Shot distribution that works against you.

by **Wobbley** 7/26/2023, 11:36 pm

Most people think that guns shoot their bullets in a single modal “Gaussian” distribution. This would have the majority of bullets hit within a bullet diameter or so of center. This would mean that a shot that breaks while pointed at say the 8 ring will land inside the 8 ring as often as not. But guns appear to shoot with a “Bi-Modsl distribution.

Shot distribution that works against you. Img_0210

This is a bimodal distribution. While there is a central thickening, most bullets don’t land on the center of the group. In short, the group is hollow to some extent with most of the impacts landing in a ring around the point of sim. One reason for this is that most bullets are over-stabilized and over-stabilized projectiles spiral around the trajectory (Mode2) and have random shot placement due to ballistic variance (Mode1). Pistol bullets are generally over-stabilized (quite a bit).

So consider the picture below. It shows the point of aim (the small cross) centered on the 8 ring at 10:30. The dotted ring represents this bimodal distribution. The diameter of the ring is roughly proportional to the distance. So at 50 it’s twice as big as at 25. What all this means is that you called an 8 on the line but you look and the shot is a 7 almost a 6. Why? If you look at the dotted circle the majority of the bullets will land outside of the ring. Couple that with a poor shooting pistol/ammo combination and you get shots way off call. This is why I try to load ammo that shoots well inside the “X-ring” sized groups at 25 yards. At 50 it will be just outside the “X-ring” group size. And it’s why when you shoot slow fire at 25 yards on the B-16 target, as often as not your score will average higher than at 50.

Shot distribution that works against you. Img_0213

by DA/SA 7/27/2023, 6:27 am

Funny how nobody ever looks for an excuse when they call a 9 or a 10 and find that it actually landed as an X.

When was the last time you claimed that an X was a flier?

Why do you make it sound like it only works against you?

Isn't there a "law of averages" associated with group size as far as printing on the target?

I'm referring to Bullseye pistol shooting, where the largest variable is the shooter, not referring to rifle shooting with the rifle clamped to the bench where the assertion that group size can only hurt you would be true.

My personal opinion is that bench rest rifle and bullseye pistol are two completely different animals.

by SteveT 7/27/2023, 8:13 am

My experience is that my pistols and loads at 50 yards have a distribution very close to normal, both in distance from center and direction. Sometimes the group is more oval but still closer to normal than bimodal. Many years ago I spent too much time shooting groups from a Ranson Rest, measuring each shot location on the target and plotting them.

I am no expert, but I understand rifle bullets take some distance to settle in and then they fly in a spiral. I have always assumed that our pistol targets were at short enough range that we were in the "relatively straight" part of the flight and if pistol bullets settle into a spiral like rifle bullets it happens past 50 yards. Even still, I kind of doubt it because our fat and slow bullets are losing velocity so quickly that they pretty much die beyond 50 yards. Some years at Camp Perry you can see the bullets hitting the ground 100-200 yards down range. If you walk out there at the end of the week you can find our bullets scattered on the ground.

And yes, I have been just as baffled by a called 8 that hits in the X-ring as a called X in the 8 ring. I'm more pleased with the X, but will stop and think about what might have happened anytime the hit on paper is more than an inch or two from my call, assuming I made a good call.

by Jon Eulette 7/27/2023, 10:42 am

Calling shots at 50 yds.......Did it look like a 10? Did it feel like a 10? You have to have both! Next time you shoot, shoot your shot. Ask yourself both questions (look and feel). If you don't have both then most likely the shot will not be on call.
When we shoot/train, we should learn to know what a 10 looks like and feels like. The feel is recoil based. Was my grip, stance and arm the same during recoil? It is a learned feeling of replication. If your arm feels different in recoil than your typical felt 10 shots, most likely it will not be a 10. Did you relax your arm during the shot? Did you relax your stance/core during the shot? Did you relax your grip?
Also slightly forced shots (not waiting for the shot break, but helping the shot off) can look like a 10 but not be a 10 because the shot was forced off.

My shot distribution is if it looked like a 10, and felt like a 10, it's a 10. Simple as that. Using excellent pistols and ammunition ensures this.

