Need help interpreting OCW development for 308 Varget and 175 SMK

[new2mud]

It is for a .308, 20.5" 1:10 twist barrel, loading 175 SMK with Varget.

I've done several Optimal Charge Weight (OCW) developments, but this one has my head scratching. I know in OCW development you're not supposed to focus on the tightest groups, but rather POI shifts--however, the data on this one is confusing to me. (Ignore the top left target--sighters)

Often the accuracy node is the charge weight that provides the lowest SD. In this case, however, 2 consecutive charge increments had the lowest SD: 43.9 grains and the next step up at 44.3 grains had SDs of 3.3 and 4.5, respectively. However, they went from horizontal stringing (43.9gr) to vertical stringing (44.3gr), and grouped around 1-1.1 MOA.

From the target picture, 43.5gr and 45.1gr (the low and high of my charge weights tested) had the highest SDs of 19.7 and 13.6, respectively, yet had the tightest groups (0.61MOA and 0.72MOA, respectively).

Could I get some collective help in interpreting this data and suggestions on where to focus next?

 

[Mram10us]

I'd be looking at 43.9.

 

[FURMAN]

I personally would do seating depth testing at 43.7.

 

[User4302021]

You are seeing weirdness because you are into high pressure. Within standard pressure limits, you should be in the vicinity of 2,600 fps with a 175 and Varget from a 20" barrel.

With heavy brass such as LC or Lapua, 43.5 is an extremely common place to end up. With light brass such as Winchester, 44-44.2 grains is extremely common.

 

[new2mud]

You are seeing weirdness because you are into high pressure.

I was leaning toward investigating around 44.0-44.1 since both show low SDs, as opposed to going toward a charge weight with higher SD.

I was not aware that OCW goes haywire when in pressure zones. Was starting to see ejector "shine" at the upper end but not the other typical pressure signs.

 

[FURMAN]

43.5 and 43.9 put 5 out of eight shots right at or slightly under .5moa. SD will shrink when you find the correct seating depth. There will be a node in-between and it is the reason for doing OCW-looking for little to no shift in POI. It would not hurt to explore 44.1 but I would do it after seating depth at 43.7.

 

[BW64]

Yep. 43.5, 43.7, & 43.9 are in the park. I bet 43.7 is your node. Maybe 43.9 but try the 43.7 first. Then go for seat depth.

BW

 

[nksmfamjp]

I'm having a hard time ignoring groups #1 and #3. Hmm...and #5, but it only has 3 shots, so I will ignore. I think is would center on the lowest sd and vari seating depths.

por maybe reshoot this whole 25 at a different depth. I would go 0.010" closer to the lands.

 

[Jud96]

Powder charge controls velocity and the consistency of your velocities. Seating depth controls group size. Find a node like you have from 43.9-44.7, pick a load in the middle of that node and use that. Then start adjusting seating depth, typically starting 0.010 of the rifling and working back in 0.005-0.010 increments until you're 0.050-0.060 off the lands. Whatever seat length shoots best, take that length and start adjusting the bullet further out and deeper in 0.002 increments until you find the seat length that works the best.

 

[rsmithsr]

(and if you had shot a ladder, you would be done now, not on here asking questions.)

 

[new2mud]

Thank you all for your advice. It's so hard to not get sucked into the tight groups even though they have high SDs.

I will focus on the low SD charge region and test seating depths. Original test was at 0.02" off lands. The sighted group was 0.04" OL and had almost same size as the same charge at 0.02".

 

[User4302021]

(and if you had shot a ladder, you would be done now, not on here asking questions.)

As if a ladder can't be messed up or misread same as any other method.

You guys crack me up.

 

[Zman99]

My 20 inch 10 twist .308 is at 43.6 varget with hsm brass, cci 200 and 175 smk. It's right at 2611 FPS. I bet you will fall right in between 43.5 and 43.9.

 

[waveslayer]

Don't waste your time with the 1 bad shot on a ladder test, jacking the whole test , comment. Ladder test is a waste of time and too hard to do unless you have 300 yards minimum and 1000 yards to test...
Anyways back to the OP. 43.7 would most likely be your node. Do some seating test and shoot 5 shots from there per group. Have you tried doing it round robin style of shooting. It would be interesting to see your shots. Your groups look like some shooter error was induced from some of the vertical and horizontal stringing