.50 Cal. Browning Machine Gun, AN M2 vs M3 Aircraft"The M3 is a version of the M2 that was designed to sustain a much higher rate of fire than any M2 variant. It was originally only configured as an aircraft gun. The M3 is an improved M2. Why improved? In order to both achieve and withstand the high rate of fire, a number of weak areas were modified. These are:
Depressors are relocated from the buffer housing to the side plates. Apparently, those two depressor tabs, which are unsupported at one end, flap around too much (and may even break or dislodge) at 1200rpm. The relocation of the depressors results in a modification of these parts: Back plate is modified to withstand greater forces. The buffer on the M3 back plate is much larger, its diameter being equal to the width of the back plate. The sides of the back plate that slide onto the side plate also have an outside shoulder that wraps around the side plate to prevent the side plates from deforming and releasing the back plate. Small recess cuts on the M3 top plate, bottom plate and side plates accommodate these shoulders. Since the M3 was only intended for use in mounts (fixed or flexible), no provision exists for trigger or spade grips on the M3 back plate. Bolt is lightened to allow higher rate of fire. Many internals were modified with new design of the bolt. Basic geometry and dimensions remain similar to the M2 bolt. The top plate bracket has also been modified, though it is very similar in geometry to the M2 bracket. Extractor & extractor cam modified. Generally, modifications here appear to make the parts more robust. The extractor has a larger curved tab that grabs more of the round than the M2. The extractor cam is wider and is secured by four rivets vs. two on the M2 cam. It also has a change in angle that is more severe than the M2 cam. Top cover is modified to grab more of the round than the M2 top cover. In addition to grabbing the round at the rear, the M3 cover also grabs at the neck in front of the link. This appears to prevent some unwanted movement at 1200rpm. The change results in a wider feedway & shuttle. Otherwise, the M3 cover is dimensionally identical to the M2 top cover. The M3 Receiver has identical geometry to the M2 receiver. The trunion is identical (in later models the provision for a front sight has been removed). The side plates are identical, except for the riveted components and a slight recess on the rear edge for the M3 back plate. All of the tapped holes and slots of the M2 are present on the M3 side plate. The top and bottom plates are identical, except for the slight recesses in the corners for the back plate."
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