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loss of accuracy between 25 and 50 yards diagnostics

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hengehold
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Post by WSR32 Wed Aug 17, 2022 11:24 am

This is just a hypothetical question to deepen my understanding as opposed to solving a problem with a particular pistol.

If you have a pistol that can produce groups at 25 yards with 5 rounds touching lets say.  But when you shoot that same pistol, same ammo, on same day, etc... out at 50 yards the 5 round  group grows to 5-6 inches (or more, something ridiculous).  Would one look to issues with the ammo?

Large groups through out all ranges fired would be more likely be an issue with the pistol?  

To be clear the pistol is being shot from a rest or a ransom rest to eliminate the shooter from the equation as much as possible.

Regards,

Miguel

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Post by NukeMMC Wed Aug 17, 2022 11:35 am

Yes, look at ammo.
Yes, look at the pistol.
Yes, look at the Ransom Rest mounting and how consistently you operate it.
Could be any or all of the above.

Think of a pistol's accuracy as a cone, with the center at the muzzle's center.  The cone grows significantly between 25 and 50yds.
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Post by james r chapman Wed Aug 17, 2022 11:37 am

Or, minimum velocity, spin rate.
.38 wadcutters are known for dying at 49 yards lol
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Post by NukeMMC Wed Aug 17, 2022 11:59 am

james r chapman wrote:Or, minimum velocity, spin rate.
.38 wadcutters are known for dying at 49 yards lol
Just like 168gr 308 bullets will suddenly drop from their ballistic flight at 800yds? Laughing
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Post by straybrit Wed Aug 17, 2022 12:08 pm

Well, if you assume that the muzzle is a point and the bullets travel in a straight line (yeah - neither are true but bear with me) then the 50yd group is twice the size of the 25 yd group. So if it's opening out more than that then the bullet is not travelling in a straight line. So the question is what stops that happening. Given that we are using rifled barrels (you did check that didn't you? :-) ) it's a reasonable assumption that the bullet became unstable at some point.  So you are essentially not imparting enough velocity and spin for long enough to keep it stable. 

So much for theory - in practice things get a lot harder - you have to deal with the internal ballistics of how the spin is imparted and how cleanly the bullet exits the barrel as well as the obvious speed and air resistance issues.

To Jim's point - there's a reason that successful model 52 shooters have very long arms.

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Post by Asa Yam Wed Aug 17, 2022 3:18 pm

NukeMMC wrote:
james r chapman wrote:...38 wadcutters are known for dying at 49 yards lol
Just like 168gr 308 bullets will suddenly drop from their ballistic flight at 800yds? Laughing
Different bullets, same issue.  WCs and 168 Sierras are GYROSCOPICALLY stable (Sg), and DYNAMICALLY unstable (Sd).    When Sd exceeds Sg, precision suffers.  The solution is to either increase velocity (more recoil, need to redevelop load), OR use a different bullet (158 grain SWCs, 190 grain Sierras),

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Post by Wobbley Wed Aug 17, 2022 6:13 pm

First, a group at 25 yards “with 5 rounds touching” isn’t a particularly great group.  5 45 shots touching can be as large as 2.0 inches. It also isn’t statistically valid, really.  Better to shoot 15 shots and see what you get.  You want a large blob about the size of the “X” ring.  When you shoot that at 50, from a machine rest, the group should be about 9 ring size unless there’s a ballistic reason the bullet is becoming unstable.  So yes you can look at ammo first. One way to confirm it is ammo is to shoot factory match ammo and see if the ammo is consistent in accuracy between ranges.  A poor shooting pistol usually manifests as as number of shots way off call.  Ransom Rests are a learning curve all of their own.
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Post by james r chapman Wed Aug 17, 2022 6:23 pm

Example 

loss of accuracy between 25 and 50 yards diagnostics Dbcaee10
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Post by hengehold Wed Aug 17, 2022 6:37 pm

WSR32 wrote:This is just a hypothetical question to deepen my understanding as opposed to solving a problem with a particular pistol.

