Hey all.  First time owner of a firearm and decided on an AR pistol.  Bought an off the shelf DD MK18 and just picked it up yesterday.

I was hoping my SB15 would fit nice and snug on the buffer tube, however it's pretty easy to pull off and on and tends to rotate as well.  Was thinking of getting the KAK SuperSig or Hex-2-C to solve this.  Do I need to buy the entire kit or can I use the spring/buffer internals from my current tube?  Apologies if that's a silly question, for the moment I'm a total newb.

Many thanks

Welcome. Thats a very nice upper!
Use all your current internals and get the $30 tube from KAK. During this time I would also upgrade to a QD end plate at the same time while your down there.

Do some inspections and see if your current buffer tube is staked or not.
Also learn what springs are going to pop and fly out and where when you take it apart. Its very easy to take apart, but you don't want to lose those springs if you can help it.

Thanks for the advice.  KAK tube on the way along with my Magpul ASAP QD, KX5 and Raptor.  Really hoping this doesn't come to an end before it's started for me considering the Black Aces Tactical debacle.  Can't own anything NFA in my state so would have to buy a whole new upper to make it a rifle.

The Black Aces letter is one thing but the Shockwave Blade one is more pertinent, I think:

http://www.ar15.com/forums/t_3_122/663692_Shockwave_Blade_brace_approved_.html

At any rate, I could never try to shoulder just a tube, even with a can tip or whatever on it,  find that simply laying it alongside cheek as 3rd point of contact works very well. Not as stable as using a brace, but COM shots out to 100 yards are quite reliable.

- OS

To prevent the rotation, add this 'locking collar' made by Odin Works onto the pistol buffer tube. It's the bomb. It's also useful for extending the length-of-pull of the brace. Or, you can by the Odin Works pistol tube combo that comes with this locking collar (which is what I'd recommend as it's a better deal than a KAK SuperSig tube+shipping, and the Odin locking collar+shipping).

Odin Works locking collar



Buy the Odin Works pistol tube and locking collar combo HERE