I get your point.

Going with the deathwatch rules for a rail rifle (because i like to use the system for comparisons)
The rifle does 2D10+10 Pen 9. With overheat rule and devastating.
Overheat we all know, devastating is that it lowers morale of squads it hits.

That damage value is roughly on par with an astartes melta gun. Les pen, more raw damage.

It also had clip size 16.

So looking at a wiki (since the deathwatch book happily only provided stats, no fluff >:(   .) It is true it has over a hundred slugs in its system.
It also states it uses an entire power cell per shot.

So i guess one would look at it more like a clip is 16 power cells and no one ever bothers with reloading the weapon since 100 shots in a heavy sniper rifle is a lot.
I think you will change the barrel long before that. Meaning the weapon might never be reloaded since the tau deem the troops should have a replacement rail gun before it runs out of ammo.

I dunno, i guess you could use a rail rifle. I'd deem it as unreliable. So it has a higher overheat/jam chance.

Oh right.
Looking i see the needle rifle. That's an accurate sniper weapon. Imperial ok as well. Toxic goodness also.
Bit weedy on the damage though.

Looking through the rules, found the lacrymole.
About as powerful as a strong, tough and very agile human. It's fellowship stat is bullshit though at less than a human on an infiltration alien.

Other alternatives.
Found a subsonic armour piercing needle rifle variant.

Here's the nomad :P Hits as hard as a regular sniper with ap rounds as standard :P

[hider=Nomad]
Fykos Forge “Nomad” Hunting
Instrument
Class: Basic (SP)
Possibly the most famous weapon Gunmetal produces, the
Nomad well deserves its superlative reputation. The designs
for the instrument are said to be the work of Angevin’s
personal gunsmith, who in turn held them from an ancient
line that could be traced back to ancient times on Holy
Terra itself. Whatever the truth of that somewhat fanciful
accolade, the Fane of Fykos makes only perhaps ten Nomads
to commission every standard year, and each is in some way
different and personalised to the purchaser, be it the grinding
pattern of the trigger or the choice of material for the stock.
Such perfect workmanship combined with the capability to
empty a carnosaur’s brainpan at two miles, they say, makes it
worth every Throne.
[/hider]

Multilaser :P

The autocannon actually only weighs 55kg's :P
About the same as a regular ratling :D

There i found a weapon i want one day.
Rad cleanser >:D
And my beloved dragon scale power armour. I still want to play a character that has this :(

Anyway.
I know you want the flintlock/laslock musket :P
With scope of course ;)

Oh well. It seems i've gone through most.
Except one.

The galvanic rifle.
While it technically is fully legal, many mechanicum operatives will think "Wait, isn't that one of our guns?" and if some skitarii (as in mechanicum armed forces) see you with it, expect a thorough questioning.
Normally this one is a no no to get, but i guess with such a crime syndicate, it could perhaps be gotten hold of.

Probably as much of a hassle as the rail rifle though.

I dunno about other alternatives.