Dunno, I have several synthetic stocks, some old style and several new checkered style, no problems with fit. Sometimes they need some adjustment so they aren't too tight, but there's no problem with the barrel channel being too small, I just put new camo syns on my 444 and 44mag Handis, also have one on my 38-55 which is comparable to a bull barrel in diameter. If you're putting it on a TDC shotgun barrel, you'll have to change the channel at the barrel swell tho, it's much bigger than a rifle barrel, been there done that too.
Maybe you have a defective forend that wasn't finished properly, what was it on originally?
Tim