I have used a steel ruler to measure the length of the gun's used on the Stug III G and Panzer IV using side profiles drawn by Hilary Doyle in the New Vanguard books (#37 and #39)
The Stug III G from October 1944 has a approx barrel length of 24mm ( from a 1/76th scale plan ). This measures the tip of the muzzle brake to where it enters the topfblend.
The Panzer IV J from December 1944 has approx length of 28.5mm ( from a 1/76th scale plan ).
To convert these measurements to 1/48th scale we ( I think - willing to be corrected here ) multiple 24 x 156 and /100.
This gives a measurement of 39mm for a Stug III barrel from the tip of the muzzle brake to the LINE where it ENTERS the topfblend.
So... take one of the PzIV barrels , assemble it and then MARK off a line 39mm back from the tip of the muzzlebrake. Next widen the hole in the topfblend if necessary and gently push the barrel in until the line marked on the barrel matches up with the beginning of the topfblend.
This may be a possible work around for you. If any guys out there actually know the correct length in 1/48th please feel free to correct what I have said