Does this make sense ...? We don't know, but it's not for us to decide. The Gripknife company offers the users of firearms vertical grips mounted on picatinny rails that hide... a knife inside.

The Gripknife can be ordered for right or left handed users. Below is some of the specification of the flagship Patriot model:

Action: Magnetic Assist ™ Spring Assist
Overall Length: 8.10"
Blade Type: Spear Point, Single-Edged
Blade Length: 4.00"
Blade Material: M2 (54-56 HRC)
Blade Width: ≈1.06"
Blade Thickness: .187"
Blade Finish: Black Nitride
Handle Material: Zytel
Handle Color: Black
Handle Width 1.50"
Handle Thickness: .96"
Blade Lock: Linear Cam™

Gripknife Patriot is available at the manufacturer's website for 495.00 USD.

Sources: soldiersystems.net, gripknife.com