I've got a blackout with effectively a 6" barrel... more or less, think of a bolt action style gun with a huge 1.75"x17" integral suppressor and a tiny 6" Ballistic Advantage barrel buried inside of it.  It's silly quiet.  

Unfortunately, the velocity is lower than I'd like.  I think  it's around 800FPS, but I'd like to be getting around 950-1000FPS.  Without handloading, is there a factory subsonic load  for very short barrels that's loaded hotter so you get extra velocity while still being subsonic?
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Also, while on the topic of ammo for 6" 300BLK barrels... what are some good supersonic loads for it?  I hear a lot of good things about 110/120gr Barnes black tips... Any other good loads that are more economical?

Why not just..you know...hand load?

I'm not aware of any optimized for that short of barrel because they wouldn't stay subsonic out of 16" barrels and people would complain regardless of whether they stated it was only for short barrels.

That and the fact 80% or more of the volume of 300blk is shot by hand loders there simply isn't a market for it.

Discreet Ballistics is what you seek. Delivery can take longer than ideal but they are worth it.

Select barrel length when ordering.

I would think that because 80% of a popular cartridge is handloaded... that would indicate there IS a market for massive improvement.

Sig makes a SBR version of the 120g.  Supposedly expands at lower velocity.  I've never been able to find any, I've never seen any videos of it, and as of a few days ago, I don't even see it on their website anymore.  

It was made specifically for the 6.75" (TACOPS)

Edit-I spoke too soon.

It's back on their website .  It gets right at about 1900 FPS from a rattler .



I handload for my Rattler and it's a loosing battle.  Most max loads are still 100-200 FPS slow than what you'd get out of an 8-10" barrel, but with the right loads and for the intended distances I plan to use it, it's just fine.

There are 2 other projectiles/ammo that expand at lower velocities in factory ammo that I know of.  The obvious Barnes TAC-TX and S&B makes a 110g version similar to Barnes.

I've been eye balling Makers and Cavity Back as they both have pretty good results from all the gel tests I've seen online - for hand loading.

Maker does, they make subs for "long" and "short" barrels.   But they have been OOS for about 2 years now.  

I was originally surprised to find I was only losing like 25 fps on 220gr subs between 9" and 6.5" barrels.  So unless you have chrono data to back it up, you are probably still over 900, possibly over 950fps out of the short barrel.

I will double check exact numbers tomorrow...

For the subs, I think Sig made some SBR-optimized subsonic duty rounds. Black cases with yellow plastic tips.

My recorded chrono numbers for 220gr SMK, 11.7gr AA1680:

12.5":  1051
9":  983 one day, 962 a different day
6.5":  973

You may have a different powder obviously... but I would chrono before concluding for sure you are losing a whole lot of velocity with subs from a very short barrel.  Possibly it is something else wrong in the calculation.
Personally I've never shot the subs past 100yd so I don't know how they compare to my calculated ballistic table past that.  I observe a 3 mil separation in POI between 125gr@~2000+FPS, and 220subs, at 100yd.

Instead of printing "subsonic" on the box print the velocity for 16" and whatever SBR length it's optimized for.

It's not really a "calculation" on my part, persay...  I'm just feeding the environment and bullet info into the hornady ballistic calculator with a 50yd zero, and then adjusting velocity until the 100/150/200 yard drops are approximately what I'm seeing in real life.  I was expecting to be around 900FPS in a 6" barrel with an advertised 1050FPS load... I'm just a little surprised it might be 800.

Honestly, as long as I know the drops I don't care too much about it other than knowing I might be leaving energy off the table by using factory subsonics in such a short barrel.  The 220gr are probably a better choice for energy retention and minimizing velocity loss due to barrel length... but the S&B 200gr are cheaper, I have a couple hundred already, and they make about 3" groups at 100yds.  Even better would be loading my own.