the Sd.Kfz. 234/3 mounted the 7.5cm KwK 51 L/24 rather
than the KwK 37, I think the dimensions of the barrel are
very close and perfectly interchangeable in this scale.
That being said, the dimensions of this piece from RB
Model are pretty good, but as you can see in the scan
above, the lathe that turned the aluminum rod into this
gun barrel has left very prominent grooves in the metal
that will likely be visible after being painted. It would
probably be very difficult to fill these grooves with any
sort of putty, though perhaps some Mr. Surfacer may do
the trick. The best I can recommend is to not drybrush
the painted barrel, and be careful with your weathering
so as to not highlight the ridges.