Jon

by **Wobbley** 7/27/2023, 11:03 am

DA/SA wrote:Funny how nobody ever looks for an excuse when they call a 9 or a 10 and find that it actually landed as an X.

When was the last time you claimed that an X was a flier?
If you’re honest with yourself. I’ve actually made a sight correction from a shot better than call but off call.

Why do you make it sound like it only works against you?
it doesn’t only work against you, but from the diagram more tha half of the distribution is away from the center. So the probability is that the shot will be outside the line.

Isn't there a "law of averages" associated with group size as far as printing on the target?
Not really the group is the group. It’s been well established that low round count groups are NOT representative at all. Even from a barrel tester. Which is why I use 25 shot groups to determine the potential accuracy of my gun/ammo combo. Many people shoot 5x10 shot groups, find one that shot an ‘X-ring” sized group and call it good but the reality is that there were 4 that weren’t accurate.

I'm referring to Bullseye pistol shooting, where the largest variable is the shooter, not referring to rifle shooting with the rifle clamped to the bench where the assertion that group size can only hurt you would be true.
This phenomenon is the main reason why Benchrest rifles haven’t progressed much in decades. Sure there are rifles that are real “hummers” but the reality is even with a “perfect rifle shooting perfect bullets in perfect conditions” the best you can expect is a group hovering around .100 inch. If you can aggregate that, you’ll likely win.

My personal opinion is that bench rest rifle and bullseye pistol are two completely different animals.
They are, but guns are guns.

by **Wobbley** 7/27/2023, 11:15 am

Jon: Excellent pistols and ammo along with excellent performance absolutely help by minimizing this effect. I’m not that fortunate, yet, but I’m working on it. One of the unstated points of this is that shooters have to be VERY critical of their gun&ammo combination. A pistol that barely holds the 10 ring was thought to be good enough. It isn’t! The gun has to demonstrate an ability to hold well inside the 10 ring in a large shot count group.

by DA/SA 7/27/2023, 12:19 pm

Wobbley wrote: One of the unstated points of this is that shooters have to be VERY critical of their gun&ammo combination.

The other unstated point is that they also need to be even more critical about their ability to utilize said equipment to it's fullest capacity!

Just another of my personal opinions...

by **Wobbley** 7/27/2023, 12:32 pm

Steve: Rifle bullets still spiral and they never stop. If anything it gets a tad worse. But in rifles, the spiral is way less than in pistols. Most pistols have static stability factors over 3. That’s why KKM have those 1-34 options and why they do well with 115 9mm.

Much of this has really only been determined due to better measurements of group sizes from electronic targets. In order for these elex targets to be used in ISSF the have to be accurate.

by 10sandxs 7/28/2023, 7:32 am

Testing is good. It builds confidence in your equipment. That said, I made master with a browning buckmark... after I shot a model 41 for two years that I couldn't break 820 with because the gun wouldn't hold 9 ring at 25 yards. I discovered this after I bought the buckmark for my wife and shot an 855 with sighters the first time I shot it... THEN I tested the 41... lesson learned...

by S148 7/28/2023, 8:17 am

Ashley/Wobbley,

I enjoy your posts very much and always read them with interest - especially the technical stuff. Please continue to do so. Thank you.

by robert84010 7/28/2023, 9:45 am

Years ago I asked about a simple question about matches in Las Vegas because I was going to visit family in the area and thought I could take in a match while there. Ashley piped in with "if I ran match there would be a match." There was no reason for that.

She can be just as big an asshole as I'm being here.

by SteveT 7/28/2023, 9:47 am

Ashley/Wobbley - I am appreciate your technical and historical knowledge of our sport and other shooting disciplines. Sometimes our opinions differ, but I always read your posts and think about what you say.

At first pass, I agree that many Marksmen and Sharpshooters would be better off spending their time practicing and training. I know I spent more time than necessary trying different guns and loads in my first few years. On the other hand, not everyone in the sport is focused on becoming the next national champion. Lots of us find the science behind accurate guns and ammo interesting.

