I took apart a FGMM 168 grain bullet today to measure the powder load. A friend told me he had done the same a while back, and it measured 42.7 grains. I found the same exact powder charge today in the bullet I disassembled: 42.7 grains.
Subjectively, the powder looks very similar to IMR 4895, which is what I am using to try to make a close match to FGMM. I don't think the powder Federal uses is exactly the same as IMR 4895, but it seems petty close. I'd love to look at some kernels under a microscope and check it against various brands of powder to really try to pin it down.
I went to the range and chronographed a series of 5 round test batches I made up last week. I used an Oehler 35P to chronograph the batches. The temperature was 47 degrees F, and it was partly sunny.
For components, I used 1X fired (in my 700 PSS) Federal .308 brass, Federal 210M primers, SMK 168 gr HPBT, and IMR 4895. I cleaned and prepped the brass including neck sizing with .336 bushing, uniformed the primer pockets, de-burred the flash hole, and chamfered the cases mouths inside and outside.
I made up batches of the following powder loads:
42.1 gr.
42.3 gr.
42.5 gr.
42.7 gr.
42.9 gr.
43.1 gr.
Incidentally, I weighed the powder by hand with an RCBS 10-10 sclae, and seated the bullets with a Redding competition micrometer seating die.
I shot a 5 shot control group batch of FGMM.
The FGMM measured:
Mean Velocity = 2679
SD = 4
The closest match I had velocity-wise was the last batch I made with the highest powder charge, 43.1 grains. It was not the tightest group I shot today, but more testing will have to be done as I hone in on the ideal load for my rifle. The wind started to pick up towards the end of my session, and my chronograph was blown over in the middle of measuring that string for the last batch.
Initially I was surprised that it took an additional .4 grains of powder to match the mean velocity of FGMM, but after thinking about it, I think I know why. I only neck sized the cases after they were fire formed in my rifle from their first firing. The body of these cases is clearly a larger volume than the never before fired FGMM cases, so it makes sense that a larger charge would be needed in my reloads (all other things being equal) to match the pressure and muzzle velocity of FGMM.
I'm tempted to try an experiment some time, and pull the bullets and powder from 5 rounds of FGMM, and then replace the factory powder with 42.7 grains of my powder (IMR 4895) and re-seat the bullet, and then chrono that against a batch of FGMM. I have a hunch they would produce pretty identical MV.
If I get around to trying that in the next week, I'll post the results.
