March 15th, 2010

IMR 8208 XBR Performs Well in .308 Win Testing with 155s

We have a “Palma/Tactical” project gun in the works built with an Eliseo chassis, Borden action, and one of Krieger’s very first 5R barrels. Initial load testing with Hodgdon Varget showed excellent accuracy potential, but we weren’t achieving the velocity we hoped to get from the 30″ medium-Palma-contour barrel. With Lapua brass and Varget we ran short of case capacity when seating bullets to mag-length, and the best accuracy came at about 2860 fps, well under competitive Palma velocities.

Mark LaFevers with .308 Win Eliseo Tubegun on SEB NEO front coaxial rest. Mark made the wood front bag-rider.

IMR 8208 XBR powder

Chris Hodgdon kindly shipped us some H4895 and some of the new IMR 8208 XBR powder, lot #1022510-4798. In beautiful 70° conditions yesterday, we did a quick pressure work-up with both powders to determine “practical max” loads with the Berger 155.5gr Fullbore bullets (#30416). The 155.5s were loaded in once-fired Lapua brass with CCI BR2 primers, to a COAL of 2.860″, which is about .010″ short of land contact.

Varget-beating Speeds from Both IMR 8208 XBR and H4895
We loaded up two rounds at each charge weight, starting with the Hodgdon’s recommended starter load for a 155gr Sierra HPBT, as found in Hodgdon’s online Reloading Data Center. (There was no load listed for the 155.5gr Berger). It’s good that we started low because we saw immediately that both the 8208 XBR and the H4895 were yielding much more velocity, grain for grain, than Varget. Though we were not shooting for groups, the 8208 XBR also seemed to have a very large accuracy load window. Moreover, the velocity spreads for each two-round 8208 charge (above 43.0 grains) were remarkably low — none were more than 6 fps, and the velocities on our two 44.0-grain shots were exactly the same — 2992 fps. (This was NOT an error — the chron registered both shots #7 and #8.)

IMR 8208 XBR powder

Of course, you can’t conclude much about ES/SD from just two shots, but the speed consistency in the 8208 was notable as you can see from the chart above. By contrast, each pair of H4895 shots varied in speed much more. For example, the two-shot ES for 43.0 grains of H4895 was 14 fps, while the two-shot 8208 XBR spread (for 43.0 grains) was 4 fps. It will be interesting to see if further testing confirms the low ES/SD potential of IMR 8208 XBR in the .308 Winchester.

We dispensed charges with an RCBS ChargeMaster calibrated and leveled on a granite bed. The 8208 XBR has smaller kernels than either Varget or H4895 and, as we expected, 8208 XBR dispensed very easily. The H4895, with longer kernels, dispensed fine in the ChargeMaster, but it took the machine more time to trickle the H4895.

As noted, we didn’t shoot for groups, but the IMR 8208 looked like it would shoot really well at anywhere from 43.0 to 44.0 grains. The H4895 proved accurate as well, at least in the high pressure ranges. We need to do 5-shot testing, over flags, before drawing any real conclusions. But right now we can say that, if you’re shooting a .308 with 150-155 grain bullets, you should definitely try the IMR 8208 XBR. It looks like a near-perfect match for the Palma-class bullets. The velocity is great, it appears the ES will be very low, and 8208 XBR packs more densely than Varget so you won’t have to run compressed loads.

IMR 8208 XBR powder

Our most important discovery was that both 8208 XBR and H4895 offered significant velocity gains over Varget, at least in this rifle. It looks like 8208 XBR can run 2990 fps in this gun without pressure issues, while H4895 may top 3000 fps. Varget struggled to get much past 2900 fps.

IMPORTANT WARNING: The stated max load for IMR 8208 XBR with a Sierra 155gr HPBT was 45.3 grains, while the stated max load for H4895 (same bullet) was 46.0 grains. Note that, in this rifle, which has a tight 0.298″ Palma bore, we hit pressure limits well before reaching Hodgdon’s “book max.” Read that again carefully folks. Our 44.0 grain “practical max” for 8208 XBR was a FULL GRAIN less than the stated max load with a Sierra 155-grainer. Likewise we started getting stiff bolt lift at 44.3 grains of H4895 — a long way from the 46.0 grain stated max. So, the combination of a different bullet, and a tighter-than-normal bore made a significant difference in pressures. This is why, if you change ANY component in a load recipe you MUST start low for safety. And never assume that a factory “Max Load” is safe or “conservative”.

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