Mid range 4756 load for .223 - rambling thoughts of a shut in.

[Bill*B]

Creating a reduced load for the .223 Remington, with SR 4756 powder, armchair method.

In the first edition of his Reloading Handbook, Richard Lee posted a table gleaned from
Vihtavuori:

Inner Ballistics Coefficients:

Variable Change Change in Velocity Change in Pressure

bullet weight +10% -4% +8%

charge weight +10% +8% +20%

case volume +10% -3% -13%

temperature +50 degrees F +2% +4%

This didn't make it into the 2nd edition. I expect that it points to the right direction, but is a bit "rough and ready".

The 2nd edition of his Handbook does show pressure tested data for SR 4756 in the .221 Remington Fireball. 7.0 grains under a 70 grain jacketed bullet generates 51,100 cup.

The Fireball has 1.45 cubic centimeters of useful capacity; and the .223 Remington, 1.87 cc. Thus, the .223 has almost 30% more case capacity.

If the VV guidelines hold, 7.0 grains of 4756 in the .223 should generate around 37,250 cup in the larger .223 Remington case, with the same 70 grain bullet.

Since I would prefer to use an 11% lighter 62 grain bullet, pressure can be reduced by another 8% (yes, I threw away a full one % of bullet weight reduction for safety, and ease of calculation). The expected pressure becomes 34,270 cup.

Let's reduce the powder charge by a half grain, or around 7%. 6.5 grains of SR 4756 should produce 29, 475 cup. Even on a hot summer day, it should run under 30, 650.

Our reduced load for the .223 Remington is: 6.5 grains of SR 4756 under a 62 grain Hornady FMJ bullet.

My range is closed due to the current virus fears, so Lord knows when I can actually test it.

[Tim357]

So which powder are you using, 4756 or 4759? Big difference there

[Bill*B]

The faster of the two - SR 4756. It's discontinued, sad to say, but I have some of it.

Have loaded 8.8 grains under 55 grain Speer SP in the .223 with excellent accuracy and about 2,000 fps - no pressure signs - but when I checked old IMR data and saw the manufacturer listing 8.0 grains as MAX in the .221 Fireball, under a 50 grain bullet, at 50,000 cup (wow!), I did a double take.

VV calculations suggest that my 8.8/55 load may be getting mighty close to SAAMI maximums. This is out of line with measured velocity, but Lyman says that 8.1 grains of SR 4756 will produce 40,300 cup under a 54 grain CAST bullet in the roughly 10 % smaller .222, so when you take into account both a heavier bullet and a stiffer charge, that 8.8/55/.223 combination may well be hotter than I had thought, and I'm throttling back.

The VV approximations are not Gospel, but they may help us stay out of trouble when wandering into uncharted waters. I have to keep reminding my self that velocity equates to average pressure, but tells us little about peak pressure.

[Bill*B]

Thank God, the range is open again and I did shoot that 6.5 grain 4756 / 62 grain Hornady FMJ load. It was pleasant and clean burning, but only moderately accurate - 5 shot groups of 1.00", 1.00", and 1.25" at 50 yards, from my light M77 carbine. Oh, well.