At the time there were limited arms (you took about 2 minutes to reload) vs able to empty a couple clips in that same amount of time, now.
There were also limited religions and limited forms of speach and press at the time too. Should the first amentment only allow freedom of religion for the religions that existed when it was written, or only apply to actual speach and hand written docuements? And I guess it should only apply to newspapers and town cryers, because TV, radio, and the internet did not exist when the bill of rights were originally written. Also, the regular people had more advanced wepons than the army did because rifles were not supplied to the troops, they only had muskets.