They could have done even more time traveler-y bullshit and not even directly interacted with John Brown.

Pop in with those weapons, full combat gear and load out, 180 to 240 rounds of ammo each, radios, scopes and binocs.

Split into two groups of two, position one team in the hills on the north side of the Potomac, a bit South of the current day (and historical?) Naval Battery Outlook, put the other team at the modern day Ridgetop Outlook, with ability to reposition if the Naval Battery Outlook team can see something they can't.

(Apologies for whipping up shitty map on mobile)

Lets say these weapons are all... Daniel Defense M4V7s, with about 1 MOA accuracy.

Let Brown start the attack, and the South of Naval Battery Outlook team can operate as Designated Marksmen doing overwatch, and plink off Armory Guards and 'local militia' with accurate fire from across the ridge... ideally, allowing JB and his crew to be able to hold the much more defensible Armory than the house he was forced to make a hasty retreat to in our timeline.

As the alarm is raised, the Ridgetop Outlook team basically lies in wait for an ambush, or if reinforcements come from an area they have bad visibility of, South Of Naval Battery team can tell them where the baddies are coming from and where to reposition.

The time travelers have dramatic advantages.

Smokeless powder.

The enemy will realize they are being shot at, but will likely have absolutely no visual indication of where from, instead having to just try to figure it out by ear.

Much, much better accuracy and range... and reload speed / time before needing to reload.

Even the further removed South of Naval Battery team will be shooting roughly 5 inch groups at worst, at the range they are at.

Aim for center mass, and thats still pretty much a guaranteed hit for each shot.

The Ridgetop Outlook team is intended to plan on repositioning if necessary, so they'll be at an even closer range.

Basically, just act as guardian angels, assist with the initial assault on the armory (JB and his crew canonically march south along the road Naval Outlook team is north of, then across the bridge), screen the entire counter attack against Brown, help put the fear of god into the inital Armory defenders... and heck, maybe bring some swan feathers or something, scatter them in with some piles of spent casings, and maybe now Brown is both a succesful revolutionary and also has the mandate of heaven.