Tamiya has recently released a great kit of the Jagdpanzer IV/70(V) (Stock 35340). This detail set is designed to enhance an excellent kit.
This whole detail set is meant to improve the gun, and only the gun. It provides an extra sprue to build the gun breech –in plastic, and provides an aluminum turned replacement gun.
The breech/loading mechanism builds together beautifully and without any fuss, as can be seen in the pictures. The metal barrel is flawless and incredibly smooth. So smooth that actually the paint peeled off the first time I painted it. Make sure to use a primer.
If you add this detail set, you are instructed to not install the torsos of the crew or they will interfere with the gun. The kit itself is designed in such a way that you can remove the upper hull after the kit is finished, I guess to be able to see the cannon detail. Or you can keep the hatches open and admire the detail if you have a flashlight.