How is heavy cargo secured in military cargo carriers?
The cargo in a military aircraft are secured in the cargo hold using chains, straps etc. connected to stowage tie-down devices, which go into tie-down rings . The following image shows a M1 Abrams tank secured in C-17 Globemaster III using tie-down devices.

 

USAF uses three different types of devices to secure cargo in aircraft, which vary in their load capacity:


MB-1 tie-down device with 10,000 lb rated capacity.
MB-2 tie-down device with 25,000 lb rated capacity.
CGU-1/B tie-down device with 5,000-pound rated capacity


The MB-1 and MB-2 are similar in their form and function, with the only difference being their load capacity.This image shows a close-up of the MB-1 and MB-2 tie-down devices:



"C-17 Globemaster III no. 5139 stowage tiedown devices" by BrokenSphere - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons.

The hook goes into the tie-down ring, while on the other end, straps or chains (from the cargo) go. 

The CGU-1/B is a 20-foot nylon web strap with two metal hooks at both ends. While one hook is stationary, the other hook has a ratchet device and can be moved and tightens the device when it is being used. 



Note: Apparently, Military designation system refers to MB-1 and MB-2 as CGU-3/E and CGU-4/E respectively.