With all of the new reloaders out there I thought it might be helpful to post a sample load workup from start to finish.

   Original White Paper on Optimal Barrel Time

    For a more complete version of Optimal Charge Weight (OCW) loading please see the links above. To put it in as short a version as possible the goal of OCW loading is to find a range of powder charges that shoot to the same point of impact and then tune the accuracy of the load by adjusting the seating depth. The result is a load that is accurate and insensitive to variations in temperature, powder charge, or small changes in internal case volume.

   I have pile of freshly acquired Nosler 155gr Custom Competition bullets in .308 that I wanted to work up a load for. I am interested in this bullet because it has a similar BC to the classic 168, but is 13 grains lighter so I should be able to push it a bit faster. In theory this should give me an inexpensive practice round with a bit more reach than a 168. My go-to powder in .308 has become AR-COMP because it meters well, is more temperature insensitive than even Varget, and it has given me good accuracy with every bullet I have tried with it. I checked load data from Nosler (bullet manufacturer) and Alliant (powder manufacturer).

    Nosler: Nosler brass, min: 40gr, max: 44gr, COAL: 2.800

    Alliant: Federal brass, 43.2 gr, COAL 2.775

   Alliant doesn't give min/max charges, instead they give "recipes." I have heard that these are max charges, but I've found there is often a fair bit of headroom above these charges. I will note that Alliant's recipes are usually pretty accurate loads themselves. I am loading using Winchester brass which has more internal volume than Federal or Nosler brass (less pressure for a given charge) so I felt I could go safely beyond the published data if I moved carefully. I want to be safe, but I also don't want a bullet moving at the speed of smell. I wanted to find the accuracy node somewhere at the higher end of Nosler's data. I decided to use increments of .5gr to work up to the range where I wanted to develop my load, and .3gr increments afterwards to find the OCW node. I load 1 of each of the pressure test cartridges and 4 of the load development cartridges. The reason I use 4 is because I concede that I am not a great shooter and sometimes yank the trigger and blow a shot.

    Pressure test charges: 41gr, 41.5gr, 42gr, 42.5gr, 42.8gr

    Charge weights for development: 43.1gr, 43.4gr, 43.7gr, 44.1gr, 44.4gr

    Because the loads of 44.1gr and 44.4gr were beyond the given max I need to make sure I am watching very carefully for pressure signs at these charge weights.

    At the range I fired my five pressure test charges without issue. I then shot my load development round-robin checking very carefully for pressure signs as I moved along. There were no pressure signs (flattened primers, ejector marks, cratering, hard bolt lift, etc) along the way. Here were the results:

    I am NOT looking for the tightest group, instead I am looking for a range of charges that all impact the same point. On targets 1 and 2 the POI is nearly identical. On target 3, if you discount the round I blew it has the same POI as targets 1 and 2. Moving on to targets 4 and 5 I notice that the POI has moved left and up slightly. As a second source of confirmation I look at the velocity number. For targets 2 and 3 the velocity increases by 17 and 18.5 FPS from the previous charge weight. On target 4 the jump is much greater at  26.5 FPS. My OCW node is at 43.4gr of powder.

    I used a COAL of 2.800 for my initial round of testing because it's close to the max length you can load into a P-MAG and I've found that the SMK-style bullets usually shoot fairly well at this length. For fine tuning the seating depth I took my powder charge of 43.4gr and loaded up 5 each at 2.810", 2.800", 2.790", and 2.780". I shot them round robin like the previous set.

    The first group was exactly the same as the rounds I had shot the previous trip. It looks like I may have pulled one of the shots in this group, but the accuracy is about the same as it was in the previous round of testing. The next group at 0.01" longer is even tighter. The group at 0.01 shorter isn't as tight, although (oddly) the group at 0.02 is pretty decent. There is probably a fair bit of shooter error here, but it looks like 2.81" definitely has the most potential, and this is confirmed by the almost as tight group at 2.800". So, I have my overall length of 2.81". The last two targets I used to check the zero shift with my suppressor on.

"Final" Load: 43.4gr AR-COMP, Winchester brass, Federal 210 primer, Nosler 155gr Custom Competition, seated at 2.810".

    To confirm this truly is an OCW loading I need to check the charge-weight insensitivity of this load. To do that I will load several rounds .3gr above and below my charge weight of 43.4gr and shoot a single group. If this load is truly an OCW load I should have a small group where the accuracy is minimally affected by the variation in charge weight.