KrisFox and his MK262 MOD3 5.56 Ammo Development Process

[KrisFox 69]

First of all I want to say, I do not advise anyone to attempt to load ANY ammunition beyond the pressure levels recommended by reputable component/ammunition manufacturers.

Even if you are watching brass and chronograph readings to estimate pressure you are getting only an estimate ie a good guess as to what pressures you are dealing with.

So now to the fun stuff.

 

Development of what I call MK262 MOD3

 

First I selected the Berger 77gn OTM Tactical as people loading it have reported better accuracy and soft tissue damage than the SMK.

I started with min loads and worked up just like anyone should when developing a load.

I used Lake City brass 1.759" max length

Rem 7 1/2 BR primers

8208XBR

Berger 77gn OTM .224

I debated using a LFC to crimp but I had good neck tension while loading the Bergers into the cases and I had no setback during testing in spite of my test rifle having a 2X power buffer spring, heavy buffer and mil spec weight bolt carrier assembly.

My barrel is 16.125" 1/7 twist with 5.56 chamber, gas system is adjustable and was tuned for optimum operation with Black Hills white box "seconds" 77gn 5.56 Match.

SAMMI uses a min spec chamber and barrel for .223 for pressure testing and place a strain gage at the mid point of the chamber.

NATO calls for a min spec 5.56 chamber and measures pressure at the case mouth.

I used Black Hills MK262 MOD1 and its Nato Max Pressure spec of 58700psi to calibrate my strain gage located 0.25" closer to the breech from the case shoulder on my rifle.

I fired 5 known rounds of BH MK262 MOD1 and used the difference between the highest reading and Nato Max to weight my pressure readings.  

I also fired 2 more that I got from a "friend" that were rumored to be second hand from actual mil inventory but I have know way to confirm or deny the claim nor would I due to legality issues.  I assume they were just more of the BH seconds...

Since my chamber and bore are not NATO min spec actual pressure should be slightly lower than the readings I got.  Now for some pretty graphics.

First 5 rounds are BH Seconds last 2 are Rumored MK262 MOD1











NOTE THE BOOK MAX FOR 8208 and 77gn SMK is 23.2gn!!!

My strain gage (adjusted with the goal of duplicating SAMMI pressure data from a min spec test fixture) confirms Loads above this point are over 55,000psi!!!  SAMMI MAX for .223



With a minor adjustment + .1-.2gn this load would duplicate MK262 in my rifle with less pressure.



The following load produces about 50 more fps than MK262 with similar pressure.



The last pic is the fired brass, 5 of each load left lowest right highest, last row of 5 is BH MK262

 

I have 30 rounds loaded with 24.2gn 8208XBR 15 LFC crimped and 15 not crimped.

 

I plan on getting 10 round strings for each than 1 10 round string 5 of each on same string to show any difference between the two.