Upper receivers, top to bottom - Type 2, Type 3 then Type 1:
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Type 1 was the original.
Type 2 has a bit more meat at the rear of the receiver, supposedly because some Type 1's had cracked at the back under full-auto fire.
Type 3 is the cheapest to make because there are less machining operations required - and consequently has the most metal of all the types, making it presumably the strongest.
You'll also notice as the type-number increases that there's progressively more meat at the front of the receiver (just under the carry-handle).
Type 1 was the original.
Type 2 has a bit more meat at the rear of the receiver, supposedly because some Type 1's had cracked at the back under full-auto fire.
Type 3 is the cheapest to make because there are less machining operations required - and consequently has the most metal of all the types, making it presumably the strongest.
You'll also notice as the type-number increases that there's progressively more meat at the front of the receiver (just under the carry-handle).
