I recently got one as part of the Fate of the Forge Star bundle and found, in common with other mantic products, it does not come with a build guide. Its not overly complicated to put together, but, just in case you're having trouble figuring yours out, here's my guide. This is going to be largely pictorial. As I said, the model isn't complicated, but sometimes it can be useful to see which way up things are supposed to go or which order its best to glue some parts together in.
The two halves of the barrels go together as seen above. Don't glue them together yet though - they need to be mounted on the main body of the gun first.
This side-view shows you the arrangement of the two shield pieces on the gun. I found it was best to glue the bottom shield to the body first, then stick the barrels on, then stick the top shield onto that piece. There's a hole in the top of the barrel section for the top shield to fit in to.
The gun barrel tips can be stuck on to the ends of the barrels once they are glued to the main body. There is only one correct alignment for them so, if they don't seem to fit right when you dry fit them, rotate them and try again.
Those ammo drums can be fitted on after the shields, but it might be easier to do it beforehand. Its not a model-breaker if you leave them till last though.
The pilot went on last on my gun. In retrospect it might have been better to glue his gun and holster on before putting him on the machine. Finally I stuck on the rear ammo drums and the tracks. The tracks also only go on one way.
From the Mantic forums, I hear some of these models turn up at people's houses pretty warped. I'm not sure which parts in particular tend to get bent, but if you get one like that its pretty easy to fix with hot water. You'll often see boiling water recommended. This might be necessary for thicker pieces, but I have used hot water straight from the tap to reshape restic in the past. The point is, a little bendage is really not that big a deal and very simple to rectify.
Feel free to ask any questions if you are building a Hailstorm cannon or indeed any Forge Father models. Later on I will be putting up some pictures of my Steel Warriors.