5-Shot Test Group's Size and Shapes

[Bart B.]

Never heard of this before......

Recently read that the US F-Class Open Team tests their loads using a 5-shot group for each .003" increment of bullet jump to the rifling. Each bullet seater settings' group will have its unique shape; typically elongated some amount and it's long axis at a different angle for a given bullet jump distance. When the group changes to a small cluster, that's the bullet jump distance to use for best accuracy. Repeating the same load bullet seating depth always has the same group elongation and angle; except for the bullet seating depth making jump distance to the lands perfect and produce a small round group.

I don't understand the mechanics of barrel vibration and bullet exit times that might cause this.

Nor do I believe that subsequent tests with the same bullet seating will produce the same group elongation and angle.

Anyone have insight to the team's test methods to verify this?

 

[Brians356]

I would rather see many 10-shot groups supporting that assertion. But I am a skeptic in all things earthly. (See my signature.)
-

 

[dcali]

I'm not sure anyone fully understands bullet seating depth and its impact on group size. Here's what I do know and/or believe:

1) a single 5 shot group is not going to produce repeatable "group shapes". It's just not.

2) bullet dispersion is largely governed by in bore tipping of the projectile. (This much, we know is fact). This is what I believe seating depth impacts. I will admit I don't have a strong explanation as to how, and neither do I have any evidence that seating depth impacts in bore tipping, but it seems to be the most likely culprit. The other would be barrel exit timing, but the math on that is suspect. My current hand wavy hypothesis is that bullets are released asymmetrically to to case wall asymmetry and will oscillate (tip back and forth) slightly as they move towards the rifling. Once they hit the lands, they stick and that's the amount of tipping you wind up with. Certain seating depths will allow the bullet to hit the lands straighter than others due to the timing of the oscillations. Note: I made all of that up and have NO evidence that that is what actually happens.

3) It is plausible that there is a gravity dependent "orientation" to the tipping.

4) Barrel muzzle movement is not purely up/down or side/side or any clean orderly geometric shape. It's wonky. That's a scientific term for "catywompus".

5) I think a lot of load development "techniques" are really just different ways of shooting a bunch of groups and picking the best ones. Some are more orderly and logical than others. But there's a whole lot that don't make any sense at all, and yet still work, just because people keep shooting until they find their tune.

Personally, this is one of the great mysteries of internal ballistics - the question "why does seating depth have such a pronounced impact on group size"?

 

[Intheshop]

Just data here guys,not anything more.This is more about developing sporters for hunting.....with a "minor" on bug holes.These are cast bullet rigs.

Take 5 sheets of typing paper and poke a pen hole in the center whilst they're all lined up.Shoot 3 or 5 shot groups with paper oriented the same(mark the top),using pen hole as aiming point....but here's the rub.Do it over a 5 day period,rain or shine....each day, shoot a group.Yes,you need a range out the back door.

By overlapping the sheets/groups was the only way I could build in the precision/confidence to say....this combination is "X" quality.No foulers,no excuses.Further,these are run up through the mag(which defines the # of shots oer group).It's a juggling act.

 

[Bart B.]

Lots of folks don't understand why several 5-shot groups across two days of benchrest matches with the same rifle:

* are not the same size, but have a 5 to 6 times spread from smallest to largest.
* don't have all group centers the same place relative to aim point.
* shoot the smallest one first in match 1. Largest one either, for that matter.

According to mechanical engineers in mass vibration analysis, most of the muzzle axis vibration is in the vertical plane. That's because the recoil axis is mostly directly above the rifles center of mass. Which is why tuner weights' position change the vibration frequency just enough to let bullets exit at the best place for accuracy.

 

[Toby Bradshaw]

Lots of folks don't understand why several 5-shot groups across two days of benchrest matches with the same rifle:

* are not the same size, but have a 5 to 6 times spread from smallest to largest.

Calling BS on this, at least for top BR shooters. Check out the 2017 FISS results. For Jeff Summers, Wayne Campbell, and Tim Humphreys (1-2-3 in the 2-gun), the average group size spread for all yardage aggregates was 1.17-fold. The MAXIMUM spread for either of these 3 gentlemen was 1.86-fold (Wayne Campbell LV100). The TOTAL SPREAD between the smallest and largest groups for all 3 shooters put together (300 shots, 60 groups, 2 guns, 2 yardages) was 2.5-fold (0.112 smallest, 0.392 largest, measured in MOA).

Edit
NB: The numbers above are standardized differences, with the smallest group in a yardage as the baseline. If you are interested in just the ratios of largest:smallest groups, add 1 to the numbers above. I.e., the average ratio of the largest:smallest group by shooter/gun/yardage = 2.17. Nowhere near 5 or 6 either way.

 

[Bart B.]

Having seen other match results from top shooters that are the base for my comments, I'll let it go at that.

What were their groups like in other matches?

 

[drumcorpschamp]

Bart,
Those group shapes and then coming into a cluster are results I get when doing tests with a tuner. Seating depth test results in my experience do not go that way.
Jerry

 

[Bart B.]

Jerry, moving tuners on barrel fronts have done that for years. Their groups horizontal spread changes very little but vertically, a lot. Positive compensation at its best.

