What's the best barrel length for suppressed? Piston or DI? I already have a Specwar 7.62 can that I will use.

I think the can is rated to 7" and will still be covered under warranty. Will you be shooting subs, supers or both? Do you care about velocity?
It is also important based on the ammo you decide to use to make sure you use a barrel length that will burn the powder in its entirety and I think it has been decided 8.5 and longer will do just that.

Mostly Subs think max range I would shoot is 200ish yards

8"

"DI"

your welcome

I'm using an 8.5'' KAK Industries barrel ($100 - on sale right now) and it runs like a top. My Specwar is pending

THIS

what ever barrel you decide on you better get a scope with alot of travel . A 225 grain hornady match bullet leaving at 1020 fps drops right at 16 moa at 200 yds from a 100 yd zero .  I have confirmed this with a chronograph , tennis shoes and a folding ruler .

Mine is 8, DI. Freaking love it.

I use a 8.5" Aero barrel (pistol gas) and a YHM PhantomLT.  

8" Ballistic Advantage DI. Shooting suppressed .300 BLK makes me want to never shoot 5.56 again. It's too fun. I love SBRs and cans so .300 BLK offers the perfect vehicle for this.

Both Piston and DI work great. DI is going to be a little dirtier than the piston running it suppressed.  You may have problems shooting subsonic rounds unsuppressed with a piston, while that will not happen with DI. There are trade off with both types. Personally I like the gun to run cleaner so I lean toward a piston but it can get a little more expensive. All of mine are 8 inch barrel SBRs and I have been happy with them.




8 inch Syrac Ordanace Piston YHM billet with YHM 7.62 LT and DI AP SBR build with a specwar

It is not decided. In fact that statement is very misleading.  Pressure is being applied to the projectile past 16". When you see factory FPS specs on ammo it is a 16" barrel not an 8". Now... Depending on several factors. The powder could actually have burned up in 8-10 inches but the gas is still expanding behind the projectile. So....regardless of all the powder burned or not you will not get commercially listed FPS ratings in a 8,10, 12 inch barrel... @ 8" you are going to loose 200+ FPS. of course the decline lessens as the barrel length goes up.
So now you are launching a subsonic 220gn @ about 800FPS in stead of 1050FPS.

Suppressors and quiet !  
Suppressors- Volume is your friend. Captureing expanding gas from the end of your barrel is its purpose. By disrupting/re directing the flow
Through the interior of the suppressor the once supersonic gas flow is slowed down and kept contained until it exits at the end subsonic. If you overload the volume available in the suppressor then you get more sound out the end of it.

Quiet- this is a function of a Suppressor with a volume sufficient to capture the bulk of expanding gas created by whatever cartridge you have chosen and capturing that gas right at the point that expansion is no longer taking place. Captureing gas that is already subsonic takes less volume than an expanding supersonic fireball still raging inside the suppressor. So.. If you use a combination of barrel length sufficient to reach maximum projectile velocity at the point the pressure is just starting to decline and... Capture all the expanding gas remnants from the end of the barrel then that is as quiet as it can be.
How you reach this goal is ??? Short barrel, long suppressor. Long barrel, short suppressor. ? Short barrel you loose some exterior and terminal ballistics. Long Barrel you loose convenience.
IMO--- Longer barrel for better ballistic advantage makes better long term sense. To each their own.

The whole thing is a trade off. With 300bk you really have to decide what you want out of it.

Like the prior poster wrote, what you want to do with it would be helpful to know.  If you want a range toy, by all means go the 8" route for maximum giggle/coolness factor.  If you want something for hunting at close range, perhaps an 8" will suffice.  If you plan to stretch hunting shots to the limits of the range of the cartridge, it might be prudent to pick a longer tube.  It's mostly a set of trade offs, so pick your most important criteria and make your selection based on that.

Do you have friends with Blackouts that you can test drive?  I thought my 10.2" barreled range toy was nice and short with my can attached until I shot my friends 8".

Blackout isn't terribly picky about barrel length.  Just about every load will burn all its powder within about 9" of barrel, so barrels from 8-10" are quite popular.

I've only run Blackout in an 8" DI upper, but I can't see there being a big difference functionally between DI and piston, except for the possibility that some piston blocks might give you some flexibility running suppressed (my Adams block has a setting that reduces gas volume (and thus back pressure as well) for running suppressed - it also has an "OFF" position for really quiet shooting).

The first step to running a suppressed 300 Blackout gun is getting a 300 Blackout gun that runs.  Not at all hard, but a lot of people start out overthinking stuff and have problems.  Build/buy the gun and make sure it runs unsuppressed, and THEN put the can on it and adjust accordingly.  DON'T buy extra-heavy buffers, adjustable gas blocks and tons of other stuff until - and UNLESS - you have problems that require these things.

I agree with what your saying about factory ammo ., but if you reload and have a chrony its not that hard to run subs at 1050 fps out of an 8.5 inch barrel it just takes time to get the right load . on the other hand super-sonic will pretty well be  a lower velocity even if you reload  2100 fps is about the best I can get with a 110 grain varmageddon out of my 8.5 inch gun but I hit around 2315 fps with my 16 inch gas gun.