It'd better be real. It's the weapon of choice in the manuscript.

It's absolutely rea- the IDF have been working on it for a couple of years now...
http://technochitlins.com/mt-archives/000282.html

Like the AK-47 and the RPG, the Nazis came up with this in 1944-45. There is a picture of one in the special section on Nazi super-weapons in one of the Time-Life WWII books. I'll see if I can find it. I think it was developed in response to the sniper wars in Stalingrad.

One such German rifle is here:

http://collections.iwm.0rg.uk/server/show/ConWebDoc.988/outputRegister/lowhtml

You'll have to replace the "0" in "org" with "o" - for some reason the site is blocking the 0rg.uk sequence.

The one I remember in the TL book was a bolt-action sniper rifle, though.

It's real all right. Firearms designers have been working on this since the flintlock, maybe the matchlock.
The bugbear was always an aiming system. Building a gun with an offset or swivel barrel is one thing a set of sights simple enough to use while robust enough to last is something else. This had to wait for the revolution in electronics. The old attempts using a sort of periscope were unsatisfactory.
They've solved one problem by designing this rig as a platform fitting already existing autoloading pistols, that of the recoil of the firearm not being in line with the stock. In common handgun cartridges the recoil would be negligable in a platform as heavy as this one looks to be. I am very interested in how the variants in 37mm and 40mm work in that regard.
I suspect that SWAT Teams all over the country are clamoring for them and not just for the shooting capability but for the video. With this platform the SWAT Commanders can see what the guy on point sees. This adds a LOT of certainty to the shoot/don't shoot decision. Add the capability to record what the shooter and commander sees. This will not only be a great training aid but has the potential to cut way down on media/politico second guessing and Monday morning quarterbacking.
This also has the potential of cutting down the number of suspects police have to kill. We don't take shots to wound armed suspects because of the danger to the Officers. If, however, the Officers are behind solid cover then trying to shoot a leg out from under a suspect becomes an acceptable risk under far more circumstances.
One more thing...as this technology matures there will be the chance to end far more barricaded suspect incidents without shooting. Imagine the day when the negotiater can tell the suspect 'turn on your TV' and the suspect can see the crosshairs right over his gonads. Think he'll surrender?