The original Aussie receivers have some issues with stretching after many rounds. This is because the receivers are only hardened in certain spots for wear but left soft for most of the receiver body to prevent cracks. The Aussie and Canadian receivers are "flame cut" instead of forged. I think this method is similar to machined billet. I'm not sure how the British receivers are formed. The Belgian receivers were originally forged from a steel similar to 1060 steel. The last Belgian receivers were cast from a steel similar to 4140 steel. The original design wouldn't allow full hardening without being prone to cracks. The newer cast receivers are made of a steel that is less prone to cracking. FN said they were able to afford upgrading the steel for the cast steel receivers because they could recycle the bad ones on site and just cast the steel again. FN was trying to compete with the stamped and welded H&K rifles in price. Early in the FAL development they made a lot of upgrades but near the end of production they cut costs of a lot of things. FN even contracted Argentina to make rifles and parts but marked FN for Belgian sales. Brazil decided to go a different route and develop their own cost cutting and compete with FN and Argentina for sales.