Background:
- I spent a year carrying an M60 in combat
- I have never seen the movie, so my comments are based only on the still image

Simply carrying the M60 that way isn’t realistic. The version shown here is the shortened and lightened version used primarily by Navy SEALs. It weighs a bit less than the 23 pounds of my full-size M60 but not a lot less. The M60 is simply not a gun you want to carry around one handed – you needed two hands and most of us preferred a sling.
Next problem. He has less than 50 rounds in the belt connected to that machine gun. The M60 fires around 600 rounds per minute (10 rounds per second!) That belt of ammo will last about 5 seconds of firing. I never carried less than 300 rounds connected together and fed to the gun – that’s still only about 30 seconds of firing, but realistically about 1–3 minutes of combat.
Given the weight of the gun, and Stalone’s physical build, firing the gun one-handed from that position would not be completely uncontrollable, but I wouldn’t expect him to hit anything.
Possible, yes. Credible, maybe. I used to carry the “pig” during live fire exercises. I wasn’t as muscular as Sylvester Stallone, But I was 6′2″, about 220 lbs, and with very little fat. I could easily carry the M60 like a regular rifle and fire it from the standing position.
Note that it worked better two-handed, as shown in the TM. But I could spray a nearby area fairly accurately with one hand. However, that’s just showing off and not the intent of this crew-serve weapon. The pig is best fired from its tripod, with an assistant gunner helping feed ammo.

Former weapons squad leader here. Normally M60s were employed from the tripod or bipod in a prone position. However, the old M60 field manual describes a set of assault firing positions including shoulder, hip, and underarm firing positions. These positions require the gunner to use both hands to control the MG and to fire in short burst. This is where Rambo fails.
The underarm position is similar to what Rambo is using except the whole “two hands” and “short bursts” things. You tuck the gun’s receiver up into the space between your upper torso and arm, and squeeze your arm against your torso to somewhat reduce the recoil.
The idea of assault positions is that as you close on the objective you synchronize your feet and trigger. That is, for instance, you fire a short burst every left step or maybe every other left step. You aim by orienting the gun with your upper body and make adjustments via the “burst on target” method.
It’s not providing the same level of accuracy as from a bipod and definitely not the accuracy of using the tripod and T&E, but it provides reasonably accurate suppressive fires during the assault, for instance while bounding the gun team forward. Keeping the bursts short lends to accuracy. Rambo and his firing at cyclic rate would be wildly inaccurate.
It’s not commonly trained because it seems unsafe to people who haven’t seen it, trained it or read the book.
One of my favorite moments as the HHC commander for a mech infantry battalion was running a range one day and surprising my XO who popped some targets at about 200 meters and I quickly dropped them with an M240 using a couple short bursts and the underarm firing position while on the move—-just like I learned when I was enlisted.
If you try to do it like Rambo, there is a high likelihood you die. If you do it like the FM teaches, it is totally legit.