Have never understood the anti-gunners' argument hinging on the "A well regulated Militia, being necessary to the security of a free State," meaning if a militia is not organized or maintained by the state, then citizens should not be able to obtain, keep, or bear arms.
If the citizens do not keep and bear arms, a militia, well-regulated or not, is impossible. A militia is "An army composed of ordinary citizens rather than professional soldiers." - American Heritage Dictionary. I believe the Founders understood that militias could come and go, may or may not be maintained or "well-regulated." But they knew that the only way to instantly form an armed mass of men quickly would require the ordinary citizens be armed and, in fact, be armed with weapons suitable for waging war.
And with regards to the argument the Founders never thought the 2A would be to prevent a tyrannical Federal government:
"Before a standing army can rule the people must be disarmed; as they are in almost every kingdom in Europe. The supreme power in America cannot enforce unjust laws by the sword; because the whole body of the people are armed, and constitute a force superior to any band of regular troops that can be, on any pretence, raised in the United States."
- Noah Webster
I'd say they understood all the reasons for the 2A.