So You Want to Build a Browning .50 Machine Gun...

M2 vs. M3


With the price of Browning M2 parts rising, many are looking to the M3 variant of the Browning .50 machine gun as an alternative. Available M3 parts are generally in better condition and are much less expensive than M2 parts are today. There appears to be, however, some confusion about the M3, its differences from the M2, and its suitability for a heavy barrel, or semi-auto gun. I will do my best to address the subject here. If I have missed anything that should be mentioned here, or if you disagree with any points made, please feel free to email me. I already consider this a collective work, and I would like to be as accurate and representative as possible.


What is the Browning M3 .50 Machine Gun?

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:

·      Left and right side plates – depressors added

·      Buffer housing – depressors removed

·      Barrel extension – channels for depressors cut through and relocated


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.


‘But I want a Heavy Barrel Browning’

 Though the M3 was never configured by the military as a heavy barrel, that doesn’t mean it can’t be. I have been using the term ‘M3HB’ to describe and M3 receiver dressed up identically to an M2HB. Yes, the M3 will accept the parts specific to the M2HB: barrel, barrel support & bushing, back plate (complete with trigger and spade grips), front & rear sights, and even the retracting slide group.


‘Won’t an M3HB fire too fast?’

Though the M3 bolt is lighter than the M2, the weight of the heavy barrel acts to slow down a full-auto M3HB to a rate of fire similar to that of the M2HB. The manner in which the M3 bolt is lightened makes it easy to add weight to it in order to slow down the rate of fire further. A semi-auto M3HB would not be affected in any way, of course, since there is no full-auto.


‘I’ve heard that M3’s are difficult to build as semi-auto guns’

Slight differences in the M3 did create some issues for semi-builders, but these have been resolved. TNW has been offering their M3-Basic semi for quite some time now, I believe they would be willing to build your M3 kit (they require you use their side plate). Recently, as solution has been solidified for D.I.Y. and third-party builders. Karma Metal Products (KMP) is now offering an ATF-approved Browning M2 Semi-auto sear kit that works on the M3. The KMP setup does require a modification to the top plate bracket, but I am now providing the modified bracket with M3-semi kits. Except for the bracket modification, all of the mods are similar to the M2. The only additional work involved is placing and riveting the depressor to the right hand side plate (2 locator holes & 2 rivet holes). KMP also offers a beautiful ATF-approved 80% semi-RHSP that will soon be available with the M3 depressor holes pre-drilled. So, an M3-semi is really no more complicated to build than an M2-semi today!


If your application requires authenticity or historical accuracy then an M3-based gun may not be suitable for you. That aside, consider the M3 alternative for your Browning .50 shooter, whether aircraft or heavy barrel, full or semi-auto.


To see general M3 kits, visit

To see new M3 kits for semi-builders, visit



Revision 1.0 8/20/2005

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