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11:15

The American assault steps off in three waves of 5,000 troops, around 200 yards apart.
Almost as soon as the first troops emerge from the hedge line, British rifles start to fire from the parapets. The volume of fire is not high - only about half a battalion is firing total, all the 1st class shots and no-one else - but there are so many American troops that a surprisingly large number of balls hit at this range. The American force troops onwards, taking one or two hits every few seconds.
Some of the American colonels of volunteers notice an odd detail - one which takes only a short time to understand. There are ranging posts out in the field, at intervals of ~100 yards, and each time the American force crosses one of the lines of ranging posts it takes many more casualties than otherwise.
Still, the attrition is slow for the first few minutes as the first wave crosses 200 yards. Then, as they reach 600 yards, the British 2nd class shots open fire as well.

This leads to a jump in the amount of shots seen coming from the parapet, and also an initial pulse of casualties. Some of the Union attackers hesitate, but the formation as a whole keeps them going.
Around 150 casualties have occurred so far, around one third of them fatal (usually due to a minie ball dropping down onto a vulnerable head at a steep angle).

Meanwhile, some way off to the left, the American field gun battery is interrupted in its cannonade when two shells whizz overhead from their forward left. The British Armstrong battery has come into play, and it is firing ranging shots - at an extremely long range by American standards, indeed their first shoot lands almost half a mile 'long' as the British artillerymen have underestimated the effect of the slight rise of ground.
Another pair of shells comes some twelve seconds later, landing some way closer to the American gun battery. Fire is returned, but halted after a moment because the guns are simply unable to fire this far. Rather than simply allow himself to be shot to pieces - as he imagines will happen - the response by the US artillery commander is to have his cavalry close in from the flanks while he limbers up and disperses his gunline. Splitting into individual batteries will allow him to keep up the fire support mission, and his command battery will remain here.
Almost as soon as the limbering-up has concluded, however, the Armstrong guns get the range. Firing at maximum rate, the six guns put out a total of twenty-four shells (both shrapnel and contact) over the next minute, and while the later shells in the shoot are less accurate than the earlier ones it leaves half the command battery guns disabled, all but one out of action and dozens of gunners dead.

The other four Union batteries keep moving, preparing to deploy again, and about 1,000 Union cavalry begin moving upslope to get at the Armstrong guns.



11:21
The first wave Union attack is now about 350 yards from the British position, and has taken something like 500 serious casualties and as many again light wounds. This first wave is starting to slow, the effect of the fire producing a considerable morale effect, but it is at about this point that the order goes up for the Union sharpshooters to drop out of the advance and begin firing back.
This order is obeyed by considerably more troops than the ~180 who should be, as some of the laggards take the opportunity to take up a position of greater perceived safety, but the snap and crackle of Enfields heartens the attacking Union troops somewhat (and especially the second and third waves, who can see it). This fire also makes the British troops more circumspect, thus reducing their accuracy somewhat, and inflicts a few casualties.
However, it is only about thirty seconds later that the first wave Union attack reaches the 300-yard range posts. This is where not just the British 3rd class infantry but also the Canadian militia have been trained to shoot, and around 4,000 Enfields total fire as the Union force passes this range marker - as does the 9-lber smoothbore integrated into the defence, firing canister.
The results are extremely nasty. The British regulars are less accurate than they would be on a rifle range, and the Canadians less than them, but nevertheless around four hundred casualties (more fatal than before as the range drops) hit over the course of a second or two. This causes the Union force to shiver, before shouted exhortations push them on - and, indeed, cause them to break into a run.

11:25
The Union cavalry runs into a British squadron of the 18th Hussars. Over the next few minutes, both sides dismount and go to cover, and a firefight develops - one which goes far worse for the American force than they had expected, since the 18th Hussars are armed not with muzzle loading carbines but with the Terry, a rifled breechloading cavalry carbine capable of a heavy rate of fire and high accuracy out to at least 800 yards. The initial squadron encountered is ultimately pushed back with heavy losses on both sides, but when the rest of the 18th arrive they use half their men to pin the Union cavalry and put in a saber charge from the flanks; the Union's cavalry is thus essentially routed.

Meanwhile, the Armstrong guns switch fire to a third Union artillery battery. By now the American artillerymen are justifiably jumpy, and when the position is bracketed a few men leave their position without orders. They survive, though the explosion of a caisson means many of their comrades are not so lucky.

At about 120 yards from the British position, the first wave Union attack has lost momentum completely and gone to ground. They are taking heavy casualties, though are now at least at a range they can cause casualties of their own.
Many of the British 2 and 1 class shots have been redirected to fire on the second Union wave, which is itself being made bloody by this accurate rifle fire at about 300 yards. There are by now thousands of Union casualties on the field, and the British and Canadian fieldworks are becoming a major psychological obstacle to the remaining attackers.

11:35
The third wave goes to ground along with the first two, at about 100 yards from the British line. These are all brave men, but are not properly trained for the situation they face - indeed, by now many of those armed with Enfields have been shot and killed by the British defenders.
Some relief is given by the heavy smoke clouds produced by the Union fire (smoothbore buck and ball) which makes targeting harder, but with so many men in a small space some hits are inevitable.
Worse, the chain of command is now uncertain - the main Union commander has become a casualty of war, and this has not yet been worked out.
At this point upwards of 1,600 Union troops are dead or seriously wounded, with about 200-300 British and Canadian combined in the same condition (and the lightly wounded fraction of the British-Canadian troops larger, due to the high portion of buckshot aimed at them).

11:45
After ten to fifteen gruelling minutes of contact, some of them being shelled by the Armstrong pieces to their north, the Union attacking force finally breaks. It has taken over thirty percent casualties and is essentially no longer fit for combat.
British casualties are considerably lighter, but the ten minutes of small arms fire has cost them nevertheless - around 1,000 are dead or seriously injured. A full pursuit is considered impossible for this reason, though the 18th Hussars are already moving in from the north (and will be delayed some ten minutes by stubborn resistance from the one Union regiment deployed north as flank guard, though relatively few casualties are caused on the Hussars by this).