what factors affect group width?

[Whiterabbit]

We have some ideas about what affects height in precision shooting. Velocity uniformity of course being the big one we always talk about, also rear bag used, I imagine we could come up with more.

But what factors specifically change horizontal group size?

I ask because I compared two loads, single variable test. One showed a 50% improvement in horizontal spread with zero impact on vertical. That's a big difference!

I know what I did between loads, but I have no idea why it would have impacted horizontal.

I'm perfectly willing to share what I did, but I first want to hear what the collective generally thinks about the factors that affect horizontal first, before I muddy the wafer with bias confirmation with respect to my load change.

Thanks calguns!

[focus]

Given stable envionmentals. Assuming sound consistent reloading and shooting skills...barrel harmonics. This paper describes a concept that may explain your observations.

http://www.the-long-family.com/OBT_paper.htm

Cheers,

[bsumoba]

Horizontal is generally conditions. Even at 100 yards, mirage and wind play a huge part in your group size. When doing load development, I generally look for the point where the group pattern starts to look more horizontally dispersed versus vertically dispersed. This can usually be done by changing powder charges. To change the group size, it is generally seating depth.

IMO, if you did this test once, it is really hard to tell if it is a one-off occurrence or if it is an actual repeatable test.

[LynnJr]

Vertical dispersion is a combination of velocity variation and bullet bc variation if your at a node.

Horizontal dispersion is the fault of the shooter and conditions.

Dr Geoffrey Kolbe explains this very well in his tuner paperwork on 22 Rimfire barrels.

[UberPatriot]

I know a guy that weighs each bullet and then groups them by weight.
He does this before target shooting because if you have five bullets and four are the exact same weight and one is lighter or heavier it will have a different trajectory.
Anyways it seemed logical to me.

[Whiterabbit]

Heartbeat and rear bag and position (also relates to heartbeat) nicely explain a baseline in my horizontal. That's definitely shooter and conditions. Mirage for me is also BRUTAL (is there anything i can do about that?) But i am having a devil of a time explaining why i reliably got the 50% improvement with a load change. That implies the baseline was further expanded by the load.

Across 5 10 round groups, 3 of one load and 2 of the other, shot over 75 minutes to try to minimize mirage, hard for me to ignore a change of that size (50%). It made for a 17% reduction in overall group size. Dominated by a shrinking horizontal in an area with zero wind.

[SonofWWIIDI]

I know a guy that weighs each bullet and then groups them by weight.
He does this before target shooting because if you have five bullets and four are the exact same weight and one is lighter or heavier it will have a different trajectory.
Anyways it seemed logical to me.

Years ago I watched a show about military sniper school. They showed a scene where a soldier was weighing the bullets before being loaded and doing exactly that.

[kcstott]

Heartbeat and rear bag and position (also relates to heartbeat) nicely explain a baseline in my horizontal. That's definitely shooter and conditions. Mirage for me is also BRUTAL (is there anything i can do about that?) But i am having a devil of a time explaining why i reliably got the 50% improvement with a load change. That implies the baseline was further expanded by the load.

Across 5 10 round groups, 3 of one load and 2 of the other, shot over 75 minutes to try to minimize mirage, hard for me to ignore a change of that size (50%). It made for a 17% reduction in overall group size. Dominated by a shrinking horizontal in an area with zero wind.

maybe you just had a good day shooting.

[LynnJr]

There is a math formula that shows what weight variation does to your trajectory and how it changes your bullets BC.
When Dr Ken Oehler and US Shooting Team Coach Larry Bartholome tested for bc variation some bullets would vary up to 20% iirc.
That is when everyone started trimming meplats to better uniform the bullets BC number.

[LynnJr]

WhiteRabbit
Mirage or heat coming off of the barrel?
For the heat you can put a small battery powered fan next to your barrel or put a shield on it to redirect the rising distortion in front of the scope.
For mirage itself the only thing you can do is lower the power on your scope. It won't change the mirage it will only lessen what you perceive through the scope.

[Whiterabbit]

Barrel Heat. Is this one of those cases of bad terminology, like "cartridge" vs "caliber"? Everyone I see online talking about the effect of barrel heat on target visibility through the scope calls it "mirage", identical (wording) to environmental mirage.

I'm definitely not talking about environmental mirage here, barrel gets hot, from near the scope sunshade all the way out the the muzzle brake, after many rounds even spread out over time.

I need to build a shield for sure and keep it in the range bag.

-----------------

It no doubt has an effect on the groups, but surely it would also be an even effect (over time)?

I shot the 5 groups interlaced: A, B, A, B, A to hopefully remove some effects of the barrel heating up and mirage. I had planned to throw out the first group A, but I didn't see a major difference.

This is the data from OnTarget:

Units are in inches

Group B doesn't change Height, but almost half width, which then translates to 17% better in overall group size.

I'd be happy to consider it luck or environmental, mirage, bad rear bag, barrel heat, etc etc etc, but given 10 round groups (plenty of data IMO to see what's going on, in fact it's too much and IMO causes overall group size to expand via fatigue) and that they were shot A-B-A-B-A and that the data clearly shows advantage to width, I'm really confused.

[kcstott]

A couple of Velcro dots and a heat deflector will take care of that barrel heat.

mirage-shields

[6mmintl]

If your shooting a service rifle (M1/M1A) you might want to look at front stock ferrel and midband/gas cyl tightness/unity, also stock condition compression if old or oily.

[LynnJr]

Barrel heat comes off of a hot barrel and mirage is an environmental issue.
And I know the difference between cartridges and calibers.

[Whiterabbit]

I know YOU do, but I see so many people call barrel heat affecting scope vision "mirage", just like the environmental issue. I could only interpret that as that they are interchangeable.

I can use barrel heat, no problem.