This could have a massive impact on history
For the sake of this answer I'm assuming the weapons in question are equivalent to 15th-century arquebuses. Rifled firearms were not suited to battlefield use until well into the 19th century, as they required hammering a lead ball down through the rifling, both time-consuming and especially difficult to do if the bore was fouled from previous firing.
But even a 15th-century smoothbore would have a considerable impact on military and social progression.
Military Effects
By the Thirty Years War, use of armor had largely dropped out of fashion for most combatants. While a set of plate could stop an arquebus ball at long range, such a set was extremely expensive to make, and still had to contend with contusions and broken bones from the impact.
In 11th century Europe, plate didn't exist yet. The heaviest armor most combatants had access to was conventional mail, which provides little protection against firearms.
These firearms are going to dominate the battlefield. They have the psychological impact of the early handgonnes, but actual effectiveness to boot, and beat bows and crossbows for armor-piercing ability. The inaccuracy of smoothbore firearms, as well, is often dramatically overstated- Renaissance arquebuses were capable of hits on point targets at 200m and massed formations farther still, but were virtually always deployed at shorter range to maximize effectiveness.
This means that melee weapons are not going away. As in the Thirty Years War, the short effective range of 'shot' necessitates the use of 'pike' (or other melee-armed combatants) for mutual support and protection. The exact proportion of the two will depend on their comparable effectiveness; but as a general rule a 1:2 ratio of pike:shot would be reasonable.
These are not untrained militia being handed guns, given an afternoon of instruction, and sent off to fight. With early Renaissance firearms, when matchlocks were the most common ignition type and paper cartridges had yet to be developed, operating an arquebus meant handling a lit match simultaneously with measures of powder, while standing in close formation with one's comrades. This requires training, discipline, and most importantly practice in formation. While it is less of an investment than the lifetime of practice needed to wield a longbow, it is more than what was provided to the peasant levies common in the Middle Ages.
Social Effects
The nature of these wonder-weapons is going to dramatically change how medieval societies approach war. With heavy cavalry no longer dominating warfare, the traditional military role of knights is usurped by infantry hundreds of years earlier than in the real world.
However, as mentioned these infantry are not simply peasant levies, either. They are specialists, trained in the operation of a specific weapons system. They are professional soldiers, but not nobility.
In the real world, these weapons, and the soldiers who wielded them, developed at a time when Europe could not yet afford standing armies but could afford bands of mercenaries for temporary conflict. In the 11th century, even kings will struggle to pay for professional soldiers to fight on their behalf. Given the very small size of medieval armies to begin with, it is unlikely that any such band will number more than a thousand strong.
Those bands of mercenaries will enjoy the social status of the Swiss mercenaries of the Renaissance, being regarded as masters of the battlefield, but few nations will be able to afford them. For most, armies raised from peasants and trained as combatants on an ad-hoc basis will have to suffice, while remaining hugely expensive. Without the economic advances of the pike and shot era, true mass mobilization is still beyond reach.
There are two ways this can go, socially, and I'm not convinced as to which is more likely. One is that the expense and societal burden of producing effective troops exacerbates the inequality in the feudal system, further concentrating force in the hands of the lords. The other is that the common availability of effective weapons, even in unskilled hands, may enable populist uprisings, which only started to occur on a national level from the 1400s on, to occur earlier. I'd say you have the freedom to explore either option, or use both in different contexts.
Castles
As one further note, stone fortifications were pivotal in military strategy of the medieval era. Most battles involved either attacking or defending some sort of fortification, rather than meeting on open ground. While the question does not specifically concern artillery, the early development of technologies needed to produce arquebuses would also enable effective artillery pieces.
While defensive structures were not rendered obsolete by artillery (although their design changed considerably- see the Vauban forts of the late 1600s), they did become less effective as a means of concentrating defensive force. Earlier access to artillery may lead to more social change through military conquest. Had, for example, the English won at Orléans in 1429, the political landscape of Europe could be drastically different today.
