HK MP5 A5 rimfire without the faux suppressor

Here are some pictures without the fake suppressor. The barrel is actually pretty thin and is housed in a barrel shroud, which is the part that actually looks like the barrel from the outside. The end of the barrel is threaded. The suppressor has a bare end that slips over the shroud and is held in place by an o-ring. The muzzle end of the suppressor screws onto the barrel threads. That is what ends up centering the barrel in the shroud.

It looks like making this an SBR may not be too straightforward. Since a shroud is used, I would guess that you could cut down the barrel and shroud, rethread the barrel and then make some sort of short faux thread protector or muzzle brake that takes the place of the longer faux suppressor so that the barrel still has support at the muzzle end and remains centered inside the shroud. I'm not an expert of that sort of thing but that's my first take on it.