Sporting muskets were more accurate than military muskets.

Modern firearm enthusiasts who hunt with 18th century fowlers (as sporting muskets were called in the American colonies) report them accurate to 60 yards against game like deer and bear. This is not as accurate as contemporary rifles (accurate to 200 yds in the hands of a skilled marksman), but pretty accurate.

The reason fowlers were more accurate than military muskets was that military muskets were deliberately made less accurate to reduce the problem of barrel fouling, while this was not a problem with sporting muskets.

A fowler was a smoothbore firearm generally lighter than a military musket. They were capable of firing various loads from "birdshot" like a modern shotgun, to bullets. The greater accuracy of the fowler was because the bullets were cast to fit the barrel more tightly.

Military muskets were built to fire 50 or more rounds in an engagement, without the black powder fouling the barrel and making further shots impossible until the gun was cleaned. Black powder is very fouling, so this required a bullet somewhat smaller than the barrel so that the gun could still be loaded and fired after numerous rounds. These gaps between bullet and barrel reduced the accuracy and range of the gun. A .69 ball was the standard military bullet for the .75 caliber bore Brown Bess musket.

http://allthingsliberty.com/2013/07/the-inaccuracy-of-muskets/

Military muskets were also generally heavier and more rugged than fowlers, and could be fitted with bayonets.

A modern enthusiast, shooting a Brown Bess (shooting offhand, rather than from a rest, and using a .715 ball, rather than the military standard .69 ball) reports a 20 inch group at 50 yards, which is not great (but not really comparable to the grouping data I report later for the fowler, because of the offhand shooting, which will increase the group size by some unknown amount, depending on the skill of the marksman). (He also reports that he shot 40 rounds and towards the end, the gun was becoming fouled and difficult to reload, which illustrates why the military used the .69 ball.)

http://www.rifleshootermag.com/rifles/featured_rifles_bess_092407/

Fowlers, were designed to be used for hunting. It was likely that only a few shots at a time would be possible against game targets, then the gun could be cleaned. Therefore, bullets were cast with tighter barrel tolerances and fowlers were more accurate than military muskets.

The first source reports shooting 4.5 inch groups at 50 yds from a .62 caliber fowler, loaded with .595 ball. Which is pretty good accuracy. (Comparison with a modern rifle: A rifle is generally supposed to group at least 6 MOA (minutes of angle) or better - lots better for a match rifle (lower is better). 6 MOA would give a 3 inch group at 50 yds. 4.5 inch group at 50 yds indicates 9 MOA.)

Fowlers were imported to the American colonies in large numbers (The largest numbers being "trade guns" to sell to the Indians for furs) from France, Britain, the Netherlands and Germany, and were also made by American gunsmiths.

Fowlers were most often in .60 or .65 caliber. They were often used for hunting deer, and were particularly useful for hunting birds like turkey, pigeons or water fowl such as ducks or geese, when loaded with shot like a shotgun.

The fowlers were also used for defense, and by the militia, although some militia were equipped with military muskets instead.

The Minutemen being armed with fowlers, while the British Regulars were armed with military muskets, may have contributed to the success of the American sniping attacks on the British column as it retreated towards Boston after the Battle of Concord, at the outbreak of the American revolution.

Fowlers were more accurate at longer ranges than military muskets. But military muskets were reasonably accurate at ranges up to 40-50 yards.

Yes infantry did stand in line and fire volleys.

At the Battle of the Plains of Abraham, in 1759, when British and colonial troops broke the power of France in North America, outside Quebec, this was how it was done.

The British force had landed in boats and scaled the bluffs up to the Plains of Abraham under cover of darkness. At dawn their 4,500 troops were drawn up in a line, two ranks deep and 1 km long, facing the walls of Quebec.

The French defenders also had about 4,500 men, but they had more militia and Native allies, and fewer Regulars than the British.

The French advanced towards the British lines from Quebec, and started firing from about 120 meters out. The British stood (and fell) silent in their ranks until the French had approached to 40 meters. Then they fired their rolling volleys, which stopped the French advance in its tracks and then sent them hurtling backwards.

Both commanders died in the Battle. The British still had to capture Montreal, but French power in North America had been shattered on the Plains of Abraham.

http://www.thecanadianencyclopedia.ca/en/article/battle-of-the-plains-of-abraham/

It entirely depends on the musket or rifle. A rifle is significantly more accurate and can be accurate out to hundreds of yards with a Minie ball such as in the American Civil war. Muskets varied in the caliber of ball and the caliber of the musket the larger the difference between the 2 the faster it could be loaded and the less accurate it would be. Muskets did not have sights on them and on rifles of the period sights were primitive if they existed at all. So there was an inherent lack of accuracy due to one simply aiming down the barrel. You could reasonably except to hit a man sized target at say 25 yards. However line combat was typically only this close when the bayonet was used. They typically engaged at longer distances between 50 and 100 in which they were far less accurate and relied on the density of fire to win the day.

Infantry lines of different types certainly existed. With different countries using different types of lines. The number of men also varied from country to country and between time periods. They typically shot until one side broke or the call for a charge was called which typically resulted in a rout.

Its a long time period between the early 17th century and the middle of the 19th century. So lots of variation. If you have more specific question shoot.