I've got two Takom 1/35 FV432's to build, and looking at the gpmg's in the kit I think a little a/m- a metal barrel, some etch perhaps - might improve it.
Or even a resin job.
I've done a search in Hannants as a start, with negative results. Probably poor search terms, but I have tried a fair few options.
Seeking advice from the (shameless flattery!) Excellent AFV modellers on B/M as to is such a/m exists, and who makes it.