Diagnosing Horizontal Variation

[PathFinder]

While working on a load for a Kimber 6.5 CM, it is exhibiting far more horizontal variation than vertical. The rifle is a Hunter with a modified stock, bonded rail, and has been full length bedded. Action screws torqued to 65 in/lbs. It has a very long throat, some of the loads are seated out to accommodate that and some are at mag length. With every bullet except the 130 HVLD, it shot much better with them seated long.
My hope is that diagnosing the horizontal issue will turn this into a solid 1 MOA lightweight hunting rifle. This is the first rifle I've had exhibit this sort of behavior. All groups at 100 yards with no serious wind.

Here are some examples of what it's doing:

145 Barnes Match Burner at 2.9" COAl, 47 gr N560



145 Barnes Match Burner 2.9" 42 gr RL16


143 ELD-X, 46 N560, 2.82"



143 ELD-X, 46 N560, 2.9"



This is the only exception to the rule: 130 HVLD, 2.82", 43.5 RL16



One logical conclusion is to just roll with the 130 HVLD and be done. But the consistent horizontal variation is indicative of some kind of special cause variation. I'd also much prefer to shoot the ELD-X or 145 BMB vs the Berger 130. I've also tried the 147 ELD-M and 156 EOL with similar results.
Possible causes I'm thinking of aside from the load:

• Shooter, likely bag/cheek pressure
• Scope mounts. Front ring to base torque was at 55 in/lb. Last night the front one snugged up a bit when I checked it, haven't shot since. They're now at 65 in/lb
• Action screws. Need to check those again, it has some modified titanium ones right now that may need to be swapped for the stainless factory ones to test.

The group above was the last of the 130 HVLD I had. I'm planning to buy more today to try that, and also try the ELD-X load again with the mounts snugged up. Will post the results once I have them.

 

[hereinaz]

I believe that there are factors from shooter position to quirks in the rifle that cause it.

I typically don’t attribute patterns in groups that small in size or groups low in number to the rifle until I eliminate the shooter as the cause. I cause many things… I have seen groups change as soon as a better shooter gets behind it. Same as rifles are magically better when my infrequent shooting friends say their 300 rum barrel must be shot out, and I suffer through a few rounds to disprove that notion as a reason for sudden 4” groups when they haven’t shot for a year.

10 round groups are a good start.

But, I have shot multiple 5 shot groups into the same point of aim/impact and put separate pieces of paper behind for the 5 shot groups.

If you cherry picked the best 5 shot group is was a .5” gun. If you picks the worst 5 shot it was a 1.0” gun. All combined it was just over 1” and no single 5 shot group was enough to do any better than reading tarot cards or as horoscope.

I have recently decided it is nigh impossible to “read” much into low count groups. I still shoot them, because they tell me something, but they don’t tell me everything.

One 3 shot group of .5 does not mean it is a .5 gun. But, the odds of a true 2 moa gun shooting a .5 group is small, so that’s where it is useful, merely giving hints at the most likely direction the data is pointing.

 

[4th_point]

I'd check parallax.

Horizontal screams shooter to me in general, as you guys have already pointed, but I'd rule out parallax first.

 

[HandgunHTR]

What scope are you using? Depending on that answer, I would then go to shooter induced.

Most likely recoil control/follow through. What is your bench and rest setup? Have you tried shooing prone or with a bipod to see if the issue persists?
When you say “modified stock”, modified how?

 

[Macintosh]

“Modified stock, full length bedded” doesnt sound like a typical bedding job, esp on a plastic hunter stock. How did the rifle shoot before modification?

 

[Kyguy]

This is the rifle with the factory stock that you built up with carbon fiber and whatnot, correct? Did you shoot it in the stock before you modified it to get a baseline?

 

[PathFinder]

To answer a few of the questions above:
- The stock was modified with Rokstock type geometry. Vertical grip, flat toe, negative comb. Plus a bunch of material removed from the forend. All done back up with carbon fiber. It's really comfortable to shoot and stays on target really well.
- Bedding is full length, the plastic was prepped to facilitate better adhesion and structure.
- I never shot the gun prior to modifications because it was so bad. The stock contacted the barrel with very little pressure, and the action fit in the stock was extremely loose to say the least.
- Shooting setup is prone on the ground. Cinder block and a short bag under the front, a taller bag or combo of the taller bag on its side and a softer bag under the rear. I've been checking NPOA just for my own sanity and there is no shift off the dot while dry firing.
- Double checked for any possibility of parallax error tonight and didn't find any. Scope is an NF SHV 3-10.

I shot the 130 HVLD load again tonight, and it strung horizontally as well. The group with that 130 HVLD load above was shot prior to the other loads exhibiting the stringing behavior.
That's now at least 50 rounds with multiple powder and bullet combos doing exactly the same thing. Pretty sure it's something related to bedding or action screws. I'll look into the screws more in the morning, but the front one is still torqued. The rear one needs a special tool to access and I couldn't find it tonight.

Sent from my Pixel 6 using Tapatalk

 

[Kyguy]

Full length bedded meaning the entire action? Or meaning the entire barreled action from rear tang to tip of forend with barrel bedded solid? I think the stock is the problem in some way but without having a baseline from the original...

I have a Remington 700 223, sporter, new production. I never shot it in the factory stock, I threw it in a Bell and Carlson and bedded the lug with generous free float. It never shot anything better than an inch for four or five shots. I decided I might sell it, so I threw it back in the factory plastic SPS stock, which has a forend pad for tip pressure and torqued it to 35 inch pounds per factory spec. I took it out to shoot it so I can give honest information to buyers. The first group with the same ammo as before was 1/2" for 5 shots. Three other groups with different ammo that was tested before resulted in everything being 3/4" or better. It's no longer going to be for sale and it's going to stay in the factory stock.

Light barrels do not always shoot better free floated. Maybe stick a couple of business cards between the barrel and the end of the stock and see what kind of change that makes.

 

[Ramem7mm]

Do you have pictures of the rifle itself and the setup during your shooting outings?

 

[PathFinder]

Yesterday I did the following:

• Swapped a hastily modified titanium rear action screw for the factory front action screw. Took the BA out of the stock to make sure the bedding looks ok and hasn't cracked or anything. It looks good. Re-installed and torqued action screws to 65 in/lb
• Loosened the scope rings, then re-torqued to 25 in/lb. I reversed the order when installing them last time, got to thinking that could torque the scope tube.
• Shot the group below. It's only 7 rounds, but looks a little more like a round, normally distributed group. Will shoot again tonight or tomorrow. That was the 143 ELD-X load, I'll play around with the 130 HVLD for testing.

Next steps:

• Install NF Ultralights w/ NF SHV 3-10
• Test 130 HVLD load (10rds)
  • If that goes well, test 143 ELD-X and 145 BMB loads again.

Last night's test group. Didn't bother to re-zero after re-torquing rings and action. If it will shoot a nice round group about that size that will be good enough.


Shooting setup as requested:

 

[PathFinder]

Update: After the following, it's back to shooting round groups:

• Change ring torque order from scope first to base first
• Swapped one of my modified Ti action screws for one of the factory ones
• Swapped the Amazon rings for NF Ultralights. No issues with the Amazon rings on another rifle that is 1 MOA for 10 shots.