.308 OCW - Not sure how to read

[davebl]

My first OCW attempt with my .308.

Shooting my JP LRP-07, front bipod, rear bag from a concrete bench. Target is at 100 yds, calm day.

Shooting 175 SMK, Lapua brass, CCI 200 primers and IMR 4064. All rounds loaded to 2.795. I shot a 5 round group (to verify zero), let the barrel cool for about 15 min and shot the first round. Let the barrel cool for 15, repeat, cool repeat. I shot two more 5 shot groups at a 2nd target and went home.

My issue is one group was nothing but fliers (yes I shot round robin). My 43.2 grain shots were horrible. I pulled the first one (12 o'clock high), feel good about the 2nd one (6 o'clock bulls) and pulled the last one at 5 o'clock low. It's hard for me not to go to the 44.4 load though. Major pressure signs at 45.3 and blown primers at 45.8, so I didn't shoot the last shot on it.

I've attached two versions, bottom has OnTarget magic applied. Thoughts from the group?

 

[Dan Newberry]

Nodes should be at ~44.4 and at ~43.2... it looks like you sabotaged your 43.2 group with loose nut on the trigger syndrome. Just kinda kidding... but re-shoot 43.2 as well as 44.4 and you should see that either charge will work well. Nodes are 3 percent apart...

A scatter node in the vicinity of 44 grains should point to OCW's at 43.3 and 44.5 grains.

Good luck. 44.4 grains is a stiff charge for a gas gun... if it were my rifle, I'd check 43.2 and see if I can't get some decent velocity (something above 2450 fps from the 18" barrel) at that level.

Dan

 

[davebl]

Thanks Dan.

1. Not sure what happened to the OP
2. Here's some groups when I was testing out my FGMM clones with different brass just incase they shine some more light

 

[jdgray]

Your on target program is seriously off! That 42.3gr load is no way a .5" group, and the others are way off too....

Or am I just way off??

 

[Signal25]

Nevermind.
Too short

 

[Dan Newberry]

Dave, interesting groups... accuracy seems to be there, especially with the FC case.

I don't know how the rifle shoots with factory FGMM brass, but it's looking like sub MOA is do-able here with the FC brass, and you could tweak the powder charge or OAL a bit to tune the other groups down tighter.

41.75 grains is the Mk 316 load spec. So 41.8 grains in the FC brass may shoot even better.

Federal brass seems like the factory annealing may be a bit short of what it ought to be for reloading the brass. If seating force starts to vary on those cases, an annealing may help them.

Dan

 

[davebl]

Your on target program is seriously off! That 42.3gr load is no way a .5" group, and the others are way off too....

Or am I just way off??

Those are ATC (Average To Center) measurements

 

[jdgray]

Those are ATC (Average To Center) measurements

Then its me!! Never heard of that, whats it used for?

 

[MrFord]

Then its me!! Never heard of that, whats it used for?

Same here. It is not mathematically possible to have a .267" group with .308 bullets at 100 yards without any bullet holes touching.

 

[davebl]

Honestly, no clue why OnTarget shows ATC. I don't know what it means, it was just the defaults on the program when I saved the image.

I would suspect it helps find deviation from a center, as ATC could you be your standard dev. of the group size?

 

[bigedp51]

Erik Cortina of Lapua Team USA uses 1/2 grain increments for OCW testing and how the groups line up vertically on the target.

Long range load development at 100 yards.

Erik Cortina - Lapua Team USA
At some point, the shooter becomes the limiting factor in accuracy!

 

[KYbulldog]

Why'd you leave out the 42gr range? Using both 168smk/amax and 178amax( in LC cases 210primers and RL15 and Varget)I have found good accuracy 42-42.5.
43.3-43.5 showed good grouping as well. Primers started to show very slight flattening above 43.3. I have not gone above 43.5 or chrony'd those yet.
(FGMM 175smk has 42.6g of something short stick, claiming 2600fps) I'd try to zero in on 42.5-/+g. btw shooting an AR10-18.5" 1/11.25twist, and those 178's flew straight/true out of that twist even though the box said twist 1/10

 

[davebl]

Going to reshoot the OCW string. Thinking of focusing on either the 41.5 - 43 range or 43.0 - 44.5 range. Any suggestions?

 

[Asperly Aimless]

Why only 3 rd groups? I look for round groups with minimal vertical, but it's hard to get round groups with only 3 shots.

Several of the groups look promising, but would paint a better picture if they were 5 rd groups.

42.3, 43.6, and 44.4 all look promising. How far do you plan on shooting once you get a load dialed in?

 

[damoncali]

ATC ("Average To Center"? ) is the average distance of all the shots from the calculated center of the group, which is a more useful number than group size because your'e not ignoring the shots in the middle.

 

[Rprecision]

Dave, interesting groups... accuracy seems to be there, especially with the FC case.

I don't know how the rifle shoots with factory FGMM brass, but it's looking like sub MOA is do-able here with the FC brass, and you could tweak the powder charge or OAL a bit to tune the other groups down tighter.

41.75 grains is the Mk 316 load spec. So 41.8 grains in the FC brass may shoot even better.

Federal brass seems like the factory annealing may be a bit short of what it ought to be for reloading the brass. If seating force starts to vary on those cases, an annealing may help them.

Dan

41.8 Worked well for me in a bolt.

The MK316 / FGMM shoots lights out in my rig

 

[davebl]

Why only 3 rd groups? I look for round groups with minimal vertical, but it's hard to get round groups with only 3 shots.

Several of the groups look promising, but would paint a better picture if they were 5 rd groups.

42.3, 43.6, and 44.4 all look promising. How far do you plan on shooting once you get a load dialed in?

My range is 300 yds as of now, I have a lead on a 500 yd spot to shoot however.

I'm firing from an 18" tube, which makes me want to chase velocity a little more.

 

[davebl]

ATC ("Average To Center"? ) is the average distance of all the shots from the calculated center of the group, which is a more useful number than group size because your'e not ignoring the shots in the middle.

Ahh. That makes sense!

Good articles from your site, btw