First question,
Debating whether to build my "battle rifle" to be an SBR(pistol until i can stamp it), or 16". Defensive/ If SHTF kinda rifle.

Next,
With this, im researching barrels. What should I go with?

Rainier Arms CHF Mountain series, Daniel Defense CHF, or BCM BFH CHF barrel?

DD is cheapest, next Rainier, then BCM. All CHF and chrome lined GOVT profile.

How far do you need to shoot? 16in has better ballistics than 14.5,than 12.5, than 11.5, than 10.5/3. Shorter than that, go to a 300blk. Most people find a 12.5 to be a great all around, but if you need to take shots at more than 200 and change then you may be wishing for the ballistics found in a 13.7-18. I personally run a 13.8 pinned to 16.1in.

As for which barrel, balance out your weight and wallet. The DD's are known for being accurate despite the lightweight profile, if you're going M4 profile I question why, so on and so forth. Rainier has some heavier profiles that are great.

Personally, if budget wasn't a concern, in my area (suburbs) I'd want a 12.5" Noveske if I was sticking with 5.56, launching 75/77grn bullets. If I was in a more rural and less populated location that afforded longer shots and likely lower volume of fire, an 18in fluted and/or mid/light profile would be nicer.

What barrel is going to depend heavily on what ballistics you need, whether you want/need something that's nice to carry or nice for/capable of higher volumes of fire, and what budget you have.

Do you plan to suppress? Do you plan to keep engagement range under 200 yards? Then make it a pistol.

Major benefits to a longer barrel are longer gas system for softer recoil, longer sight radius if you are using irons, and higher velocity delivering any given muzzle energy or transsonic range further out. If none of those are important to you then shorten it up! Just plan to get a can too because short barrels are obnoxiously loud, especially if you shoot indoors most often.

I am a big fan of 18" barrels, and not going shorter than a 16" barrel unless I'm SBRing a 12" barrel.

I live in the northeast where my surroundings vary from city to mountains. I'd expect most engagments to happen under 300yards, lots of trees here. But i want a good all around fighting rifle. Id expect to be shooting 55-62 grain military ammo.

I love all my SBRs, but if I were you, I would go 14.5, pinned, for the rifle you sort-of described.

I have 14.5s with Rainier Match, DD, and BCM.  The Rainier is my fav, just because of accuracy.  Pick either the Rainier, or the DD IMO

I have gone through quite a few Ar15's, and have settled on 18" barrels with a rifle length gas system as being the best compromise of compact, velocity, and with the rifle length gas system and an adjustable gas block, they are almost as smooth shooting as a 20" rifle.  I'm just not a big fan of the shorter gas systems.   If being super compact is a huge priority, I would go with a bull pup rifle like a Steyr Aug, FN FS2000, or something like that.

Yeah, I agree with the above post.

If you're going to stack and depend on 55/62g ball, then go with a 14.5" minimum length AR or a bullpup.

I like the idea of a pistol build (SBR/compact gun), but outside of a truck/trunk gun or range toy I see no point to have them. Unless you live in really tight quaters a 16" seems like the perfect do it all barrel length.

I'm a fan of DD barrels for few reasons;  they come ready with pre-pinned tapered pin to have the most secured method on the gas block, cold hammer forged, chrome chamber, and smooth barrel extension, very accurate, great finish and little over gas port, smooth shooter and price is right and much better if you get them on sale. (Got mine from aim surplus on sale around thanksgiving days last year)

I say 16"  for an overall applications.

I would stay within the realms of 10.3" - 12.5" for a short-barrel in 5.56. If you plan on going any shorter I would go 300 blackout. For barrels I'd stick with BCM, DD, or Noveske. If you are deadset on an SBR I would run it with an SBA3 as a pistol until your paperwork comes back. Or just leave it as a pistol. To be honest in this day in age with braces pretty much functioning just like normal adjustable stocks there isn't much reason to go through the hassle of SBR'ing anymore unless it's just what you want to do.