I used to have the same problem with the Moritat in my group. It can be hard to keep up while keeping to the spirit of the restrictions.

However, there are several options available throughout the game lines which allow the Moritat to kick ass.

From the Inquisitor's Handbook, consider:

-Bolo Knife - 2d5 damage instead of d10 increases the chance of scoring Righteous Fury. Comes with a built in Pen 2.

-Lathe Blade - a wonderful upgrade, adds +2 dam, +3 pen to any suitable melee weapon, and makes it withstand power fields.

-Composite Bow - silly as it sounds, it's an Accurate weapon, meaning it can deal a lot of damage. Maybe have someone build the character some Mono arrows to negate the Primitive quality (if it doesn't seem too silly for your tastes).

I see you already know the joys of a Tox Dispenser - truly an invaluable tool for an enterprising Moritat. Consider letting the character upgrade the toxin so that all Toughness rolls against it are made at a penalty (inspired by the overhaul Toxic weapons got in BC).

In Rogue Trader, you'll find two real gems. The first of them is the Groxwhip, a killer weapon dealing d10 +3 damage with Flexible and Tearing qualities - just slap a Mono upgrade on it and it'll last you a long time. The other one is an exotic weapon called Fractal Blade. It deals a measly d10+1 damage, but has an impressive Pen 7 and withstands power fields. Both can be found in RT corebook.

Then, there are the Anointed Blades in Demon Hunter, which range from meh to absolutely awesome, depending on how much your game focuses on battling the daemonic.

I don't know how the character (or his Inquisitor, if that matters) would feel about using Eldar weapons, but Shuriken pistols and catapults are really good ranged weapons. Then, there's the Harlequin's Kiss (also in RT corebook) which is one of the most devastating melee weapons in the whole 40k line. Very assassin-like, as well.

Forget about the Purgatus Crossbow, it sucks. From Ascension, consider the Neural Gauntlet, a truly horrible weapon capable of incapacitating living targets with one strike.

For non-gear upgrades, there's a cool talent in DW and BC called Flesh Render. It changes the Tearing quality on melee weapons, letting you roll two additional dice instead of one.

Morangias said:

I'll have to respectfully disagree here, as the Purgatus (and it's less-powerful IH cousin the Catechist) are decent weapons when engaging a psyker or daemonic entity from range.

Purgatus (50m range, 1d10+5 dmg, Pen 2, Reliable/Sanctified) is the preferred obviously, as they are always Good Q. and doing a single point of damage results in the stakes becoming lodged, dealing an extra 1d5+1 dmg ignoring Toughness/Armour AND the stakes force Daemonic Trait-creatures to make Warp Instability Tests (at a penalty) each round they're lodged/increases the Threshhold for any Psy Powers by 20. Rare? Sure. Slow? Kinda, depending on what talents you have. Expensive? Oh heck yes, but not the sort of weapon you use everyday. Useful for putting a hitch in that Sorcerer's attempts at warp dickery? Most definitely.

The Catechist is a similar weapon - basically the same really - but has less range (40m), less damage (d10+4) but better Pen (3), loses the Reliable trait (but gains the Tearing trait against psykers, not that a Moritat would really gain from that), and requires greater damage be done to lodge the bolts (must do at least 5 damage, removal dealing only 1d5, still ignoring Toughness/Armour). Still not a bad choice for the Moritat that may have to deal with a psyker on occasion.

In any case, they have their value, especially the Purgatus, and I can't really agree that 'they suck'.

Too situational for my tastes. Sure, there are moments when this is a complete game changer, but IMO it's not reason enough to carry around an otherwise very mediocre weapon.