Seems to be kind of a "platform gun" that shoots a really long, really thin platform in a straight line to wherever you hit.
Should be possible, but it'd need a specialized model for the chain and would either create a million laggy static shapes or have about three
really stretched links in the chain over a hundred or so studs.
Quick mockup of what I think he wants -

The gun shoots a projectile
If it hits a person, it'll do damage but not much else - perhaps a 'pull' effect
If it hits a wall, it will create a very long 'chain' between where you fired it
You can walk on this chain, so can other people
If anybody shoots the chain, it breaks and falls apart