Was going to post the same about continuity. Bearer's bonds job with the armored car resulted in burned guns and gear in the van with an improvised incendiary device from Chris.
There are some good documentaries on the making of Heat that discuss the technical aspects of the firearms, the training consultants for the actors (SAS and LAPD). They actually conducted pre-operational surveillance on the bank with the SAS guy as if they were really going to do it, without notifying the bank. They spent a lot of time learning close quarters marksmanship with live fire, then team-level shooting and moving techniques from the same consultant, while the LAPD guy taught the police actors how to shoot their way.
There was something they did with the firearms to escape the Hollywood BFA effects. There was some mastering to kick in more bass, but they had originally dubbed in gunfire sounds to cover the real recordings. Michael Mann realized immediately that something was horribly wrong and missing from what they experienced when filming, so I think they used original sounds, just re-mastered by their audio tech army, which was substantial at the time.
As to the figure 8 bungee sling method for covert carry of carbines, it was taken from SAS operations in Northern Ireland for work in and out of cars. The bungee routes through the top sling loop and the slot under the tube portion on the Fiberlite stock, and the running ends of the bungee are often taped together or sewn. They also did it with the MP5K as a car gun for close contact, since the IRA would run a lot of vehicle checkpoints to intercept SAS and Det14 personnel in their campaigns against the occupying Brits.
I saw Heat on disc when I was on the DMZ with 1-506th Scouts in Korea in 1996, and was totally blown away by the film on all levels. The cast, cinematography, special effects, story, plot, setting, score, and especially the technical advisory work told me that someone with some serious operational skill sets had woven many things into the film that I felt should not be seen by the public.
A few years later, we watched the bank scene as part of the battle drills class in the Special Forces Qualification Course during our classroom lessons in the Small Unit Tactics Phase at Camp McCall. The AAR comments from the cadre were interesting, to say the least.