As for High Masters talking about this, Jonathan Shue at Camp Atterbury talked about needing to test your guns and ammo to find the best load, springs and magazines because those can easily turn a 1" gun into a 4" gun or worse. Note that this is my memory and interpretation of what he said, not a direct quote. I have heard plenty of other very good shooters say essentially the same thing. Buy all the points you can, then go earn the rest.

by DA/SA 7/28/2023, 9:59 am

I need to take some composition lessons from SteveT!

My questions and comments are sometimes a bit too direct and to the point... I'm more used to business dealings than social dealings.

My apologies if I offended anyone.

by JHHolliday 7/28/2023, 12:11 pm

I tried to send a similar message earlier but it didn't go through: try again now.

Wobbley thanks for answering my question about the bimodal distribution. I, for one, am here to learn, and have learned a lot from you and many others. Thanks for taking the time.

Seems like no matter the field online forums frequently descend into ad hominem attacks. Much more often than in-person discussions, methinks.

In any case do some here believe there is harm in learning too much? Would better understanding the mechanics of shot pattern distribution or other technical information detract from skill development?

Back to the OP: Wobbley is the bimodal distribution a result of a circular shot pattern resulting from barrel/bullet characteristics? ie, if you mounted a barrel in a rigidly mounted vice and sampled many shots, is the distribution not a (3 dimensional) bell curve, but a curve with a circular peak shaped like a volcano?

by **Wobbley** 7/28/2023, 2:48 pm

The “bimodal” pattern is based on a “radius from point of aim.” That makes it two dimensional plot. In actuality it is as you describe, more like a volcano. To determine if this phenomenon is (A) real, (B) bullet driven, (C) barrel driven, (D)….. would require multiple guns, mucho ammo and tons of time….remembering that barrels, yes even pistol barrels, have a finite life the costs rise to the level of “is the juice worth the squeeze?” If I was pitching this to a technical manager saying I wanna spend a boxcar full of money on this test….I don’t think I’d be successful. Especially if we know about it, or have a reasonable suspicion that it is true, we can mitigate the effect by making the Gun/Ammo combo as accurate as we can and paying attention to our efforts. And that’s all I have ever said on this anyway. You the shooter still have to deliver a good shot on the target center.

by only_8_ring 7/28/2023, 3:22 pm

Wobbley, that's actually super cool. I mentally model my performance as a sum of 2D gauss ("normal") distributions. Ideally the mean for the gun and my "wobble" mean are the same, so the "mu" is the same for both. But there's some sigma_equipment and some sigma_human, and those get added.

I'd be interested to see some real-world data backing up this bimodal distribution idea. Even out of a machine rest, my equipment seems to conform pretty well to a 2D gaussian distribution across the target.

by **willnewton** 7/28/2023, 10:05 pm

I have banned a member of this forum for making deliberate attacks against another user, continuing to do so even after a strong warning from an admin, and posting their own public admission that they knew full well what they were doing. Rob also reviewed the situation and agreed it was warranted.

Rob asked that I clean up this thread and I have done so, removing the unneeded commentary.

Thanks to those that made supportive and positive comments. You may find your posts in this thread have had some partial or full deletions, but only to bring us back on topic, not because you were part of any offense.

Thanks to those with experience that are willing to help give others a hand up, rather than trying to throw other folks below them off the ladder.

by **Founder** 7/29/2023, 12:08 am

Personal attacks are never tolerated on this forum. Thanks for taking care of business, Will.

Ashley, I'm not smart enough to understand bimodal patterns, but as the guy on this forum who has probably the cheapest kit, I'll do some rest testing to see if cheap equipment and ammo is capable of that winning performance.

I want to see if my volcano pattern can land in the X-ring at 50 yards.

I don't have the variety of guns and ammo to prove bimodality, but I think I can prove that cheap equipment can get a competitor to a performance level that they can be happy about for at least 25 years in the sport.

