All such mounts you speak of were usually jury-rigged at the unit level, in the field. The crew/section/platoon would scrounge the weapons (.50s or .30s) and their mounts (both the guns and cradles off tanks were popular targets) and have their LAD weld up a mounting strong enough to withstand recoil and general abuse.
Incidently, this went for all sorts of armoured vehicles, not just carriers; I have photos of an incredibly complex .30 Browning mount welded onto the front of a 12th Manitobas Lynx scout car, and most Canadian Ram Kangaroos had at least one if not two such jury-rigged mounts on their turret rings.
It's interesting to note that the .50 Brownings supplied for AA use on the turrets of all Shermans were usually discarded by Canadian tankers; no doubt many of these ended up in such service as we're discussing. I've also been told, though, that in some circles the .50s weren't all that popular because of their size, recoil and the vast amount of space needed for ammunition storage; in many cases, the .30s were far more versatile in all respects.
I can only think that a .50 on sustained fire from a carrier would shake the poor thing to bits... but nonetheless, we've seen the pics! This is one of the pics to which you are no doubt referring:
These are unidentified members of the Lake Superiors clowning about in the vehicle park after they were stood down in Holland. There's no reason to suspect that this T-16 carrier might not have carried this armament for many months.
If you watch the Norman Summer episode of the CANADA AT WAR series (available through the NFB), you will also see glimpses of standard Universals so armed, and I've seen one more pic with the cradle mount for one welded onto the bulkhead behind the front compartment.
Be advised, though, that in standard infantry formations, MOST carriers went unarmed other than the crews' rifles, Brens and Stens (and perhaps a PIAT). With notable exceptions (Wasps, mortar and Vickers MG carriers), they were used as utility vehicles, not fighting vehicles. Crew strength varied depending on mission (three or four is a good number to go with), and if you insist on building an armed carrier, stow all personal gear OUTSIDE the carrier and fill the inside with ammunition boxes!
If you have any more questions, don't hesitate to ask.
Geoff