so I am insterested in the dissapator type upper, I was thinking I should be able to convert my stock hbar 16" carbine A2 upper to dissapator profile.  

My plan: get new low-pro, under-the-handguards gas block.  replace front sight block with new gas block using current gas tube, install old front sight block at correct position in front to use full length handguards.  I believe I need some notchs in the barrel for to move the front sight block forward.

Please point out problems with my plans.

TJ

I'm not 100% sure but I think the diameter of the barrel from the front sight to the muzzle might be a slightly smaller. You might have a problem with the front sight being too loose to mount farther out on the barrel.

there is a couple of ways you can go on this.

first, if you want to use your original front sight block remember it is drilled for taper pins.  you can get around this by drilling for straight pins. also, as ridge said the barrel in front of the gas block area is a bit smaller. this may or may not cause alignment problems.

now the easy way to do this is to buy a front sight base that mounts using set screws instead of pins or to drill and tap the holes in your sight tower for set screws. all the front sight base is doing in this configuration is holding the handguard cap in place and the screws can do that just fine.
plus, with the added benefit of being able to adjust the alignment of the front sight tower you wont have to worry about excessive windage on the rear.

best of all, you wont have to worry about drilling pin notches in a perfectly good barrel.