They don't really keep up with that right now. Astartes Armour is 8, 10 in the body, and they usually have a total TB soak of 8. Your hellgun dishes out 1d10+4 Pen 7, but gets reduced by 9 [11 body] or more.
After accounting for Dodge, Cover, Fields[the things that cause 'invulnerable' or cover saves vs that AP], you're hard-pressed to deal anything, and lucky to dish out 4 to a space marine when you do manage to do any damage. Its worse against Daemons, as they tend to be even higher toughness, with less armour to ignore. You could easily run out of ammo in that weedy "little" 10kg pack before you down a single Astartes.
It doesn't need much, but perhaps an overcharge setting granting Tearing and Felling would do the trick, without increasing how fast it vaporises normals?
Well the thing is, TT rules are what they are for balance reasons. In the fluff a Space Marine is a nigh-unkillable god of war, and a single squad can pacify a sparsely populated planet on their own. The RPG draws more from the fluff than from the TT, so game balance between armies is often flat out ignored.
Anyone remember a little article that came out in the White Dwarf called Movie Space Marines or something? They had like WS, BS, S, T and I 6 and 2 wounds each and their bolters were S6 and Rending. That's what a fluffy space marine is on the TT.
Regardless, a hellgun might not punch through a Space Marine power armor, but it does quite a number on human-sized ones.