Reloading for a new .338-06 with a 24” barrel (Shaw barrel on Marlin XL7) but IMR4320 loads are slow. All rounds fired over a chronograph with 180, 200, 225 grain bullets. Using load data from Nosler, Speer and Hodgdon the max loads are running up to 300 FPS slower than advertised. Starting load values lag approximately the same. However, loads with Varget and Benchmark perform very closely to the published values. I have been using CCI primers with all loads except for one batch with RP which appeared to be less accurate. The max 4320 loads showed no pressure signs at all (primer, brass, bolt lift, etc.). Have you ever experienced a similar result? I have been reloading for 30 years (rifle, shotgun, handgun) in over a dozen cartridges but this is my first foray into the 7mm + range. Thanks for your input.
Low pressure powder lot?
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AJC1
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Every barrel is different and unique. Some are slow and some fast. Have you completed your break in where things speed up for the first 100 or so shots.
Blue68f100
Member
Haven't used that powder. A new barrel takes a few rounds to break in and smooth out. Velocity should increase over time, 100 rounds or less depending on lapping. You may have to tweak your load after the barrel settles down.
Have not reached 100 rounds yet. The first rounds were fired using Shaw’s ‘break-in’ instructions and were turning in velocities very close to those expected using Varget. IMR4320 seems to be a traditional powder for the round but I should also mention that the powder stock I have is older / this is a freshly opened metal can as opposed to the current ‘poly’ canisters. The powder looks and smells normal.
AJC1
Member
If it's truly a slower lot then you may test and find out you can use more. After your barrel is broken in you may test and find that you need a little more to match velosity. If your graphing and looking for pressure signs you can safely work up till you get velosity or pressure.
Blue68f100
Member
I had a old metal can of IMR 4350 that did not have the solvent smell, but did not the bad smell either. I knew this was ending its end of life, just since it was 40+ yrs old. I had used this powder in my Rem 7mm Mag for decades. the SD were running 1 & 2 on a 10 shot test, where a new current batch were in the 12-13 spread. The velocity was lightly lower but nothing to be concerned about, <25fps. The extremely low SD made some nice groups.
That is what I thought also. I am not chasing 20 or 30 FPS and actually would be satisfied at 100 FPS slower than advertised with max loads as that would easily be within the realm of reasonable. It just seems a bit odd that this particular powder seems so slow when the others I used are close to book values. Thanks for your input.
Update - I have incremented the IMR 4320 loads up to approximately 3.5% over the reloading data I have for 180 grain bullets. Velocity is within 100 FPS of advertised and within the range of other powders that were delivering near advertised values. No pressure signs have been noted on any loads. Do you see any major concerns with loading this powder lot at this level and then starting over when I open up a new can of IMR4320? It does run closer to advertised velocities with 200 grain bullets.
FROGO207
Member
As long as you use them at around the same temperature that you made them at you should be good to go. If you leave them in a hot car in summer weather they might be a little "energetic".
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