Cutting .338 LM Barrel from 30″ to 17″ — Velocity Loss Revealed!
With Randy Wise’s recent record-setting ELR performance with a .338 Lapua Magnum Savage rifle, we thought our readers might be interested in a .338 LM velocity test conducted by our friend Bill Marr, Editor of Rifleshooter.com.
READ FULL .338 Lapua Magnum Barrel Cut-down Velocity TEST »
A couple seasons back, Bill did a fascinating barrel cut-down test on a .338 Lapua Magnum rifle. Bill, a skilled gunsmith who runs 782 Custom Gunworks, cut down the barrel from 30″ to 17″ in one-inch increments. During the cut-down process, Marr measured velocities at each barrel length, shooting four rounds after each cut. As you’d expect, there was a huge change in velocity from long to short. Speeds were measured at the muzzle with a Magnetospeed barrel-mounted chronograph.
If you’re looking for max velocity with the .338 LM, go long. With 250-grain Sierra MK bullets, the peak velocity Bill measured was 2942 FPS at the full, 30-inch length. This decreased pretty steadily down to 2547 GPS at the shortest 17″ length. That’s an average decrease of 30.4 FPS per inch from 30″ to 17″.
Bill also tested 300-grain Sierra MatchKings. This is interesting because Randy Wise ran 300-grain bullets (Berger Hybrids) in a 26″ factory barrel for his ELR record. Peak velocity was 2833 FPS at 30″, 2799 FPS at 26″, and 2492 FPS at the shortest 17″ length. Interestingly, velocity at 29″ was higher than at 30″ for the 300-grainers. Bill notes: “The 300 SMK load showed a slight increase from 30 to 29″. I’ve recorded this in other tests and it seems to be more common with a heavier load. I suspect it is primarily due to the small sample sizes being used along with the relative proximity of muzzle velocities in adjacent lengths.”
At each new (shorter) barrel length, velocity was measured with a MagnetoSpeed chronograph using two different loads, 250gr SMKs with H4831sc and 300gr SMKs with Retumbo. Four shots were fired at each length with each load, a total of 112 rounds.
Load #1: 250gr Sierra MK, Lapua brass, CCI #250 primer, H4831SC, OAL 3.720″.
Load #2: 300gr Sierra MK, Lapua brass, Win WLRM primer, Retumbo, OAL 3.720″.
.338 LM Barrel Cut-Down Test Results Summary
.338 Lapua Magnum with 250gr Sierra MatchKings
After shortening the barrel from 30″ to 17″, total velocity reduction for the 250-grainers was 395 FPS, an average loss of 30.4 FPS per 1″ cut. The amount of velocity loss per inch rose as the barrel got shorter, with the biggest speed reduction, a loss of 55 FPS, coming with the cut from 18″ to 17″.
Start Velocity: 2942 FPS | End Velocity: 2547 FPS | Average Loss Per Inch: 30.4 FPS
