Need advice on my OCW results

[mdmp5]

So the other day I performed an OCW test for my .308 loads. I kind of went all out on this one; I loaded 14 different charges of RL15, from 40 to 44.3 gr, 3 rounds a piece for a total of 42 rounds.
Brass: Lapua, FL sized with a .335 bushing, yielding around .002 of neck tension.
Bullets: 168 SMK seated 0.05" from the lands.
Rifle: FN SPR A1a 20" bbl

Here are the results:

It seemed to me that I had discovered not one, but 3 possible OCW loads. Targets 1, 2 and 3 have very similar impact points, however the loads are very slow, evident by the low POIs. The next OCW is 42 gr, since targets 6, 7 and 8 have very similar POIs. The last OCW I counted was 43.3 gr, which was evident by targets 10, 11 and 12. I tried to be very objective by not getting "bowled over by the grouping of target 11 (which was 0.1 MOA), but I could not discount the fact that the 3 triangulations of targets 10-12 were all slightly low and right, and were on target.

Today, I chose to try out the 43.3 gr load with different seating depths, from 0.00" to 0.07" from the lands. However, the results were a but ambiguous. I Shot 3 rounds of each depth at 8 targets, and repeated the test with a .336" bushing to see if less neck tension yielded different results. Almost all groups shot sub MOA, with the .335 bushing yielding better results, and more groups that were around 1/2 MOA. However, the results were a bit lackluster, as I expected at least 1 group to be super tight on target.

My question is, based on the results of my OCW, do you guys think I picked the right charge to tune, or should I stick with the 42 gr load, which was my original load for a while straight out of the Sierra manual?

BTW, I sighted the rifle with Fed GMM 168 before I commenced with the test, so any low POI charges were slower.

 

[mdmp5]

Re: Need advice on my OCW results

Here are the results from today, 43.3 gr of RL15, seating depths from 0.00" to 0.07". Target 1 is 0.07" and target 8 is 0.00" from the lands. .335 bushing on the left, .336 on the right.

 

[mdmp5]

Re: Need advice on my OCW results

bump for some input

 

[Bugholes]

Re: Need advice on my OCW results

based upon first set of targets it seems you have an OCW between groups #5 and #8. With #6 and #7 being in the middle of this node and also being better accuracy, I would look there. If I were to do the test you are doing I would be looking from 43.5-46 RL15 or Varget with the 168SMK. also dont be afraid to jump the SMK more. you never know if your distance to lands measurement is correct. Shoot a load that has potential and tinker with your seating depth to see if groups gets better or worse, then carry on your testing at the best depth while fine tuning your powder charge. Just my $.02

 

[bennybooboo]

Re: Need advice on my OCW results

First off- thanks for not posting some dumbass thread about "what product/load works best because I don't want to find out for myself."

Going back to your original question, I might look at load #7 for an OCW. The reason I say this is that loads 6,7,8 give you a decent window to work with regarding charge-weight and group-center location.

Whenever I have test results like this and it seems like I'm splitting hairs I move out farther. Usually I do my work at 200 yards because of the whole settling-in/going-to-sleep theory (I'm a firm believer that it does exist, especially with boattails).

You didn't post this, but based on high-pressure signs, where is YOUR max load? Where do you start to get a stiff bolt lift, cratered primers? Who knows, you might find another window with a higher velocity. And remember: group center consistancy is a result of finding the ocw, group size is a result of proper seating depth adjustment.

Thanks again for posting something worth reading.

EH

 

[mdmp5]

Re: Need advice on my OCW results

Thanks for the advice guys. From experience, I kind of expected ambiguous results. But the fact that I found what I perceived to be 3 ocws, it validated my understanding of harmonic interference.

I followed dan newberry's instructions pretty tightly, so I determined my max load by the nosler manual and went 1 increment above that. At 44.3 gr, I didn't notice anything extreme, but maybe some slight primer flattening. Perhaps I will heed your advice and work higher, this time out to 200 yds.

But do you guys see 43.3 grains as an ocw too, or was I mislead by the small group? Forgot to mention that there was a shitload of mirage in both sessions.

 

[flashhole]

Re: Need advice on my OCW results

What distance did you shoot at? Did you round-robin the shooting or just shoot 3 of each in sucsession? You did a lot of work but if you shot at 100 yards you defeated the basic premise of the OCW test which is to determine a charge range where any charge within that range will give you equal results.

When I do the test I do it three to five times but I do it at maximum distance (at least 200 yards and preferably longer and I shoot one shot of each charge weight per test. The results are generally pretty telling.

 

[mdmp5]

Re: Need advice on my OCW results

I round robined the test at 100

 

[okiefired]

Re: Need advice on my OCW results

IMO, between loads 6 and 7 would be a very good shoot all day every day easy on the rifle load, without much (if any) seating depth adj. You're right, its hard to ignore load 11. What tries to discount it is the fact that on loads 10 and 12, the group POI is conciderably higher and to the left. Think I might try those three again at greater distance. All that said, if you're not getting any pressure signs and are looking for a 1k accuracy load, I'd at least try the next couple charges but keep an eye out for pressure. Good to read about someone using the OCW correctly. Good luck.

okie

 

[DaveV]

Re: Need advice on my OCW results

If you shoot 5-shot groups, your statistical data will be a little tighter. Just a thought.

 

[mdmp5]

Re: Need advice on my OCW results

I don't think 5 round groups would help shed any light in this situation. The point is to triangulate the shots. Not to mention, more rounds on target can only widen a group and shift the calculated POI, corrupting the data.

 

[mdmp5]

Re: Need advice on my OCW results

Okie, on targets 10 and 12, the groups are in the 4th quadrant, just more pronounced than target 11.

 

[okiefired]

Re: Need advice on my OCW results

Hey, no argument from me on 10, 11, and 12. Thats why I'd run them again at greater distance.

okie

 

[mdmp5]

Re: Need advice on my OCW results

I hear ya. I am gonna run 42-45 gr at 200 this week sometime, and I am gonna try 42 gr at 0-0.08" depth at the same distance. For a while, I was running 42 gr only bc I got the load from the sierra manual and it does shoot pretty well. However, it is obviously slower than Fed GMM, and I wanted a load that was at least as quick as that one.