I understand how polling works, and while it is useful and valid information, it is folly to conclude that it must be 100% true and accurate representation of everyone's opinions. I dont care how carefully those 1000 who are polled are picked, it is simply not possible for them to accurately represent everyone.
I don't think you understand how polling works then. Not only is polling explicitly not about being 100% true, but there is no claim that any poll represents everyone. The polling sample is supposed to accurately represent the preferences of the population it was drawn from. This has been tested a lot. They do.
To be clearer, polling is most often reported at the 95% confidence interval, which means that the odds of the population's true preferences are 95% likely to fall within its margin of error. Considering the overwhelming results of those polls, even if you expand your confidence interval to 98% and above you still have massive majorities of Americans in favor of expanded background checks.
If you're simply talking about what the majority of Americans support however, a result that far from 50% means that the odds a majority of Americans support those expanded background checks is going to be somewhere around 99.999%.
Long story short, the polls are right. The support for those checks is absolutely overwhelming.