AequitasThere is currently no better way to simulate the time needed to prime, load, and fire the rifle grenade. A delay after switching to it would result in people running around with it loaded.
Yes, people could run around with the rifle grenade loaded just like they can run around with bazookas and machineguns. But that doesn't give them any advantage - on the contrary, their crosshairs will be screen-wide and they'll have to wait for a long time for them to recenter to get an accurate shot. Whereas a smart player only switches to the rifle grenade when he needs it, and even with the delay is ready to fire faster than the noobs running around with it.