I blame the avengers.

One of my PC wants to play a Hawkeye-like character. He's thinking of an arch-militant, armed with a high-tech bow and a vast arsenal of arrows. Ok, I'll let him try it, who knows, it may be fun. However, when we looked up what kind of monition there is for a bow, we only found explosive arrows…
So, I have to conceive his stuff myself. Here are my ideas; could you guys give me advice or tips?

-Bow: I'd start with the basic bow, upgrade its penetration value to 2, its range to 40m and remove the "primitive" quality. Then, I would add a fire selector, and a red dot laser sight, and give it the "accurate" quality. Based only on components, that makes a rare weapon, but the upgrades I gave push it into extremely rare territory.
-Arrows: I'd make available the base arrows (silent), + arrowheads that mimic each grenade with no or special damage. Ie: anti-plant, blind, hallucinogen, smoke, stun, web…+1 damaging arrowhead (not yet made my mind up on which, but loud). Those arrows would lose the zone quality, and there effect would only apply to the one hit.

I'd let my player start with the bow, and three types of special arrows he can choose, +the silent ones and the damaging ones. In exchange, he would not get his usual starting weapons, or his starting acquisition.
 

I like the idea; my suggestion for arrow types would be the following
 

• Inferno Arrows - No change to the damage, If hit the target my make an Agility test or catch on fire
• AP Arrows - Increased Pen to 3 or 4
• Snare Arrows - Does no damage but gives the bow the Snare Quality
• Poison Arrows - Give the bow the Toxic Quality
• Shocking Arrows – Gives the bow the Stun Quality
• Krak Arrows – as the grenade

Also maybe a possible upgrade would be for a stronger draw, the bow has a strength requirement which will then increase the damage and pen accordingly.
 

Perhaps consider the bow as a melee weapon, so that it's damage would be base damage + strenght modifier?

 I really like the idea of the SB modifying the damage of the bow. After all, the stronger you are, the more tension you can generate and the more force will be behind the arrow. However, I would cap it at some point (maybe at 5?) and require a trade armouer test/aquisition to increase the SB damage cap. Every bow has a max draw that, if exceeded, would simply snap it in two. 

 Even going beyond just giving arrows plain "Toxic," you could have a lot of fun with applying various chemicals, poisons, etc. with various effects to the arrows, and given the potentially limitless potential of discovering new applyable-liquids, it could make for a very fun constant side-quest for the PC. 

I would suggest making several bows with different strength requirements to use.

• Standard Bow: Stats unchanged, Strength required 30
• Next step up: +5m range, +1 damage, +1Pen, Strength required 40
• Next: +10m range from standard bow, +2 damage from standard, +2 Pen, Strength Required 50
• Final Bow: +15m Range from standard, +3 damage from standard, +3 Pen, with Strenght Required 60 (also this would be a large bow similar to a heavy weapon in size so maybe require it to be "Braced")

 A simple solution is to just let him have grenade arrows. Same stats as the grenades but with the range of the bow.

grappling hook arrow. maybe net arrow although i don't see how that would work

Provided that he got the training for it and was either able to purchase or loot one, perhaps the player might also want to consider getting the krootbow listed at the bottom of p. 146 in Rogue Trader: Into the Storm? 

I tested the bow this week-end. I finally décided the following profile:

group: base

range 40m

pén:2

Clipsize:1

reload 1/2

"accurate","reliable","composite" (home-made, meaning the player adds his strenght bonus to damage)

red-dot laser sight and fire selector

strenght needed : 50

It was… way to powerfull.

My player had 5 arrowtypes:

silent: 1d10 damage

explosive:2d10 damage (can't add his strenght bonus) (zone(3))

inferno: 1d10, target must pass agility test or catches fire

web: no damage, gains snare

E.M.P.: no damage, puts all electronic out for 1d5 turns (zone (1))

he had a BS of 53. Thanks to the fire selector, he could shoot 4 arrows before having to reload his bow,meaning he could aim four times before having to just reload, shoot, reload, shoot

When he aimed, he had a BS of: 53+10(aim)+10(accurate)+10(red-dot-lasersight)+10(arch-militant (he choose the base group))=93. Most of his shots made (3d10 + 5 (SB))damage… But we had great fun, so i'll let him have it. If any other Gm gives this bow to a PC, be warned: it hurts.

It sounds like accurate combined with the fire selector is the issue, I would not have allowed the fire selector. But, if you had fun with it then that works too.

Yeah, but once I started tracking what arrows he had in the fire selector, it was less of an issue. He would try to prepare for anything, so often he had only two damaging arrows ready, the other two beeing very often an E.M.P. and a snare. But as I sead, it was a great game.

I would change the fire selector to some sort of higher tech version of multiple quivers, making it so changing arrow type is full turn. maybe drop red dot unless its some sort of cross bow

As an arch-militant by rank 2 he will be reloading it as a free action if he can't already.  The only issue I see with the bow is its more versatile than a bolt pistol with a fire selector.  As a free action with rapid reload he can notch and fire any specific arrow type he is carrying to deal with any specific enemy.  The power of choice is a powerful one.

The only way to limit the power of the bow (without modding its stats down or those of the arrows) is to limit the number of arrows and type he can carry at any given time.  Since he wants a Hawkeye style character he needs the quiver.  One full quiver may carry 10-15 arrows.  Limit him to one quiver and have him detail which arrows he is carrying at the onset of the adventure.

Over the course of an extended mission finding additional arrows may prove difficult.

My thoughts

actually, given the complexity of the bow, my idea was that it's reload time could never be brought below 1/2… But I like the idea of the quiver. Seeing how this is the year 40k however, I think he will be allowed to carrie a big number of arrowheads (like 80 or something like that), but the shafts would be limited to 25-30.

 Of course then you can have fun with archeotech items like a nanoforge quiver-equivalent that creates arrowheads capable of doing whatever, so long as you keep it filled up with raw materials.