My OCW Test (.308 Win), Help me decide on a load

johngfoster

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Happy 4th, Everyone

I made it out to the range today do to an OCW test for my 308. First a bit of backround: I'm was running 175gr SMK in front of 44.4gr Varget in Lapua brass. It is a pretty accurate load, and chrono'd around 2760 out of my 25" Bartlein barrel. But... pressure signs! Ejector swipes on the case head on almost all my brass, some really light, others pretty heavy almost obliterating the writing on the case head. Primer pockets were starting to get quite loose, not to the point of primers being able to be pushed out with a folded out paper clip, but very loose going in. This after only 3-4 loadings. On one occasion at an informal match, after 80+ rounds of this load, I was starting to get a very stiff bolt lift. So I felt that it was time to find a load that isn't so stiff.

For my test, I continued to use Varget and 175gr SMK with Lapua brass as my components, but I started at 42.6gr and worked back up to 44.4gr in 0.2gr increments. I shot 5-shot groups in a round-robin fashion at 100yd. I recorded each shot's MV at 15yd with my Competition Electronics chrono. Because they were shot round-robin style, I had to record the numbers manually and calculate average MV and SD/ES separately. I charted Ave. MV against charge weight and entered all the data into an Excel worksheet, but don't know how to add that to this post. I didn't measure distance from aiming point, because it was pretty small, and I'm not exactly sure how to find the center of a group (the point to measure from) accurately. Also, when I look at my groups, the elevation of the perceived center of the group doesn't seem to change as I would expect it to as the charge weight increases. Here's a picture of my targets:
dsc0324f.jpg


Some of the data: (charge weight/average MV for 5 shots)
42.6gr/2623.4 fps
42.8gr/2634.6 fps
43.0gr/2657.6 fps
43.2gr/2656.6 fps
43.4gr/2680.4 fps
43.6gr/2686.8 fps
43.8gr/2711.0 fps
44.0gr/2717.4 fps
44.2gr/2718.4 fps
44.4gr/2739.6 fps

I have quite a bit more data (S.D., E.S., group size, and a graph) but it's on an Excel spreadsheet and I don't know how to get it in this post. Open to help here. I'm thinking 43.0 to 43.2gr or around 44.0gr, but the group at 44.0 was pretty big (0.962") and it is not that far from the high-pressure load of 44.4gr. What do you think?

Edited to add: Excel Screen Shot (thanks c_bass16)
308ocwscreenshot.jpg
 

RobertB

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Re: My OCW Test (.308 Win), Help me decide on a load

I think I would go with the 43.6. But thats me just looking at that. I would try again a time or two and see if it comes out the same. Or just try that load by itself a few times and see how it does
 

johngfoster

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Re: My OCW Test (.308 Win), Help me decide on a load

I wish I could post my Excel spreadsheet. You would see that 43.6, while the best group, is not on a flat spot on the graph, nor does it have the best S.D. or E.S.
 

wirehand

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Re: My OCW Test (.308 Win), Help me decide on a load

In order to really see the different POI vs. POA changes in the representitive charges the test is better done at a farther distance.

The speeds that most are getting 175 SMK to perform have been between 2600-2680 as far my research goes.

Without measuring those groups and finding center it is difficult to say, since they are so close together.
 

Grump

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Re: My OCW Test (.308 Win), Help me decide on a load

If I were you, I'd re-test 43.0 and 43.6, plus/minus .2 again. Six x five, for 30 more bullets used. That's cutting the intervals as fine as is reasonable already, for final testing.

If there were no wind or other variables with 43.8 to explain the shots on the right end of the group, I might not even bother with the 43.6 load. Too close to a suspected scatter node. Plus, that extra 52 fps (more or less, based on your follow-up test) might come at the cost of barrel life.

One 5-round group per load is a small and statistically fickle sample size.
 

MitchAlsup

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Re: My OCW Test (.308 Win), Help me decide on a load

I would take the 43.6gr recipie and do bullet seating experiments.
 

gcoulsey

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Re: My OCW Test (.308 Win), Help me decide on a load

I would retest 42.8 - 43.6
looks to me like 43.0 43.2 is in the middle of a POI cluster.

Anything past 43.6 looks to be moving again..??
Maybe try it at 200 yrds with no wind.
 

