Load Development – Grouping, Velocity & Pressure

When developing loads, I have been using start and maximum load data as published by powder and bullet manufacturers. And cross referencing with various experiences of people as posted on message boards such as the THR, and comparing them to published data.

I assumed that I should work up from a published starting point until I found the best grouping at the highest velocity, without exceeding the maximum load listed for that particular powder/bullet combination. And then of course I would inspect my shell casings and primers for signs of pressure.

But what I have not considered until now is the published maximum pressure indicated for the maximum load; and factor in the test barrel length as it compares to my barrel length.

I found an Excel spreadsheet at closefocusresearch.com for “Calculating Barrel Pressure and Projectile Velocity in Gun Systems.”

Since I use a chronograph, I decided check my results using the above referenced spreadsheet.

For this example, I recently tested .223 Rem using A2520 powder, Hornady 75 gr BTHP-M bullets, WSR primers, mixed FC cases from 1.740” – 1.750” and a COL of 2.25” with a light taper crimp. My gun is an AR-15 with a FN 5.56 CL 16” barrel with 1/7 twist, and running a 5.56 suppressor. On a 72* F, overcast and humid day, below are my results, 5 shots each at 50 yds:

23.2 gr – Avg 2499, ES 52, SD 19, 0.635”
23.5 gr – Avg 2534, ES 48, SD 19, 0.980”
23.8 gr – Avg 2569, ES 34, SD 15, 1.500”
24.1 gr – Avg 2601, ES 91, SD 33, 1.200”
24.4 gr – Avg 2642, ES 32, SD 12, 0.600”
24.6 gr – Avg 2526, ES 334, SD 130, 1.600” (WTH – something not correct here)
24.8 gr – Avg 2685, ES 21, SD 8, 1.970”

My best group was 24.4 gr at an average speed of 2642 fps, and I could not see any signs of over pressure. However, using the above referenced spreadsheet and assuming that it is accurate; at 2650 fps and a 16” barrel, I would have almost 68,000 psi with a 75 gr bullet. Using Western Powders’ published max pressures (see below), the spreadsheet says my max fps for .223 REM at 54,602 psi would be 2475 fps. And for 5.56 NATO at 61,890 psi it would be 2529 fps.

Below are the published numbers for this powder/bullet combination from Western Powders 7th Ed.:

• .223 Rem: Start 22.2 gr (2610 fps), Max 24.7 gr (2863 fps), Max Pressure 54,602 psi - 24" barrel, 1/12 twist, WSR primer.
• 5.56 NATO: Start 23.7 gr (2764 fps), Max 26.4 gr (3039 fps), Max Pressure 61,890 psi – 24” barrel, 1/7 twist, WSR primer.

If I am pushing a 75 gr bullet out of my 16” 1/7 barrel at about 2650 fps, I am taking a risk that it is way over-pressured, even though I cannot see any signs of over pressure? If anything, maybe accelerated wear on my barrel, etc?

Would I be better served with the best group at a velocity that does not exceed published pressure for my powder/bullet combination? For this example should I settle with 23.2 gr with an Avg of 2500 fps? This would fall within the range of maximum pressure for both .223 REM and 5.56 NATO.

I am sure there are other variables that I have not considered or completely do not understand.

Blue68f100 said:

I have some loads developed in the 70's that are now shown to be over pressure by published loads. I have no indications of over pressure, so I leave them as is.

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I appreciate the feedback. I found an article mentioning the same thing, that many currently published maximum loads were higher in the past.

I also found a tech article from Hodgdon titled - DOES MY SHORTER BARRELED RIFLE REQUIRE DIFFERENT RELOAD DATA?

The first thing to remember is that the chamber dimension does not change based upon application. A 223 Remington chamber is the same whether the gun it is in is a handgun or a rifle. The chamber dimension determines the pressure. So, the pressure is the same when fired in that chamber in a rifle or a handgun. The barrel length has no impact on the chamber pressure and hence the reloading data (powder charge and pressure).

The length of the barrel will change the actual velocity you observe. In general as the barrel gets shorter than standard the velocity will lower, and as the barrel gets longer than standard the velocity will increase. This does not change the reload data (powder charge and pressure), just the velocity.

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And another thing occurred to me after my original post. I had chronographed Hornady’s 75 gr BTHP Superformance Match factory rounds (P/N 80264). They are advertised as 2930 fps at the muzzle using a 24” test barrel. My 5 shot average from a 16” suppressed barrel with 1:7” twist on a 68 F day was 2636 fps. This is almost identical to my hand load mentioned earlier in this thread. Obviously Hornady would not sell a factory round that if used in a 16” or shorter barrel would be significantly over pressured.