what is the shortest 300blk barrel I should get? I want something as short as possible but effective. I want it for a personal defence weapon and want it to be able to reach out and hit hard at least 200Yrds.

Anything in the 8"-10" range is fine.

I went 10" because a 10" 300 BLK has about as much muzzle energy as a 16" 5.56.

Sort of amused by the 200-yard personal defense concept , but seriously I think going as short as a seven inch will meet your needs.

Here's a possible approach to the problem:

Determine what bullet(s) you would want to hit with and have expand at 200yd.  Find out from the bullet maker what velocity is needed for expansion (for example:  Sierra 125gr pro hunter needs 1700fps; Nosler 125gr ballistic tip needs 1800fps.)  Now, determine approximate muzzle velocities obtainable with different length barrels and run a ballistics program to determine what velocity you will get at 200yd.  If enough velocity to expand the bullet, you will have your answer.  I use the Nosler 125gr for deer hunting.  Even with a 16" bbl, the calculations show the bullet should only expand out to around 190yd, so 200yd is gonna be sorta "ify".  Bottom line, if using something other than Barnes copper bullets designed for the 300 Blackout, I don't think you can get reliable expansion at 200yd from any short barrel 300 Blackout.  Using the same Nosler 125gr bullet out of my SBR (9.5") my load is only good for expansion out to about 160yd due to less muzzle velocity from the shorter barrel.

Excellent post.

I am going for a10.5 barrel. How reliable is a 7.5/9.5 inch barrel  with cycling?

No problems whatsoever.  You really need to throw out all of what you know about 5.56 when looking at 300 BLK.  Short is just not a problem with the 300 BLK at all.  Of course, you want to pick a good manufacturer, but as far as the physics of cycling goes, a 300 BLK gets all it needs for reliability from an 8" barrel.

Some manufacturers tune their barrels so that they don't reliably cycle subsonic loads without a suppressor.  This is independent of length.  You just need to know what you're expecting to do with the barrel and pick one that matches your needs.