Confusion about OCW

[SJshooter]

Sorry to post yet another topic asking for help, but...

I did an OCW test today. Varget, 168SMK, and Winchester brass and primers. I was shooting my Savage .308 with sporter ss bbl.

I loaded 41gr to 44 grains in .5gr increments (7 sets). I shot these round robin and analyzed it on OnTarget. Charge weights 43 and 43.5 have almost the exact same group center... But they're also the largest groups. Like 2.6 and 3.5 inch large groups. WTF?

My question is, do I chose one of the other 2 consecutive charge weights to fine tune, or do I start fine tuning the 43-43.5 range?

 

[trailrider121]

Re: Confusion about OCW

When I was using my Savage 308 win, about same components as you I found Varget with 45-46gr was an OCW charge area. It is on the upper end . Lower end ocw around 44-44.5grs.

Can you post a photo of target? How far off the lands? maybe someone else can help. Try loading ocw charges in .3 to .4grs. each step. It will give you better idea.

 

[Bob 964]

Re: Confusion about OCW

Probably not gonna tell you anything you want to hear but, since you asked, I will tell you what I think. First, I presume you did your OCW test at 100 yards, as recommended by Dan Newberry. If not, you should do your next OCW test at 100 yards. Second, 0.5 grain increments are a bit too much for a .308 OCW test. I recommend 0.3 grains next time out. Third, you are looking for 3 consecutive groups that come closest to hitting the same POI. Fourth, Hodgdon recommends a maximum charge weight for Varget and the the 168 SMK of 46.0 gr. (compressed). Depending on whether you saw any pressure signs, you may be able to bump the upper limit of your next OCW test by a couple of grains. Finally, if you were shooting at 100 yards, then I share your concern about the maximum spread of these groups. Perhaps bipod hop or some other technique flaw caused the groups to be so large. Make sure you are letting your barrel cool for a minute or 2 between shots. A hot barrel can cause your POI to vary.

I have had similar experiences. If it were me, I'd chalk this one up to experience and redo the test. It's gonna be hard to have any confidence in your results. It might be worth a few minutes of your time to (re)read Dan Newberry's instructions for doing the OCW test to make sure you are following them to the letter. Here's the link.

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

Good luck!
Bob

 

[NoFail]

Re: Confusion about OCW

^ yeah, and let cool for 5 minutes between each "round robin" of the same charge weight.

 

[boeger42]

Re: Confusion about OCW

Personally, I use the OCW at 100, the ladder between 300 and 500 and group shooting at 100...If the load is the load it should be good at all the ranges.

From the link provided, thanks Bob, click the OCW vs. Ladder and then on the Chris Longs Pages link. There you should find some theory about how one can get a large spread at 100 and a smaller spread at longer range. I've heard several theories so I conclude that there is more than one cause for the phenom?

I have a 308 now with 20" 1in12 that shoots normal at 2400 fps but bump it up to 2650 fps and it begins to spread at 100 but is 1" capable at 400 meters. Since it came with a surefire and no crown, i substitued a harmonic tuner that seem to settle it down.

Good Luck...

 

[SJshooter]

Re: Confusion about OCW

Alright, so I repeated the test in .4 gr increments, and I found that I had the same POI from 44.4,44.8, and 45.2. The 2 best groups were 44.4 and 44.8 with the best being 44.8.

I then repeated the test with 5 rounds each of 44.6, 44.8, 45.0.
I did the test round robin, waited 5 mins between rounds (at least) and took my time. All the shots felt really good too.

BUT!!!... I'm really pissed because it looks like I shot the target with a fucking shotgun. WTF?

 

[trailrider121]

Re: Confusion about OCW

Do you have any means to post a picture? I would like to see groups.

 

[RyeDaddy]

Re: Confusion about OCW

I'd do a seating depth test at 44.8, according to theory that will be your OCW. Tune seating depth to shrink groups. Once you've done that load some tuned ammo at 44.8 grains and then a few at 44.6 and 45.0 again(at the same length you find to work best with 44.8) and shoot them all at the same target and see what you get. How shotgun-ish did this turn out, 6 inch pattern at 100? Larger or smaller?

You can color the bullets of the 44.6 and 45.0 loads with a Sharpie (I use red and blue) and it will color the hole the bullet leaves on target. Makes it easier to identify what's what. They should all hit around the same POI and make a reasonable group like you expected. If you find that one of the charges is scattered all around, move your chosen load 0.1 grains in the other direction and see what happens. For example if you did it the same way as before and find 44.6 is a larger group, then move up to 44.9 or 45.0 for OCW. That will help with stability.

For knowledge's sake, when you're all done run some tuned loads over a chrono and see what the velocity spread looks like, if the ES and SD are too high it won't matter what kind of groups the load makes at 100 unless that's as far as you intend to shoot the rifle.

I would also be interested to see pics of your original OCW round robin and the subsequent targets. I'm no expert, but have used OCW to develop two loads now that work well.

FWIW, my OCW with a 16.5" barrel 308, Win brass, CCI primers and Berger 168 hybrids is 45.0 grains of Varget, so I think you're on the right track.

 

[boeger42]

Re: Confusion about OCW

I'm making a bunch of assumptions:
The loads are consistent, clean, trimmed, charged, seat, primer, neck sized, head space, concentricity, etc.
There's nothing loose on the gun, head space is correct, etc.
The barrel is normal, i.e. longer than 20", 1in12, cleaned, blah...
ES your talking about is within a couple of inches at 100 yards.

So...

After you flup on the seating and velocity as mentioned by RyeD, shoot a 300 yard ladder. Start below 44.6 and finish above 45.0 (not to exceed max of course). If you confirm minimal vertical dispersion between 44.6 and 45.0 then you have confirmed your OCW at 44.8.
If you confirm your OCW but don't like the group size, you'll need to start looking else where. I've been here before and it sucks, but here's how i got passed it but only after review, replacing optics, bore scoping the barrel, and validation of the assumptions listed above:

I found my hand loads at 43.9, 175gr vld's, ES >30, SD around 10fps, were averaging 2650 fps. I got a box of Black Hills Match and a box of LakeCity both 175's and shot them over the chrono. Lake City speeds were 2600+ and the groups were large, 1.5 moa. The Black Hills were .5 moa but the velocity was down 2450-2500'ish with an ES of 101fps over two boxes.
Today I still shoot my 43.9 hand loads. I accept the fact that there is a stabilization issue at 100 yards. On a good day I can expect the following from this rifle:
.5-.7" at 100
.9-1.5" at 300
1.0" at 437
1.5" at 500
haven't got to the 1k yet

Patients and Logic...Apparently this is going to be a learning experience...Good Luck! and keep us posted on progress.