I don’t know of many preppers who are prepared for an artillary duel.
Or even something small like 60mm mortars.
If a military faction wants your resources in a war for all the marbles, you ain’t squaring off like in Bosnia or Afghanistan. You are getting World War Two or Desert Storm and they won’t give a crap if the building you were in has to be leveled or not, so long as they don’t risk taking casualties.
One problem with that scenario.....if the SHTF, beyond the early days, most of the trucking industry will stop, which means much needed supplies like fuel don't get delivered. Unless, this military faction has deep supply lines and the ability to defend them.
The problem with this is that the US is a rather large area and it would be difficult to conduct operations very far away from things like refineries. Eventually, your tanks, APCs, and IFVs would become little more than stationary cannons.
Sure, you could do things like wars past, and walk/pack mule everywhere you go and hump your mortar systems, machine guns, and rocket launchers. Unfortunately for them, any formidable force would probably be spotted in small rural communities. I would also imagine that large numbers of the military/NG would desert in order to be with their families (after looting the armories and ASPs of course).
You currently have a large force of service members/veterans that have experience conducting COIN. This means that you have large numbers of people that understand the basic concepts behind things like IEDs, as well as, more advanced concepts like improvised rocket launchers, pressure plate initiators, and explosively formed penetrators. A whole lot of havoc could be wreaked by a relatively few individuals.
Sure, I would agree that large military supply depots and things like the strategic oil reserve could keep a large military force going for awhile. But if you start talking about a large breakdown in overall law and order, it won't be long until some of those same militants loot the larger force's supplies for personal gain, or just sabotage it; these guys do have families that don't live near the base. It doesn't take a lot of imagination to figure out this force can be used to violently destroy the area you grew up.
If law and order wasn't established within the first few months to a year, I could foresee insurgent forces long before large military factions.....at least until someone manages to co-op several smaller forces and provide some semblance of law and order. Especially since there are so many competing interests. I'm picturing something along the lines of Somali warlords, and tribal/insurgent forces similar to Iraq and Afghanistan.
One of the reasons America is as large as it is, is because of the ties that bound us. There is a reason, the US doesn't encompass all of Canada and South America. After the shooting starts, how much similarity is there between say Dallas, TX and Manhattan, NY? How difficult will it be, during civil war, to raise a force that aligns California with Kentucky? Againg, you are looking at various warlords, vice large standing armies squaring off like in WWII.