This blog article is combined from several emails from Doc Gunn.

Thomas also agrees & loves the 300 blackout for the reasons listed here.

  1. 556 is only 55-67 grs, while a 300 blackout/blk is 110-220 +/- gr bullet
  2. you can use your standard ar15 mags for 300 blk
  3. same bolt, same mag-well, same everything other than you just simply add/swap for a 300 blk upper
  4. suppressed the 300 blk is a joy and almost no felt recoil. subsonic ammo suppressed, hopefully we never NEED anything like that, but it is fun at the range.
  5. MAKE SURE you NEVER put a 300 blk cartridge into your 556 upper. CLEARLY label every 300 blk magazine and NEVER put 556 ammo in those specially designated mags. I have camo dutch taped wrapped around my 300 blk mags and I have them labeled with a lable machine “300 blk”. I also label my 300 blk upper on both sides.
  6. Ballistically, the 300 blk has better ballistics & wind “bucking” ability than the 556/223.
  7. Thomas Recommends a “Freedom Rifle” for everyone in your family chambered in 556/223. After than, you should get at least 1 300 blk upper.



If you put a 30 cal bullet into a 556 chamber/barrel, it will explode damaging the gun & hurting the operator.

some will chamber, if fired will explode your gun/face.

some will chamber, if fired will explode your gun/face.
Doc Gunn Recommends designating “300 blk mags”. For example:

You will never have an issue so long as you never use 300 blk in your ar15 upper.


WHY you need a 300 blk

Nice thing about the 300 BLK is all you need is a dedicated upper, which can be had for around $300-350.  Should also have dedicated magazines that are visibly and physically distinguishable to prevent 300 into 5.56 chamber, which can result in a detonation, destroying the weapon and posing a serious danger to anyone near.

Ammo can be made from .223 cases and any .308 rifle bullet, including hardcast for low-velocity rounds.  Pistol powders are used as propellants.  300BLK reloading dies are cheap.  ($40.00)

As for ammo availability in a worst case scenario, put your 5.56 upper back onto the lower receiver.

Total cost of conversion is no more than another quality self-defense pistol.

As for a superior caliber, I still prefer the 6.8 SPC.  The bolt for the 6.8 is different than the 5.56 (although they both use the same bolt carrier.)  There are dedicated magazines for the 6.8.  I have not tried to load 6.8 ammo in a 5.56 magazine, but the 5.56 mags might work satisfactorily.

I have uppers in 300BLK, 6.8 SPC and 9x19mm.  (9mm needs a $180 magwell conversion block and colt-style 32 round metal 9mm mags from Brownells.)

I REALLY like the 9mm in an AR platform!  Great practice round for ARs, and safe to shoot steel.  (115 at 1400 fps vs 1160 from a 4″ barrel.)

I recommend a multi-caliber AR pistol lower, then adding a 9mm PISTOL upper from Rock River Arms.  SWeeeeT!  Also good to suppress a 147 9mm at no more than 1000 fps

Doc Gunn’s recommended 300 blk ammo:


My best personal handload for the 300 BLK is a Barnes 110 grain TTSX under 20.0 grains of H-110 (Rem 7.5 primers, cases trimmed to 1.368” and an oal of 2.245”) at a chronographed 2,324 fps and 1,312 ft-lbs energy at the muzzle (still has 2,046 fps and 1,023 ft-lbs at 100 yards.)  This was from a 10.5″ barrel. This load is still going 1,546 fps and has 583 ft-lbs energy at 300 yards.  It is an excellent accurate hunting round with both expansion and penetration.


My Other 300 Handloads:
110 Hornady V-MAX should be similar to the 110 TTSX, but a with more fragile, fragmenting bullet that is less expensive to load in bulk.  It is an excellent self-defense load when penetration through barriers is not required.  It is available from several sources (including HSM) as a factory load at reasonable prices and is very accurate.
17.0 grains of VitaVuori N100 gives an estimated 2,200 fps using a 110 grain projectile.  (It is bulkier than H-110 and not as much will fit into the 300 BLK case.)
Cheap Blasting, Fun, Courses:

150 gr Hormady BT-FMJ loaded to an oal of 2.122″ under 17.5 grains of H-4227.  Rem 7.5 primers.  (Not chronographed, but it is powerful and very accurate with no pressure signs.)

SUBSONIC Loads (from reliable Internet sources):
Accurate Powder Data: (We might want to decrease these loads slightly as the claimed velocity is around 1,050 fps. I’d try 10.6.)
220 grain Sierra HPBT MK at 2.089″ oal, maximum of 10.9 grains of Accurate-1680.
210 grain Sierra HPBT MKL at 2.170″ oal, maximum of 10.9 grains of Accurate-1680.
Hodgdon Powder Data:

220 grain Sierra HPBT at 2.260″ oal (Probably cannot be loaded that long and still fit into mags?  I do not load longer than 2.245″.) 10.5 H-4227 or 9.9 grains of H-110 (or Winchester 296 which is the identical powder to H-110.)

Also an excellent link from Sierra bullets:


This Sierra link mentions 2 excellent loads for the 300 BLK using a supersonic 125 grain bullet that will closely approximate 7.62x39mm ballistics with velocities of 2,200 fps compared to the 2,400 fps we expect from the Soviet load.  The hunting version uses their 125 grain SPT Pro Hunter, loaded to an oal of 2.000″, under 18.7 grains of H-4227 or 17.2 grains of Lil Gun.


Freedom Munition’s 147 grain FMJ 300 BLK is reasonably-priced loaded ammunition but is very inaccurate, grouping 4+ moa in my 300BLK which is capable of at least 1-2 moa with good ammunition.  Buy it only for cheap blasting ammunition or as an alternate source of 300 BLK cases.
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One Response to Why you should consider a 300 blackout for your ar15

  1. Doc Gunn says:

    Great job Thomas!

    Thanks for your work Spreading the Sunshine! Not only in Firearms and Training but in other areas of self-reliance and self-sufficiency as well.



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