 

[Tim Singleton]

Never heard of this before......

Recently read that the US F-Class Open Team tests their loads using a 5-shot group for each .003" increment of bullet jump to the rifling. Each bullet seater settings' group will have its unique shape; typically elongated some amount and it's long axis at a different angle for a given bullet jump distance. When the group changes to a small cluster, that's the bullet jump distance to use for best accuracy. Repeating the same load bullet seating depth always has the same group elongation and angle; except for the bullet seating depth making jump distance to the lands perfect and produce a small round group.

I don't understand the mechanics of barrel vibration and bullet exit times that might cause this.

Nor do I believe that subsequent tests with the same bullet seating will produce the same group elongation and angle.

Anyone have insight to the team's test methods to verify this?

These guys have it figured out. This comes from many thousands of rounds being fired. Short range guys have been doing this for decades
Seating depth is the key

 

[Depart123]

I don't think it is any great mystery that a barrel vibrates like a tuning fork. It likewise is no great mystery what this pattern looks like since it shows up on the paper if you shoot enough shots at the same point of aim for it to be apparent.

It is neither a stretch of the imagination that things like: charge weight, bullet seating depth, and powder burn rate affected by powder temperature, primer sensitivity, brass thickness etc. will all have an affect on the exact moment at which the bullet exits the barrel. Is the barrel in the middle of it's transit or at the end of its travel when the bullet exits?

Bullet tipping as it enters the lands will affect dispersion for sure, but the harmonic patern of the barrel will be unchanged. If the dispersion is so large that the harmonic pattern is indiscernible then none of this is going to matter to you.

The confusion comes when you don't shoot enough bullets at a single point of aim and then start trying to read the tea leaves.

POI shifts are a completely separate problem with a few causes in common.

 

[Bart B.]

Tim,
Good picture. Would like to see a dozen groups with .011" bullet jump at all 3 charge weights.

 

[Tim Singleton]

Here are five. The charge weight varied thru the day. Don't remember right off by how much. Wasn't a lot
This gun and barrel is a shooter. You still have to pull the trigger at the right time tho
They don't magically all print at the same poa

 

[Heman]

Bart, glad you posted this here. Maybe the guys whose data I showed you that you didn't want to believe will chime in. Jeff Hopkins you around? Or two of our other F-Open shooters will chime in with it. Dwayne Draggoo or Kevin Hill you guys on?

Also a correction I said it was two of our shooters that are ON the US team. And the data I showed you from Jeff Hopkins was 3 shot groups. Please get the story correct if you are going to re-post.

 

[Bart B.]

I await their comments.

 

[Toby Bradshaw]

Having seen other match results from top shooters that are the base for my comments, I'll let it go at that.

What were their groups like in other matches?

Normally when someone throws out a SWAG and I respond with actual data, I don't feel obliged to do their research for them. But what the hell.

Tim didn't shoot unlimited at the 2017 FISS, but Wayne and Jeff did. Two shooters, two yardages, 20 groups, 200 shots.

Average standardized difference between largest and smallest group by shooter/range = 0.95
Average ratio between largest and smallest group by shooter/range = 1.95
Smallest group (MOA) 0.143 (Wayne Campbell 100M3), largest group (MOA) 0.519 (Jeff Summers 200M2).
Maximum standardized difference between largest and smallest group across both shooters at both yardages = 2.6
Maximum ratio between largest and smallest groups across both shooters at both yardages = 3.6

Still nowhere near 5-6.

Aggs and multi-guns are won with consistency, not a combination of tiny groups and big groups.

 

[Bart B.]

Meangreen,

I dont think a group pattern reflects where the bore axis pointed for each bullet hole. All bullet's barrel time, trajectories and atmosphere they go through are not exactly the same for each shot.

 

[chkunz]

Each bullet seater settings' group will have its unique shape; .............. Repeating the same load bullet seating depth always has the same group elongation and angle;..........

This is the part that I was hoping to see addressed, the ability to repeat the shape of the groups with the tuning. It seems that the replies are not focused on this critical point.

 

[Kevin]

I can say with certainty that not all US F-Open team members test the same way. We have different equipment and cartridges amongst us. Some use groups, some use ladders, some do both. Some test from 1000yds, some from 100. Some in between.

But what I will tell you is that most of us can find an accurate (1/4moa@100) load in approx 100-120 rounds after break-in.

That load will remain constant for the life of the barrel and component lots. The lands can be chased if necessary with seat depth to maintain a sufficient group size to be competitive.

We shoot 20 shots for score in up to 20 minutes. If we have a good load, then wind is our biggest enemy.

The results on target are due to understanding the bullet exit timing relative to the sinusoidal vibration of the barrel. You cannot find this without testing. However, the testing doesn't have to burn up the useful life of a barrel either.

 

[xcesiv]

Bart, glad you posted this here. Maybe the guys whose data I showed you that you didn't want to believe will chime in. Jeff Hopkins you around? Or two of our other F-Open shooters will chime in with it. Dwayne Draggoo or Kevin Hill you guys on?

Also a correction I said it was two of our shooters that are ON the US team. And the data I showed you from Jeff Hopkins was 3 shot groups. Please get the story correct if