For those interested, here is a pic of the firing-pin holes in the breech-face of a Greener 'Empire' shotgun, in this case my .577 x 2 3/4 double rifle conversion:

For comparison, here is an early British 12-bore hammer shotgun:

I hope this points out the difference. If using the latter action for a DR conversion (...and I wouldn't!), then bushing the firing pins would be mandatory.

Doublegun:
If the dimensions of the firing pins and their holes in your Beretta 409 are in the same ball-park as the average bolt-action rifle, then I would suggest that bushing them is unnecessary. Bushing is not really done to handle escaped gas (though it does form an effective 'valve'), but rather to support the primer-cup so that it doesn't push the firing-pin back and rupture into the oversize hole.