what is the shortest 300 blackout suppressor on the market

Well, not sure about the shortest, but the effectiveness of a suppressor relies on 2 things...total volume and number of baffles. You can have a REALLY short suppressor that measures 4 inches long, but it could have a 3 inch diameter (28.26 cubic inches) and maybe 2 or 3 baffles and be similarly effective as a suppressor that is 8 inches long, 1.5 inches in diameter (14.13 cubic inches) and 6 baffles. However, that short, fat suppressor will look ridiculous and might interfere with sights. No one would buy it.

As for me, I have a SAS Reaper that is 7 inches long, 1.5 inches in diameter and I think was designed for .300 blackout that is rated to take 300 Win Mag pressures. I use it on a 6.8 SPC and it works very well for me.

Wilson Combat does make a shorter/fatter suppressor to look into.

The AAC SDN-6 is one of the best choices.

LG, that 4"x3" suppressor would look remarkably like an oil filter.

MErl, you're probably right and honestly, I hadn't thought about it. I was pulling numbers out of my....well, the air.

Probably the Liberty Chaotic, measures 6x2 inches.

http://www.silencershop.com/shop/liberty-chaotic/

Gemtech 300blk 6.7 long x 1.5 diameter

The TBAC has a 5 inch lightweight suppressor that works great for subs if you're not concerned about ultra quiet for supers.

https://thunderbeastarms.com/products/30pss