WaterWar.net

Assassin gametypes require small weapons that can be carried for to 1-2 weeks without detection vote for your favorite here

Not on the list, but I vote for the XP 20.

I have Duel 20"s a broken 10 and a 215 and while nice the 20 is nice it's to large for pocketing and the 10's grip gets in the way as well

I think this all depends on how large your pockets are, but I don't know if I'd choose any of these.

Squirt Gun - not enough output or range
SS 10 - not enough output or range
XP 15 - haven't used
XP 15 (2000 EDITION) - haven't used
XP 215 - maybe
water warriors squirt gun (better quality than normal ones) - still a squirt gun
Hydro Fury - haven't used
Micro Burst - piston guns aren't good for assassins when you have to be quick

My choices:

#1 Storm 500. This has the best stream lamination, and output is actually around the 1 oz/sec range. Opponents will know when they've been hit.
#2 Water Warriors Goblin. Slightly larger and less lamination than the above option. You also have to pump more frequently, but I don't expect to pump at all unless I'm taking out multiple targets. With that being said, the Goblin offers the greatest range for a gun its size and output is close to that of the Storm 500.
#3 Point Break. Basically an SS 25 on the inside with a vastly improved outer shell. Super comfortable and super durable, but not as powerful as the above options. Call me crazy, but I actually think the Point Break is a good (note, I didn't say great) blaster and is superior to all the guns on your list that I've used.
#4 SS 25. This would have been my go to back in the day since I've never had an XP 15 and didn't have a 215 until recently. The 25 seems vastly superior to the SS 20 despite being practically the same size. It's just a lot more durable, which isn't saying much since it still pales in comparison to all the above.
#5 XP 215 probably better than the 25, but I never tested them side by side so I wouldn't know. Other blasters are more comfortable to use, especially the Point Break.

Other thoughts:

The Micro Burst seems like it would be a good choice, but again, I don't know. The Max D 2000 is very powerful for its size and would be a good choice if you had larger pockets. I like the XP 20, but I still think I might go with a Storm 500 over it. I have always hated the XP 220. I've used about 6 and every single one seemed less powerful than the XP 20s I've used, and the design is much too cramped for my liking and wouldn't fit well into a pocket.