Ashley, even if the bimodality is the pattern, wouldn't it be more likely that the shot lands at the mouth of the volcano instead of at its base? So when that shot is called a loose 8, it's less likely that the bad shot by the shooter would correlate to the loosest shot in the bimodal results of the gun/ammo?

by **Wobbley** 7/29/2023, 8:55 am

Rob: yes the bullet will hit nearer the mouth of the volcano than elsewhere most of the time. About 40% will likely be inside the volcano…. That helps… it also helps if the volcano is tall and has a small mouth diameter.

And yes cheap equipment shooting cheap ammo can be very competitive. I was doing that when I shot rifle. I scoured the “reject racks” of gun shops for years looking for beater pre-64 model 70s to build match rifles on. Found some too, and my best 45 pistol was bought used at a gun shop.

by **Founder** 7/29/2023, 1:14 pm

My mindset is that if my hold was good, but I broke the shot as a loose 8 that ended up a loose 7 because of the bimodality, I wouldn't dwell on it as breaking the shot as a loose 8 is not part of my shot process. I focus on the next opportunity to hit that next 10.

by DA/SA 7/29/2023, 2:57 pm

Founder wrote:I wouldn't dwell on it as breaking the shot as a loose 8 is not part of my shot process. I focus on the next opportunity to hit that next 10.

Ding, Ding, Ding! We have a winner!

Don't focus on or analyze undesirable shots.

by Wes Lorenz 7/29/2023, 10:42 pm

Ashley,
If a fast twist barrel contributes to to over-stabilized projectiles and bimodal distribution, how do you explain a football and it's spin rate leading to better accuracy? How about a baseball - fast ball vs. knuckle ball?
Just don't understand your OP vs. my life experiences.
Thanks,
Wes

by crmath 8/7/2023, 5:45 pm

I hesitate to post this reply because I'm not an expert on the subject of Bullseye Pistol. I do understand error analysis fairly well and someone might find my take to be interesting.

The bottom line for me is that bullets do precess about their nominal trajectory just as Ashley describes and the 'volcano'-shaped distribution describes the effect. I also agree with Ashley (and everyone else) that better ammo results in better groups. Upon superficial examination of the literature (courtesy Google) I'm convinced that precession error is systematic, not random and the effect on our groups is mostly translation, not dilation. If true, this explains why expert ammunition testers don't notice circular patterns and that the effect of precession on group size is not as serious as it first appears.

My analysis became a little longwinded, so I attached a PDF. I hope you can open and read it.

If there are any well-informed ballisticians out there, please chime in.

Happy reading!

Bob

Attachments

: Bullet Precession, Group Size, and Location.pdf You don't have permission to download attachments.(430 Kb) Downloaded 23 times

by SteveT 8/7/2023, 8:18 pm

Interesting analysis, and good sources. Thanks for sharing.

One question it raises with me is can we assume the precession "rotation" always starts in the same direction when the bullet leaves the barrel or does it start in a random direction, in which case it could hit the target anywhere in the circle even if the velocity was the same shot to shot. If there is a flaw in the muzzle it might be somewhat consistent, but I suspect that tiny differences in the base of the bullet would influence the direction the bullet starts to precess.

Another thought on the whole topic is if the bullet is tipping as it precesses in a spiral we should see that in the holes on target. I have definitely seen oblong holes with 38's, even when shooting decent groups, but with 22, 9mm and 45's when I am looking for a double and using an overlay or Eagle Eye, the holes are very close to round almost all the time. I doubt I could see a 5 degree tilt, but I suspect more than 10 degrees would start to be visible and we would comment that holes in the target are usually slightly oblong. Would a few degrees of tilt impart much of a spiral to the path? I don't know the answer, just wondering.

by **Founder** 8/7/2023, 8:45 pm

Keep in mind that even a poorly thrown football that doesn't have a tight spiral travels generally in the direction it is thrown. Sure the aerodynamics are poor, but the mass is still generally traveling along the same trajectory.

I also agree that the shooter is able to impart some fault in the .38spl WC bullet's spin through error in follow through, because I've seen the evidence. However, it's my opinion that most of any distance from the x wasn't because of spin disruption, but the point of aim when the bullet left the barrel.

It's my opinion that the ability to observe this phenomenon is only possible when shooting from a machine rest.

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