I'm still rather new to arduino and I'm working on a pretty large project that needs to MG996s to run at a button press to push a mechanism. I ran a simple line of code with the arduino being powered by 6AA batteries and I got some stuttering and even a moment where the servo would glitch and spin. I want to make sure I can have them be reliable for hours on end so what would be recommended to power them?
What is the rated voltage of the servos? Usually hobby aircraft type servos are 4.8V to 6.6V, 9V is way over the max. (MG996 datasheet) How are the servos actually powered. Post a schematic, please.
Is the battery ground connected to Arduino ground as well as servo ground? Like so.
By 6AA do you mean 6xAA batteries in series? Because this is way too much for powering even any Arduino board. For that you need 5V and 6xAA batteries will give you about 9V.
Do you have a schematic of your circuit? Because, when powering your project from batteries, you will generally use a buck or a boost converter, depending on your battery setup.
Also, the MG996R runs with 4.8~7.2V, so you'll need a voltage supply in this range to power the servo.
Your explanation suggests that you imagined that you could power the Arduino with your 9 V battery via "Vin" or the "barrel jack" and use the "5V" pin to power the servos.
Notwithstanding the grossly misleading descriptions on the Arduino website proper, you should not power the Arduino in this fashion for any project using anything other than the Arduino board itself and perhaps half a dozen indicator LEDs. 
The on-board regulator of the UNO, Nano or Mega 2560 has no heatsink - it is basically useless. You should power these via the "5V" pin with something approaching 5 V but not exceeding 5.5 V. A USB "phone charger" or perhaps a "power bank" is a suitable source of 5 V, if the total current draw is less than 500 mA, then it can be plugged into the USB jack. If you need more current (as you certainly would for a servo) then the 5 V supply should go directly to the parts requiring that current and also connect to the "5V" pin. Obviously you also have to connect all the grounds.
You do however have to be careful not to connect a 5 V source to the "5V" pin when the USB connector on the UNO or Mega 2560 is connected to a PC. No problem for a Nano which is the more practical counterpart of the UNO.
However it is better to provide a separate supply to the servo - four "AA" cells is generally the standard.
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