300 BLK load development: 125, 135, 175 and 220 SMK with H110 and IMR 4227

The 300 AAC Blackout (300 BLK) is a popular alternative cartridge in the AR-15/M16 M4 (AR) community.  Known for its ability to deliver heavy subsonic bullets for standard AR magazines while cycling the action in suppressed applications, it also works well with lighter supersonic loads, offering a similar external ballistic performance to the 7.62x39mm Russian (check out my 300 AAC Blackout Review from September 2012).

300 blk ammo only

A few manufacturers offer bolt action rifles chambered in the 300 BLK.  These guns have had mixed success.  The two key features that make the 300 BLK excellent for the AR platform, make it difficult to work with in a bolt action rifle; a short case and use of both super and subsonic  bullets.  The short case length doesn’t necessarily effect feeding, since the bullet of the loaded cartridge is similar in length to the 223 Remington, but it can make ejection difficult from many bolt action rifles.  On some guns, the short case can simply snap out from under the extractor if using a constant tension spring type ejector.  The ability to shoot a wide range of bullets is a mixed blessing.  If you aren’t worried about being on the cutting edge of accuracy, the fast twist needed to stabilize the heavier bullets, means you are spinning the lighter loads way too fast.  This can lead to poor accuracy from lighter loads.  Keep in mind that the benchrest crowd, the hot-rodders of this sport, feel the best accuracy is on the edge of stabilization.

300 blk custom rifle away

Prior to building a custom bolt action rifle in 300 BLK I did a lot of homework on loads and load development.  I found some commercial manufactures (mostly off the record) either discontinued bolt guns in 300 BLK, pulled them from the market after the product was announced, or declined to introduce them due to accuracy and feeding concerns.  Reading a few different reports of accuracy issues with the cartridge in general (you’ll notice many of the sub sonic groups published online are shot at 25 yards!- more on this later), I came to the realization, that most of the “good” groups found with the cartridge were shot with either a special chamber (not a 300 BLK which has a long throat to accommodate the heavy bullets) or a slower twist rate to work well with the light bullets (SAAMI specifies a 1:8″ twist, click link).

The design of the cartridge itself is a bit of an issue as well.  The low case capacity requires the use of powders with a fast burn rate like H110 and IMR 4227.  These are powders that are typically marketed to the magnum pistol crowd, not the rifle folks.

Left to right: 308 Winchester, 30 AR, 7.62x39, 7.62x40WT, 300 AAC Blackout 125 OTM, 300 AAC Blackout 220 OTM (subsonic), 300 AAC Blackout 125 Accutip, and 5.56x45mm

Left to right: 308 Winchester, 30 AR, 7.62×39, 7.62x40WT, 300 AAC Blackout 125 OTM, 300 AAC Blackout 220 OTM (subsonic), 300 AAC Blackout 125 Accutip, and 5.56x45mm

Since I wasn’t trying to build a wildcat 300-221, which is what a modified 300 BLK would effectively be, I went with a standard chamber cut by a Manson Reamer.  As far as twist rate is concerned, I wanted the flexibility of being able to shoot the light and heavy rounds, so I ordered a custom barrel from Bartlein with a 1:7″ twist.  Anecdotal accounts from other shooters indicated this would provide the best accuracy with the 220 grain Sierra MatchKing (SMK).

300 blk side

The rifle, as pictured above, includes the following parts from Brownells:

For testing and evaluation purposes, I used 125, 135, 175 and 220 grain Sierra MatchKings (left to right in photo below) over H110 and IMR 4227 powders.  The case manufacturer, Nosler or Jagemann (JAG), is listed on the table below.

DSC_0821

All shooting was done prone, from a bipod with rear bag.  Groups were 5 shots (unless otherwise indicated) at 100 yards.  The target was a 2″ orange dot printed on paper.

300 blk from magazine

Rounds were fed from a AICS style 223 magazine from Accurate Mag.

The loads shown below are only considered safe in the test gun!  Please take the time to read the disclaimer below and note that this data was gathered in a bolt action rifle and NOT a semiautomatic rifle.  Also note when examining published materials you will find a wide range of loads that are deemed safe.  Most of the reloading manuals show lower data, I suspect this is related to the wide variety of ways to form brass for the cartridge and how that may cause an increase in pressures.

For reloading information: WARNING: The loads shown are for informational purposes only.  They are only safe in the rifle shown and may not be safe in yours.  Consult appropriate load manuals prior to developing your own handloads.  Rifleshooter.com and its authors, do not assume any responsibility, directly or indirectly for the safety of the readers attempting to follow any instructions or perform any of the tasks shown, or the use or misuse of any information contained herein, on this website.

300 blk loads H110 and IMR 4227 bolt gun

Examining the table above, note the wide range of accuracy, velocity and standard deviation achieved with the different loads.  The best 5 shot group was .334 MOA while the worst was 3.787 MOA!  The 1:7 twist worked fairly well with the heavier, 220 SMKs, with groups ranging from 1.082-1.477 MOA.

300 blk target 1

Note: In the target above, the groups labeled “6.5 123” were shot with a 6.5 Creedmoor and are not relevant to this post.

300 blk target 2

300 BLK target 3

The targets above represent the loads shown in the table.  By cross referencing the group size shown on the target with the group size in the table, you can determine which group was shot by which load.

135 SMK .350 inch at 100 yards

This is the best group achieved with the 300 BLK and 135 SMK at 100 yards.

The 300 BLK is an interesting cartridge for the rifleman.  In many ways, it is the anti-rifle cartridge.  Rather than having a steep shoulder and overbore case, like the 20 Vartarg (a 221 Remington necked down to 20 caliber), it has a relatively straight design that requires fast magnum pistol powders.  The twist rate needed to accommodate the heavy subsonic bullets makes finding accurate super sonic loads difficult.

On the up side, firing the 300 BLK from a precision rifle is a pleasure.  The gun doesn’t move, there is even less recoil than an AR because you don’t have the mass of the bolt carrier reciprocating in the rifle.  Running a rifle like this suppressed would be a hoot.  I understand why the 300 BLK caught on in the AR and I also understand why didn’t take off as much in the bolt guns.

One of the benchrest guys I know asked how I liked my 300 BLK rifle.  I told him about my load development experience and he asked why I didn’t use a 1:14″ twist and a reamer with a short throat like he planned on doing.  I explained to him he wouldn’t really have a 300 BLK anymore.

I like my custom 300 BLK.   With the right load and A LOT of tuning, the rifle shot well with both supersonic and sub sonic offerings.  I plan on doing more work with it and reporting back in the future.