Additionally your assertion that his reloading of his weapon allowed children to escape is not accurate. There are assumptions that this MAY be the case, but no official word yet from the police. Additionally, even the sories that make this assertion state that it was eithter his weapopn jamming, or him making an ERROR while reloading that allowed them time. Changing the magazine on a firearm (barring error) is an action of at most a couple seconds and will not give anyone time for a meaningful action. With practice there are many people who can change magazines so rapidly you won't even notice them doing so. Lanza reloaded frequently leaving many partially full magazines behind.
if changing magazines is so easy, why are you opposed to limiting their size? obviously you can just swap in another 10 rounds when you need more bullets without even thinking about it, so if it's so easy to do you should not be against changing the capacity.
conversely, when a majority of guns used in crimes are proven to be from straw sales, it should be a no-brainer that making straw sales to be a federal crime should be easy to pass. it should also be a no-brainer to extend background checks to all purchases with no exceptions because it has been proven time and again that criminals go for the easy route when it comes to buying guns, and that means buying guns for which there are no checks, and buying guns from straw sales. but the recent failure to pass any regulations proves we'd rather protect the rights of criminals to buy guns rather than protect the citizenry from those criminals.
Magazine capacity:
I don't need a reason to keep magazine size unregulated, you need an compelling reason to make them illegal, and you don't have one.
Straw Sales:
They are not "conclusively proven" to be the main supplyer of weapons used in crimes, they are widely believed to be, a significant difference, and IT IS ALREADY ILLEGAL.
The easy route:
What part of "It is already illegal to supply a felon with a gun" are you not comprehending? There are no exceptions to this law. If I buy a gun, and give it to my buddy who did felony time, I have commited a crime. It is a crime today, it was a crime yesterday, and it will still be a crime tomorrow.