Indeed, the 18-45 male population is probably where 80% of the firearms are. I would guess the pct of 18-45 with guns is more like 30-35%--much higher east of California and south of the Mason-Dixon line. I don't really expect we'll have to fight the British again. We have, in fact, beaten the bloody British back twice. Anyone remember the War of 1812? The Battle of New Orleans? (OK, we'd probably lose that one this time, but let 'em try and come up the Houston Ship Channel!)If I read the data correctly, according to census.gov (Age and Sex Composition in the United States: 2011 - People and Households - U.S. Census Bureau) there are roughly 62,349,000 males age 18-45 in the U.S. If we took a representative sample equal to the ratio of gun owners against the population as a whole (roughly 314,000,000 according to Population Clock) we get a value in the neighborhood of 16%. If we applied that 16% to the 18-45 male demographic we get a total of 9,975,840 males of age that own guns. So we take the patriotic 10% of that and we arrive at a final number of 997,584. Nearly four times the number that fought in the Revolutionary War.
My point is that nearly 1 million partisan fighters is a nightmare for any invading force and I guarantee you that a lot of determined women and a small army of crusty, old farts with plenty of fight left in them will swell those ranks not only in number but in experience.
Again, I wouldn't bet too heavily against those odds.