"Advance on the Mechanicus forc-CLANG-WHAT THE FUCK WAS THAT?!"
The Transuranic Arquebus is a type of weapon used by Adeptus Mechanicus Skitarii Rangers. Like the Flechette Blaster, it occupies its own niche as it is neither a Stubber (Ammunition and quality material is beyond any known Stubber, that and it hunts down tanks) nor is it an Autogun (Again the gun is too specialized in firing only one type of ammunition whereas most Autoguns are more flexible in that regard). Known for their range, precision, and efficiency, these weapons fire a shell of depleted transuranium. That sounds really fancy but GW fails again, transuranium elements are just those elements with an atomic number above 92. These bullets made out of hyper dense and atomically unstable elements allows it to puncture a tank from one side to the other, the resultant pressure wave also pulping any biological creatures inside. This is of course fucking stupid, DU projectiles in reality are touted for armor piercing capability (thanks to aforementioned density as well as self-sharpening capability) and additionally a special property known as pyrophoricity. Put simply: when a DU penetrator hits the armor of the target the tip of the projectile begins to vaporize under the heat generated from the impact, so when the projectile actually breaches the fighting compartment a tiny moment later the now-vaporized metal sets everyone on fire! "Transuranic" just means "atomic number above 92", nobody said it was limited to the modern Periodic Table. Or even to natural elements. This could be particularly fun, however, and not quite as fail as it first seems. The island of stability theory states that some stupidly heavy elements should be possible, and stable (compared to some insane elements we've made in the real world). This is very good, because in the wh:40k universe ammunition tends to be stored in large amounts for long durations. You don't want to open up an ammunition crypt and find out your ammunition has decayed... Or gone critical.
Given it's similar look to the Galvanic Rifle, it is likely powered in a similar fashion, making it the Imperial Railgun.
7th edition crunch
On the tabletop, it is an AP3 ARMORBANE sniper rifle. Keep in mind, Armorbane means you roll an additional D6 to determine penetration. Sniper weapons are S4 against vehicles by default, so a roll of two 5s means you're glancing Baneblades from ANY side. However, this does require a fair bit of luck to achieve, so Arc weapons are more reliable for bringing down Monoliths or Hammerheads. Transports (and whatever's in them), on the other hand, will not be so hard to crack. There is literally no vehicle this anti-tank rifle is incapable of theoretically making go boom pretty much across the board.
It has a ludicrous 60" range, so with enough of them you can bust open that METAL BAWKS and force those Mary Sues to foot slog their way through your fire lanes the rest of the game. Not to mention, anything that uses this gun has Relentless, so just walk sideways and get yourself a nice peek at some side armor if you're that worried. Two units of Rangers with 2 each camping both corners of your side should be able to ensure that at least one of them catches sight of side armor. Keep in mind that the average result of 2d6 is 7 (which will glance AV11 and penetrate AV10). So while it is possible to penetrate AV14, you are better off sticking to targeting transports to get the most out of this gun.
It also works beautifully with a Scryerskull Perspicatus on an allied Tech-Priest Dominus. Light up an enemy vehicle, re-roll armour penetration against it with your Arquebi. If taken in a Dominus Maniple, you can re-roll to-hit as well.
Unfortunately despite the benefits, it perhaps has the least synergy with your Skitarii. Straight up do not take it on Vanguard. Rangers are better since the non-specialists also have Precision Shots. However, you are better off taking them in minimal squads with two Transuranic Arquebuses. Having 3 in a unit sounds enticing, but keep in mind that requires having 7 Rangers sitting around doing nothing but wasting points.