338 Lapua Magnum Load Development

By Mark V. Lonsdale, STTU

While the following information is based on current load development with .338 Lapua Magnum, it applies equally to load development for any caliber.

When evaluating any product, whether it is ammo, barrels, actions, scopes or stocks, it is necessary to first eliminate any of the other variables in the platform. In other words, for ammunition development, it is necessary to ensure that the rifle is accurate and has a proven capability to shoot 0.5″ groups at 100 yards and sub-MOA at longer distances. If the rifle is not capable of repeatable precision accuracy, then the ammo testing becomes flawed and invalid.

20200213_Acc Lab

.338 Lapua Magnum built on a Stiller’s TAC338 action, Bartlein Heavy Palma barrel, Piercision Muscle Brake, bedded into a McMillan A5 stock, with an Accu-Tac bipod. Scope is a Nightforce ATACR 7-35x56mm MOAR. 

The next step in load development is to document everything including: loading components, COAL, muzzle velocities, extreme spreads, standard deviation, and group size. Range testing should be done under the same conditions from the same position. Again, the goal is to eliminate the variables.

One error that many shooters make is not documenting and tabulating every load and group. They tend to ignore the bad groups and only record the good groups. You will hear shooters bragging about the half-inch group that they shot, while neglecting to mention that the other nine groups were all over one inch. The other error is stopping when they hit that first half-inch group and not shooting another four groups to see if the first group was an aberration or is repeatable.

One of the interesting points that came out of the .338 Lapua testing is that virtually every load from 92 grains to 100 grains of Vihtavuori N170 with 300 SMK bullets grouped sub-MOA, with a large number being 0.5″-0.6″

5-shot groups; 98 grains N170 with 300 SMK (left); 100 grains N170 (right) both with Peterson brass and Federal Magnum Match primers (215M)

Warning: Anyone working up a load for their rifle should start with a conservative load and gradually work up to the desired accuracy and velocity while watching for signs of over pressure. Two indicators or over pressure are the bolt handle becoming difficult to lift and flattened primers.


It is important to document all groups, even the ugly ones. Keep in mind that on some days it is just the shooter throwing fliers. Shooting tight groups requires total concentration and and a stable, repeatable, natural shooting position. All these 5-shot groups, using H50BMG, were shot off an Accu-Tac bipod. The first group was the 94 grains, on the right, which required re-zeroing the scope.


First group with Vihtavuori N570 pushing the 300 SMK. This load showed signs of excessive pressure and the group is not up to standard, so future test loads will have less powder.

Final word of advice – if the rifle is not a match-grade precision rifle with a proven capability of repeatable 0.5 MOA accuracy, then it is unrealistic to expect half-inch groups. The STTU formula for success is Stiller’s actions, Bartlein barrels, Piercision brakes, McMillan stocks, and quality optics such as the Leupold Mark 5HD or Nightforce ATACR. Most of the shooting is done at 20-25X magnification and off a bipod.


.375 CheyTac (rear) and .338 Lapua Magnum (front), both with Stiller’s actions, Bartlein barrels, and McMillan stocks. Badger M5 bottom metal and Accu-Tac bipods.  


About Mark V

Dedicated shooter, seeker, traveler, teacher, trainer, educator
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