Note that Resicast sells just the gun breech as a separate kit, for much less money.
I too am perplexed that Tasca never included a breech in their original kit, despite the fact that the breech and guard fills nearly half the commander's hatch. I wonder did they originally intend to do so, as the Tasca commander figure is perfectly posed to avoid the breech guard while standing in a Firefly hatch. IMHO a Firefly with open hatch and no breech just looks incomplete, especially when the breech guard was painted a surprisingly conspicuous white, as confirmed by a German newsreel film of a Firefly knocked out and on its side.