed. This is all just visual feedback. The character could flash red if you want but in the end the player needs to know they've connected their shot with the target and a knockback allows the pacing to change. As the player checks whether the shot has finished the job.
Key word in video games in my opinion is video. Successive images to depict movement. The more you explore movement and its meaning in your game the more you're exploring the element that separates video games from any other number crunching card, board, dice games, etc.