Please email us at techsales@opticsplanet.com regarding your question.
Unfortunately, Barska does not provide internal dimensions for this quad rail. I would recommend reaching out to them directly for more information.
https://www.barska.com/general-request
The Barska AR Quad Picatinny Rail will fit any rifle that utilizes a delta ring. It is a 2-piece free float handguard that locks into the upper receiver with the delta ring.
The holes will be the more rectangular options now offered by Barska.
No, this will not fit flush and will leave a gap between the handguard and the front sight.
It seems to use the existing delta ring assembly and barrel nut and looks to clamp around it, but Barska provides zero information on their website as to installation or any real specifications. I would highly recommend reaching out to them directly. Here is their contact information:
https://www.barska.com/contactus