Swan

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Re: My OCW Test (.308 Win), Help me decide on a lo

I'd load up 5rds each of 42.9, 43, 43.1 and 43.5, 43.6, 43.7 and blast em away at 300yds. See which group holds vertical the best and secondary, groups the best. Thats your load.
 

johngfoster

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Re: My OCW Test (.308 Win), Help me decide on a lo

How do I add an Excel spreadsheet to this post? I tried looking for the "File Manager" link on the full reply screen as it says in the FAQ, but I don't see it. Someone help please.
 

Seawalker

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Re: My OCW Test (.308 Win), Help me decide on a lo

http://optimalchargeweight.embarqspace.com/#/ocw-instructions/4529817134

You might want to try this link for Dan Newberry's Optimal Charge Weight and follow the instructions exactly as he outlines how to do it?

I have done Ladder tests at 300/400 yds. and while they work, it is easier to do the OCW test as Newberry lays it out and has yielded good results for me with several different powders, bullet weights, and rifles in different calibers.
 

johngfoster

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Re: My OCW Test (.308 Win), Help me decide on a lo

Thanks for the link, Seawalker. Interesting read.

I just figured out how to add a screenshot of my Excel Spreadsheet. Adds a bit more data and makes the choice a bit more complex. The data suggests 44.0gr would be a good load based on ave. MV, ES and SD, but the sample size is only 5 shots, AND, it's still pretty close to the somewhat over-pressure load of 44.4gr, AND the group is one of the largest (if not THE largest if you go by mean radius) so... Looking at the targets alone, it is tempting to pick 43.6 gr, but it is at the beginning of an up-sloping part of the graph. SD and ES are OK, but not the best. 43.0 still looks promising, maybe 43.1gr? Maybe I should load up some 44.0gr and test it at 300 or 400yd to see if it is just that the 175gr SMK needs to settle down a bit and go to sleep?
 

Seawalker

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Re: My OCW Test (.308 Win), Help me decide on a lo

John, yes you'll need to shoot the longest range you intend to use the load at when you have decided your OCW along with 1% above, and 1% below and they should group sub moa.

It seems contrary to what we usually think, but I believe you would get a better test by only using three rounds/charge weight. It makes it easier to triangulate where the bullets are hitting on the target, and to plot the POI. You can always repeat the test a second time if you want to get more data and a verification.
 

Enios

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Re: My OCW Test (.308 Win), Help me decide on a lo

I would repeat the 44.2, IF I wanted to shoot at 1K. Never been much on the Round Robin or Ladder test. 44.2 works great in my rifle with Lapua Brass. 44.8 with Winchester. Load work up I foul rifle with a couple of shots with like powder then will shoot 5 shot group and run a couple of wet patches with Kroil and shooter choice, dry and cool before next group. After 25 rounds I repeat cleaning a second time and a third if really dirty powder. Trying to keep conditions as close to same as possible. I like to load 10 of each charge and repeat the test on a second day before I decide on anything. Have found that has helped about chasing the white rabbit down the hole.(Yep that's a lot of shooting but I've had way to many look good the first time that didn't pan out and it saves a bunch of trips to the range. Wish I had a place where I could load and shoot at the same time could save a lot) One other thing IMO your rifle might not start to open up till after 30 to 50 rounds and some even more but IMO if each group doesn't start off as close to possible as the first you are loosing something in the test.

If you have a load that was shooting good and the chrony speed when it was, I would check it to see if it's still the same. My bet would be it's shooting faster now and you just need to back off the powder some because it's hotter in the heat.

Anything I develop in winter has to have from .2 to .4 drop in charge for summer if it's a max or near max load. The 175's tend to do best out of my rifle around 2710 fps it's a 24" 1/10 twist.

Here's a pretty little program for free that will give you about as much information you should need on group size and what not....

http://www.ontargetshooting.com/features.html
 

johngfoster

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Re: My OCW Test (.308 Win), Help me decide on a lo

Would backing down only 0.3gr be enough of a drop to keep from getting pressure signs?
 

Enios

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Re: My OCW Test (.308 Win), Help me decide on a lo

It would be worth a shot! What I would look at the most is when it shot good, how fast was it moving. Think if you have that info when you drop enough powder you can repeat it. I've seen it between .2 and .5 grain. If you get back to that speed and still don't get the accuracy something has changed in the rifle or reloading process. I had to drop .2 grain in the summer on my 175's in Winchester brass to maintian 2712 and keep the same accuracy.
 

crumpett

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Re: My OCW Test (.308 Win), Help me decide on a lo

Yes, .2gr does make a difference, that is what I drop my 338lm from winter to summer matches to maintain FPS. Sounds crazy but that's what it is.