So whats the deal with the Rattler. I think its cool, but in my head why not just use my MPX?

It seems with that short of a barrel you really can only use Barnes 110 tac-tx going about 1,750 fps, or Lehigh 194 ME subs going 800 fps. Nothing else will expand/fragment?
So, is it really worth it? Why not just use 147 subs in an MPX or 124+P+ going 1,400fps in an MPX?

I agree its "more firepower and more effective than 9mm" but is it really THAT much better? Why not get a 9" or 6.75" blackout MCX?

Those Barnes rounds are expensive to buy.

Am I missing something?

It was designed around a potential contract for the military, the military set the restrictions/requirements and it really was to be as small as possible and yet still be reliable. It is perceived that this is intended to replace the MP5.

Yes the Rattler is a bit constrained on what rounds will perform out of it but it does have an easier time at penetrating armor than 9mm does and they don't exactly have open access to the hollow points yet and typically have to use FMJ.

With the right load of 300BLK, it should still be able to get to 100+ meters, while still penetrating soft armor and having a little better of ballistics than 9mm FMJ.

They sell it to us as Sig has been on a kick lately of selling their contract weapons to the general public.

Replaces MP5K, MP5SD, MP7.

Offers rifle perfomance/ballistics within 200-300 yards. Reduced flash and blast compared to shorty 5.56 rifles.

I like its looks and feel better than MPX, but for the price point - It should come with a conversion kit for both 5.56 and 9mm! - It wouldn't be that hard for them to do that would it??  No such conversions (not hard is it barrel assy and some 9mm mags/bolt/adapter?)  300AAC is cool round for PDW but not for cost Much more practical for blasting so much as 9mm or 5.56/

I reload 300AAC and Barnes bullets alone are more than complete 5.56 ammo

FWIW, the Rattler ballistics are closer to 1900 fps for the 110gr TAC-TX, 1900 for the BH 125gr TMK, and 925 for the Leheigh 194gr.  The 110gr TAC-TX is the only load I would run in a Rattler for serious use, and should perform well within 100 yards...and by well I mean will balistically outperform any PCC.