One thing to remember: the skill, in Dark Heresy at least, is "Silent Move", not "sneak". This is not some generic "hide in plain sight" skill, that gets bonuses if you have concealment. It is the ability to move without generating too much sound. If the enemy is looking in your direction and there is no cover or concealment you do not get to use the skill. As people have said, reliably getting to within 3 metres of someone is unlikely, unless in perfect circumstances (heavy cover to get close by behind the guy, and then hope he doesn't turn round while sneaking the last stretch... oh, and none of his mates do either). Close environments present more cover, but also limit approaches more. Someone properly wanting to protect something is going to make sure the approaches are limited, so they can be easily covered, or that there is a lot of empty ground to cover.
Also, remember DH doesn't have many hard and fast rules vis-a-vis modifiers, combat exceptions aside. Armour and lots of gear doesn't give an automatic penalty to Silent Move, for example, but probably should do in most circumstances. Bits of gear bang into each other, you have less flexibility etc. This is entirely within the purview of the GM.
As another note, as it is "Silent Move", just because you fail does not mean the enemy realise you are sneaking to attack them. If they cannot see you (because you are behind a wall, for example) they will just hear something. This will likely put them on alert, or they might investigate the sound, but only if they have some reason (or they have caught the sneak-er in an exposed position, such as sneaking from position to position, or they have turned round and found him behind them) will they necessarily instantly presume they are under attack.
I would say one way to help regarding full-auto: change it to the pre-errata status of it being a hit for every 2 DoS like Semi-Auto. Apparently this was a mistake in the original printing by Black industries, but it would certainly help with the sheer number of hits, and further emphasise the waste of ammo.