Help with OCW - Pretty Data Included

[Fedge]

Happy 4th! I ran my first OCW today and I'm having some trouble forming a conclusion with my data.

Rifle: Rem 700 5R .308
Load: 175gr SMK, 2.92" OAL, FC Brass, WLR Primer
Powder: Varget

Test was conducted at 100 yards with sufficient cooling time between shots. Shot round-robin.

Given my understanding of the OCW method, it looks to me like the groups to focus on are 43.5 and 44. Both of these appear to have small POI shifts relative to the groups above and below them. I guess expected to see more of a difference in POI throughout the test, considering the 200fps spread. As this is my first stab at this, I'd love to have a some feedback from more experienced eyes.

Targets as shot:

All groups:

Centers of all groups:

Hard numbers:

 

[AMG04]

First off, great shooting. The rifle seems to be treating you well. I like the data you included in your post, but believe it or not you still need more data...

Half grain increments may be a little too large for conclusive data. I generally run 0.2-0.3 gr jumps on my OCWs. With the data presented it looks like you may be in a node between 43.5-44.0grs, with the high end of the node being close to 44.3ish. I say 44.3 because the 44.5 gr group is almost at the same POI, and has very little vertical spread.

The Extreme spread and Standard Deviation for the velocities seem a bit erratic (based on the graph), that may also muddy the waters. At an OAL of 2.92" are the loads jammed into the lands? I ask because if you are loading close to the lands, some may in fact be jammed and others jumping which could cause the wide ES in velocity.

I would recommend loading 3-5 rounds at each of the following: 43.2, 43.5, 43.8, 44.1, 44.4
I would also check the relationship between the lands and your OAL.

Then get out and have fun shooting the test again. That may help fill in some of the gaps and clear up the results.

 

[Bill Larson]

Depending upon your useage....hunting rnds... Competeing with 20 shots.......i would carefully load 9 rnds. At 43.5 and see if it repeats..... I do not believe in wearing out a barrel to est. Unneeded results....

 

[Dan Newberry]

a scatter node at 42.9 would point to an OCW at 43.5 grains. that's looking like where to be.

43.5 is the norm with Winchester brass, with Varget and 175's.

The fact you're using Federal brass is a little curious, but the results speak for themselves. Perhaps you have a lighter than average batch of FC cases.

Dan

 

[Fedge]

Half grain increments may be a little too large for conclusive data. I generally run 0.2-0.3 gr jumps on my OCWs. ... At an OAL of 2.92" are the loads jammed into the lands?

I was worried that 0.5gr was a bit too coarse for the test, but I was a bit limited on range time so I went with it. I did my best to determine the OAL to the lands using the "cleaning rod method" and I got 2.92" so I believe that I am just kissing the lands at that length. Would you suggest shortening the OAL?

a scatter node at 42.9 would point to an OCW at 43.5 grains. that's looking like where to be.
43.5 is the norm with Winchester brass, with Varget and 175's.
The fact you're using Federal brass is a little curious, but the results speak for themselves. Perhaps you have a lighter than average batch of FC cases.
Dan

Dan, I'm unsure of the significance of using FC brass. Other than 50 or so pieces of Winchester brass, the FC stuff is all I had available to me. Could you explain what differences it could account for? Is it a difference in case capacity/pressure?

I had some extra time to head to the range today (before I saw all these helpful replies...) and I loaded up 5each of 42.5, 43.0, and 43.5. The results are similarly interesting. I can post them later tonight when I have a chance to scan the target.

This is my first bolt gun and my first real experience with the precision game in general so I'm in info-sponge mode. Thanks again to all.

 

[Fedge]

So here's today's three groups of 5 shots. They were shot one group at a time (not round robin). I had a very stiff tailwind today (25 mph gusting 30) so I did my best to be as consistent as possible. Unfortunately I wasn't able to shoot my 44gr load so that data is missing.

 

[AMG04]

The "cleaning rod method" is a good start, but it's not an exact science. The overall length you chose should have the bullet just kissing the rifling, but due to tiny variations in bullet dimensions or other factors it is hard to say for sure that all rounds are just barely touching the lands. My guess is that some rounds are lightly touching the lands, some jammed slightly, and others jumping. All of these scenarios influence the extreme spread and standard deviation in velocity.

You can certainly back away ten thousandths (0.010") to ensure all rounds are jumping or add ten thousandths if you want to jam them. That may help tighten the velocity spreads.

If the goal is to jam the bullet, I like to use +0.010" so that I know all of the rounds are jammed. In your case, 2.92" has you touching so you could load to 2.93" to ensure that all rounds are firmly in the rifling.

I try to avoid loading to touch the rifling because it's hard to account for all variables. Whether you jam or jump you want enough distance to ensure that all rounds are doing the same thing.

 

[Beluebow]

I agree with AMG04 for the most part.Either jam or jump just touching/kissing is a fine line esp if your not sorting bullets base to ogive.As a general rule vertical is powder charge i.e. the 43.5gr load could go up .1-.3gr to pull it together at that seating depth "maybe".The 43gr load is horizontal which sometimes indicates seating depth.A .003 depth adjustment one way or the other will show you which way to go.