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The .300 blackout is the latest "in" cartridge amongst shooters. It's basically a shortened .223 (5.56 x 45) case shortened to 1.368" and blown out to handle .30 caliber projectiles. It's ballistics are close to the 7.62 x 39 mm Russian round in the same barrel lengths. It's big claim to fame is that it fits and functions in standard 5.56 NATO magazines, and AR platforms with the standard 5.56 bolt face.

It is a well balanced cartridge, gives good ballistics in barrels as short as 9", and makes an excellent sub-sonic round loaded with bullets in the 180 to 220 gr range and used with a suppressor. It is very popular for use in Personal Defense Weapons (PDW) and has an effective range of 100 to 200 yards depending on the bullet weight.

There is a lot of information on this cartridge on the Internet so I won't cover the history or load data here, but approximate ballistics are given below. Note that performance varies greatly between different load data. ...

Approximate .300 Blackout Performance
Bullet Wt	9" Bbl	16" Bbl
110	2190	2400
125	2050	2300
135	1700	2100
150	1600	1950
180 (subsonic round)	1600 (950)	1800 (1050)
200 (subsonic round)	1450 (915)	1650 (1050)
220 (subsonic round)	1300 (920)	1500 (1050)

Commercial brass is fairly expensive but you can make your own from 5.56 brass. There are a number of firms that offer reprocessed military brass but the quality varies a lot. The biggest problem is with neck thickness and concentricity. If you used reprocessed brass ensure that all the cases come from the same headstamp lot, and the neck thickness specifications are correct, and get a guarantee from the manufacturer.

If you have both 5.56 and .300 Blackout systems BE VERY CAREFUL not to mix up your .300 and 5.56 magazines. It is possible under some circumstances to chamber and fire a .300 round in a 5.56 chamber. If you do this you will have a VERY bad day. Clearly mark all magazines and AR uppers used for .300 Blackout and if possible use a distinctly different magazine, either color or brand.

As far as I know all the information presented above is correct and I have attempted to ensure that it is. However, I am not responsible for any errors, omissions, or damages resulting from the use or misuse of this information, nor for you doing something stupid with it. (Don't you hate these disclaimers? So do I, but there are people out there who refuse to be responsible for their own actions and who will sue anybody to make a buck.)