Good enough for people who group up to fire musket volleys and if they don't know the possible rate of fire and what they've seen is already mind blowingly faster, chances are the colonial militias with a few professional soldiers or veterans would scatter.

It isn't the volume or rate of fire that is really that important.

They ar15 armed time travellers have dramatically more accurate weapons (they are actual rifles, with... rifling), able to make slowly paced (by our modern standards, very rapid by the standards of a ramrod reloaded weapon), controlled shots from much, much greater ranges.

They also have smokeless powder.

This will basically be seen, or rather heard as 'fucking magic.'

At that point in time, you as a soldier or gunman or hunter would be expecting to be able to identify gunfire both by the sound, and by the large volume of smoke created by even a single ... well, often, not even a rifle, more of a percussion cap musket.

Even revolvers of the time produced much, much more smoke than modern day revolvers or semiautos.

An enemy force (or the local militia) being shot at, accurately, effectively, with distant sounding gunshots, but no visible plumes of smoke... would be a totally alien experience and likely utterly terrifying.

...

Basically, you don't make your tactics leverage volume of fire suppression... you operate as mobile, long range ghosts calling down accurate, invisible thundercracks, whose actual source location cannot be determined.

Make each shot count. No full auto at all. All semi auto, fire as you are exhaling.

Entirely unironically, most people of this era would experience that as some kind of supernatural intervention.

...

More directly to your question:

Those are not machine guns.

Machine guns, as you are thinking of them, would be like an M249, ~~RPK~~ (derp, I think an RPD would be a better example), something with belts or drums of 75-200 rounds of ammo.

What is pictured in the image is ... some kind of AR15 pattern semi-auto rifle, with 30 round magazines.

Generally speaking, basically all civillian variants of the AR15 are semi auto only, the military M16A4 has a 3 round burst mode, but not full auto.

Barrel overheating would only be a problem for most AR15s if you are intentionally doing a 'burn down', which is more of a stress test thing than something that is likely to happen in any engagement.

If you are just mag dumping like a COD player, which in real life you would have to do semi-automatically because most AR15 style weapons an average person can get their hands on is not full auto capable...

... yeah barrel heating could be a problem, but that would be a very stupid tactical decision with this given kit.

One or two bullets to the body is gonna put down your entirely unarmored opponents, who likely will have no idea where you even are.

No reason to pray and spray, conserve ammo instead, that way if you do get spotted, then you can lay down some suppressing fire in a leapfrogging retreat/repositioning.