For the United States.
I believe The Only "War" to Break Out would be a Race War... Combine the Riots of Rodney King with New Orleans Disaster. Raise the level of Violence 300%.
Like 2beararms said, it would be social collapse..loss of power, water, public services that will fuel crime and violence.
Speaking of that.... You are better off stock piling a diesel Generator and fuel ...save the extra $1500.00 on a AR Pattern Rifle and invest in some basic survival gear...water purification etc.
Forget scrounging... The "Street Thugs" READ; WHOLE NEIGHBORHOODS of Like Minded Individuals, will incapacitate and relive the LEO of their firearms/ammo... The Police that DO Stay on and work... most, like New Orleans will leave to check their own keep. 2beararms is correct about Military forces, But, I feel most of the National Guard will not even show up for duty, leaving the other half that do, to follow orders or become looters themselves.
DO NOT Underestimate the weapons of the inner city gangs... They are VERY Well Armed. They Do Not carry their Best out on the street... They Steal and horde AR's, AK's...etc etc... When they get Busted for $100.00 of Drugs its the BullSh*t .380 Nickel Davis that gets Pinched.. "Another gun off the Street"... Right, They have 60 assault rifles in hidden away.
To Keep the topic on its course.... Look for the MOST popular Calibers... .22LR, .9mm, .38/.357. .40 .45acp.....223 and 12 Gauge Shotgun. If you have the money, purchase some inexpensive firearms...you may have to arm a trusted friend, relative or even barter with.... OR, If the "Authorities" come around like in New Orleans and start to Confiscate Guns for YOUR Safety... Wouldn't it be nice to hand in your $50.00 Raven .25 acp Sh*t Gun and a .20 Ga Single Barrel.... leaving your Kimber and AR-15 hidden in the house?
Far Better to plan a plan as they say... Fort Up with trusted people... All the Stores that make it thru riots and black outs are forted up and guarded by armed store owners and their FAMILIES...
High Ground, Roof Tops....
Paranoid? Hmmm Maybe, Maybe Not... LOL, But Just watch the News... Themz are Crazy Times!
I work as a Consultant for a Security and Medical Extraction and Evacuation Company... I live in the Philippines. These are scenarios that EXpats, Wealthy and Politicians Plan for EVERY Day in Most 3rd World Countries... Complete Social Collapse. Believe me, They have routes OUT... Its the Same everywhere.. The Gangs, Political Extreme Parties... They are well armed with loose weapons.. Stolen from the Police, Military or like the Drug Cartels in Mexico, They are actually the Police Themselves selling the military class Guns...
Stay Safe... Captain