Installing an aftermarket inner barrel is pretty simple and mostly a harmless procedure, but like most operations, a closer look at the body before performing any surgery can reveal a lot! Let's take a look at the MK23 hop up assembly!
Below, we have an example of how the standard MK23 barrel and hop up components sit in the hop up assembly. Nothing out of place, right? Notice the groove to the left of the hop up rubber. That groove serves the purpose of helping to keep the barrel in a fixed position and helps to prevent rotation.
The problem with the majority of barrels that are not specifically stated to be produced for the MK23 (generally anything longer than 133mm) is that their groove placement is further along the barrel, which becomes a problem when we see how the hop up assembly looks beneath the barrel.
Do you see it? The little extrusion or nub next to the where the hop rubber would sit. This is the notch that catches the groove in the barrel that helps keep to keep the barrel in position. The problem with this piece of the hop up assembly is that when using a longer barrel with a different groove placement, the curve of the barrel sits directly on top of the nub as the nub has no where to go, this causes the barrel to not seat correctly and can result in it being off center as well as putting stress on the hop up assembly itself.
Up until now, the common fix to resolve this issue has been to use a crafting knife or rotary tool to remove the nub from the assembly so that the barrel seats flush in the assembly. You can see the result of this in the image below.
Community members have generally advised Tokyo Marui users to not perform this mod as it does result in permanent damage to the hop up assembly and have even suggested filing out a groove on the aftermarket barrel if the barrel is cheaper than the assembly part. So whether you perform this mod is up to you, there really is no obvious negative regarding performance by undergoing it. If you are using a clone MK23, you shouldn't hesitate to do it.
Recently however, manufacturers / retailers have started to notice the MK23 communities desire for longer barrels and have started to accommodate by providing longer barrels with the MK23 groove placement, allowing direct plug and play use. Two obvious example are the Maple Leaf Crazy Jet 150mm and 138mm barrels designed specifically for the MK23.
Whatever you choose to do regarding the nub and groove placement, you at least can now make an informed decision about it!