I just had a look on the site and couldn't find their rules regarding cosplay props. I'm hoping to cosplay Yao Fei (Arrow) and/or Hawkeye (Avengers) in the future and don't know what to about a bow, I want to buy one because it will look much better but the lowest strength i've found is a 20 pound draw, which although is weak, is still a weapon. Of course my arrows will be fake. So my questions are

1) Is that allowed ?

2) If not, can I use a larger string to decrease strength or is it still a weapon?

I think your best bet is to contact the cosplay team for Expo directly and ask them. I am pretty sure there is a contact email for them on the Expo forums.

I went as Merida and Kate Bishop (Y!Avengers Hawkeye) last year and took bows that were strung with string. (8 I never had any problems. I think it says in the rules somewhere that you can't string it properly or use anything which would make it possible to fire a projectile, and if you want to, then you can only do so for photoshoots.

From what I remember when I asked is that if it is a real bow it must be unstrung.

Bear in mind, while you are part of an event you are still in the public eye. A real bow is classed as a real weapon so under the law if anything where to go wrong you would be arrested and charged for carrying a real weapon. Real bows can be dangerous if mishandled. So its better to have it unstrung or do what I did for my Hawkeye which was to make a prop bow. It still looked the part and was perfectly safe.

Of course, its best to check with the cosplay team directly.

I'll echo what's been said; if it's a real bow it must be unstrung, if it's a prop/fake bow you'll be alright. IT CANNOT be capable of firing something with any deal of force.

http://www.mcmcomiccon.com/london/wp-content/uploads/sites/2/2013/03/MCMWeaponPropsRules.pdf

It's pretty much all here. You would be carrying an offensive weapon if it were strung or not. You might get away with if it weren't strung.

It might be better to make/commission a con safe one than using an actual bow.

Thanks for all the help guys, i'll avoid the real one i was looking at and create one, cheers