A couple of things to consider; most states consider an individual if armed when committing a burglary an armed robber or armed home invader, not a burglar.
The US Department of Justice has some great statistics although dated data (1997) on who was or not armed when committing a criminal offense. Warning the DOJ article is very long.
One of the interesting things disclosed in the study is where criminals are getting their weapons.
Most states also use the deadly force criteria of; a person is authorized to use deadly force to prevent serious injury or death to self or others.
http://bjs.ojp.usdoj.gov/content/pub/ascii/fuo.txt
Fewer than 1 in 10 offenders serving a
sentence for selling or carrying illegal
drugs and 1 in 30 inmates in prison for a
property crime -- burglary, larceny, fraud,
or destruction of property -- had a firearm
with them while committing their current
offense.
8% of drug offenders and 3%
of property offenders armed while
committing their crimes
In 1997, 14% of State inmates who had used
or possessed a firearm during their current
offense bought or traded for it from a retail
store, pawnshop, flea market, or gun show.
Nearly 40% of State inmates carrying a
firearm obtained the weapon from family or
friends. About 3 in 10 received the weapon
from drug dealers, off the street, or through
the black market. Another 1 in 10 obtained
their gun during a robbery, burglary, or other
type of theft.
From 1991 to 1997 the percent of State inmates
with guns who acquired them at a retail outlet
fell from 21% to 14%. At the same time the
percentage reporting that they used firearms
furnished by family or friends increased from
34% to 40%. Between the two surveys the Brady
Handgun Violence Prevention Act of 1993 was
enacted. The act requires background checks
for persons purchasing firearms from federally
licensed firearm dealers. Changes in how inmates
obtained firearms, when the two surveys are
compared, may or may not reflect the requirements
in the Brady Act. Inmates may have procured
their firearm or entered prison before the Brady
Act became effective in 1994.