There has been a little confusion about what this handguard adapter does.  

1) The Tacamo Handguard adapter goes between the barrel and the gun on a paintball gun which has 98 barrel threads.
2) This adaptor allows the front barrel shroud for a US Army Alpha Black to be attached.
3) The small rail section on this adaptor is on the top side.
4) This adaptor was originally designed for the Tacamo magazine conversion kit for the Alpha Black.  The magazine conversion kit for the Alpha Black is the same as the kit for the Model 98, but with addition of this piece so that the Alpha Black front shroud can be used.
5) If you are looking for a way to rail mount attachments on the Alpha Black front shroud, you need this : Alpha Black Rail  , which will let you mount things like handles and grips.