First go with Optimal Charge Weight.

Tom D

Well-Known Member
So I thought I’d try the OCW method and see what happened. I have been meaning to work up a load for the 7-08 with 162 amax of which I have a lot. I was using N550 and according to the Vihtavuori load data the max for a 160gn bullet was 41.8, there was no data for the 162 amax. As the amax has a soft thin jacket I guessed that max may be a little beyond the Vihtavuori figure, they do tend to be fairly cautious.
The OCW method is shown here:http://optimalchargeweight.embarqspace.com/ocw-instructions/4529817134

I followed these instructions pretty much to the letter, selecting 43gns as the maximum charge, this gave me an initial charge of 38.7 then 2 more sighters / pressure gauges at 39.4 and then 40.2 with the first 3 shot group at 41.0. After these I went up in 1% intervals (0.4gn) until I reached my max of 43.0, as there were no signs of pressure at this point I tried one more group at 43.4. The results are below. They are not so easy to interpret as you might think since they all grouped fairly consistently low and right. I guess I should chose group 3 and work with that, although I’d prefer a bit more velocity.


Anyone else tried this?
 

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Muir

Well-Known Member
Never. I just pick the powder that gives the velocities I want at the lowest pressures and go from there. I set my starting point and load 50 cartridges in identically prepped, FL resized cases. Then I shoot them, recording the results. At some point I fire ten rounds over the chronograph to check the speed. If the speeds are acceptable and the accuracy good, I stick with it.

That's nice, consistent shooting you were doing but in truth i can't see a spit's worth of difference in any of the groups. #3 has two touching but that could have been totally random. I don't think this tells you much other than the loads were nicely consistent and that changing the powder charge had zero effect on accuracy. Were I you, I would load 50 of the #3 (if that's the one you think is right) and really scrutinize them. JMHO~Muir
 

Tom D

Well-Known Member
According to the OCW method no3 is the one as the two either side of it also have a similar POI. It’s not supposed to be about the size of the group at this point. It’s a slightly odd way of thinking about it. It makes sens in that if the two groups either side also group in the same place then the middle load ought to be tolerant of slight variations in charge weight or temperature. I’ll maybe try some fine tuning and see where that goes.
 

Tom D

Well-Known Member
Yep it was 100m, i could stretch it out to 200 maybe, I have always tested at 100 and then tried the selected load further out.. the problem with developing loads further out is the wind.
 

Cottis

Well-Known Member
I have done both Muir's way and more recently have gone the OCW route. For me, OCW is much better but they both work fine. You just get to your destination with less fuss with OCW i reckon.

If you read about the theory behind OCW, 3 and 6 are clearly your nodes as far as I can tell but it is easier to see if you plot the triangulated POI's for each charge weight on the same target. Your results are a spitting image of mine and if done properly, it should be that way if I understood the theory correctly.

My chosen weight (which was also the lower of the two nodes I found) was the marginally worse group of all initial charge weights using book standard COAL. It was probably just me on the day. I significantly reduced group size by loading a few rounds 20, 15 and 10 thou from the lands and the longest OAL gave marginally the best groupings. All were under 1/2" and since then, the rifle has shot hundreds of this charge weight without issue.

I did shoot some rounds over a chrono afterwards once I had got the weight and seating depth where I wanted and they were bang on 150fps under the book max for that powder which was about what I expected. I was quite impressed and feel it gives me plenty of poke, accuracy etc without being near pressure that could be damaging to me, the rifle, case life etc etc.

I am properly sold on OCW

Good shooting by the way.
 

Tom D

Well-Known Member
Cheers, I did chrony all the loads, although the round robin method of shooting means that you can’t rely on the chrony to calculate the ES and Average. Still the ES for the slower 3 groups was very low 10,5 and 6 respectively, it was around 20 for the faster groups. Average velocity ranged from 2544 to 2717 over the 7 groups.
 
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