I have recently acquired a few SLAP and SLAP-T rounds and I was wondering if anyone has ever fired any saboted projectile out of their 50?  The manual for my AR50 warns as to not fire a saboted round because of the break but doesn't the military fire SLAP rounds from a Barrett M107/82 which has a muzzle break fitted?

The military doesn't fire SLAP from the .50's anymore. The angle of the brake may make the  sabot come back towards the spotter.
Nearly all manufacturers chamber their rifles to accept standard ammo. The SLAP round has the sabot so far forward that the round more than likely will not chamber all the way, or even allow the bolt to close without a lot of force.

So the old Barretts had a different chamber?  If thats the case, could a Barrett with a "Sabot chamber" use standard ball ammo?  Worst comes to worse, they will look cool on the shelf.

There were specially chambered rifles that had SLAP stamped on the chamber end of the barrel near the barrel serial number. Those were the only military barrels to be stamped that way.

There were very few civilian guns chambered for SLAP.
Standard ammo will fit easily in the SLAP chambered barrels. The 1/4" more of freebore will be very bad for accuracy shooting.The huge jump to the lands and grooves isn't very condusive to accuracy. It's one of the big trade offs with the SLAP barrel.