Short Barrel 338 Lapua Magnum Load Development Results

September 26, 2014 Bill Marr Cartridge Selection, Reloading 0

Designed to bridge the gap between 308 Winchester/7.62 NATO and 50 BMG, the 338 Lapua Magnum (8.6×70) has developed an impressive following with long range shooters.  When I built mine, I settled on a 22″ barrel, which is short for this cartridge.  Accuracy International does offer a 20″ barrel 338 Lapua from the factory, but you’ll notice most shooters running barrels from 27 to 30″ long.

Left to right, 338 Lapua Magnum, 300 grain Sierra Matchking, 308 Winchester, 175 grain Sierra Matchking, 223 Remington, and 62 grain M855 projectile.
Left to right, 338 Lapua Magnum, 300 grain Sierra Matchking, 308 Winchester, 175 grain Sierra Matchking, 223 Remington, and 62 grain M855 projectile.

The rifle was built with the following parts from Brownells (I will be covering its construction in another post):

Custom 338 Lapua Magnum rifle.
Custom 338 Lapua Magnum rifle.

After quite a bit of research I decided to try to develop an accurate and relatively fast (2600 FPS+) load using the 300-grain Sierra Matchking (SMK).  For comparison purposes, factory Lapua 300 grain ammunition is advertised at 2725 feet-per-second from a 27″ barrel.

I used four different powders: Reloader 25, H1000, Retumbo and US869 (I don’t post complete load information, however, if you take a look at the powder companies data you’ll be headed in the right direction).

Brass is new Lapua.  Primers are Winchester Large Rifle Magnum (WLRM).  Since my gun uses CIP length magazines, overall cartridge length is 3.708″, which is .020″ from the lands.

For each of the 14 loads posted below, I loaded 3 cartridges.  A MagnetoSpeed barrel mounted chronograph was used to gather velocity information.  All shooting was done prone, from a bipod, with a rear bag.  The target distance was 100 yards.

Please note, I understand the limitations of 3-shot groups, however, during initial load development, they can lead you to an optimal load, without unnecessary barrel wear and component cost.

Load Powder Primer Bullet Velocity (FPS) SD Group (in) Comments
1 Reloader 25 WLRM 300 SMK 2346 4.5 .859
2 Reloader 25 WLRM 300 SMK 2405 4.6 .988
3 Reloader 25 WLRM 300 SMK 2433 8.4 .748
4 Reloader 25 WLRM 300 SMK 2463 7.8 .365
5 Reloader 25 WLRM 300 SMK 2488 12.2 .823 MAX
6 H1000 WLRM 300 SMK 2607 8.5 1.340
7 H1000 WLRM 300 SMK 2641 14.2 1.009
8 H1000 WLRM 300 SMK 2673 4.0 .866
9 Retumbo WLRM 300 SMK 2733 15.3 .570
10 Retumbo WLRM 300 SMK 2764 7.4 .612
11 Retumbo WLRM 300 SMK 2798 10.0 .346
12 US869 WLRM 300 SMK 2576 31.7 1.052
13 US869 WLRM 300 SMK 2678 32.1 .860
14 US869 WLRM 300 SMK 2719 9.6 .314 MAX

I was shocked by how low the SDs were with Reloader 25.  Unfortunately, the velocity wasn’t there.  If I had a 26 or 27″ rifle, I would be exploring it further.

Reloader 25 provided exceptionally low SDs , however, the velocity wasn't there.
Reloader 25 provided exceptionally low SDs , however, the velocity wasn’t there.

H1000 had low SDs, however, accuracy was poor.

US869 offered a shockingly accurate load, but it had minor pressure signs and large SD.

This was the best group so far. .312" at 100 yards. I was shocked the US869 performed so well.
This was the best group so far. .312″ at 100 yards. I was shocked the US869 performed so well.

That leaves Retumbo (an elusive powder to buy lately).  With relatively low SD, high velocities and good initial accuracy.  None of the loads I tested exhibited pressure signs.  Plus, they didn’t seem to be that hard on the brass. I was actually shocked I safely broke 2600 feet-per-second with any of the loads.  Approaching 2800 feet-per-second with a 300 SMK from a 22″ barrel seemed like a dream.  I guess dreams can come true.

For the phase of load development, I’ll be working up some more loads with Retumbo and posting the results.

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