If you have a pistol that can produce groups at 25 yards with 5 rounds touching lets say.  But when you shoot that same pistol, same ammo, on same day, etc... out at 50 yards the 5 round  group grows to 5-6 inches (or more, something ridiculous).  Would one look to issues with the ammo?

Large groups through out all ranges fired would be more likely be an issue with the pistol?  

To be clear the pistol is being shot from a rest or a ransom rest to eliminate the shooter from the equation as much as possible.

Regards,

Miguel
I have been shooting highpower rifles at 1000yds for about 20 yrs. Here are a few principles that may help with the OP question. In the rifle world there are a couple things that can happen to cause a cone of fire to become bigger than a linear projection. An example of a  linear cone of fire would be a shot group of 1 MOA (1”) at 100 yds and as you extrapolate that out it continues to maintain a 1 MOA cone of fire for maybe out to 1,000 yds to pick a number. 

Variable 1: velocity variance. The variance of the muzzle velocity (MV) becomes increasingly more important as you get farther from the target. A deviation of 20 fps in the MV is not a big deal at shorter ranges (100-300 yds) but at 1000 yds that variance is much more pronounced because the time of flight is longer and the vertical bullet dispersion on target becomes exacerbated. Many successful The short range (100-200yds) benchrest rifle shooters just throw powder charges and do not weigh each one. Contrast that with 600-1000yd f-class and benchrest shooters weigh powder charges to the 0.01 gr for best noticeable consistency. Velocity variance shows up as a vertical dispersion on target. 

Variable 2: bullet stability. A bullet may become unstable a yaw/pitch due to a transition from super-sonic to sub-sonic Additionally, a bullet can be unstable if it is not spinning fast enough to be stabilized, even though it may be supersonic. This bullet instability can result in flyers that are vertical or horizontal in nature, or somewhere in between. 

How does this relate to pistols? I doubt that variance in the MV will cause a non-linear dispersion between 25 & 50 yds. 
As far as stability, you will have to consider if the pistol is transitioning between super-sonic and sub-sonic between the 25yd line and the 50 yd line. Or, is the bullet unstable for the entire flight due to lack of spin and therefore tumbling?

Sorry for the long answer to your short question. 

-TH

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Post by Wobbley Wed Aug 17, 2022 8:12 pm

Except that most bullseye ammo is loaded well below sonic velocities.  Much of the problems in bullseye pistol ammo is poor bullets ( for a lot of reasons).  There’s a reason most HM shooters shoot 185 JHP.  It’s because it’s a stable and known quantity.  A pistol bullet has to be easy to load straight (key), function through bullseye guns flawlessly, fly straight and stable to 25 and/or 50 yards.  The JHPs are relatively easy.  SWC and lead SWC may be great or difficult.
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Post by Allgoodhits Wed Aug 17, 2022 8:19 pm

If it is factory ammo, try a different factory ammo. If it is a handload, try some loads -2/10 and some +2/10 gr to see what the group change from 25 yds to 50 yds does. I know of no load that is accurate at 50yds that is inaccurate at 25 yds. There are numerous loads that are accurate at 25 yds and not so much at 50 yds. Are the atmospheric conditions the same? In other words is the 25 yd test indoors or protected, and the 50 yd test has other atmospheric conditions, like outdoors?  Temperature similar during testing?Maybe not complete science, but double the distance, double the group size.
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Post by WSR32 Wed Aug 17, 2022 8:20 pm

Thanks everyone.  Lots of good insight.  

I read more on Sg and there are plenty of online calculators for it.  There seem to be no calculators or formulas for Sd.  Further this does not seem to be a number post/listed/or advertised in any of the bullets I load.  Is there a resource for Sd that I am missing?

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Post by NukeMMC Wed Aug 17, 2022 8:29 pm

Asa Yam wrote:
NukeMMC wrote:
james r chapman wrote:...38 wadcutters are known for dying at 49 yards lol
Just like 168gr 308 bullets will suddenly drop from their ballistic flight at 800yds? Laughing
Different bullets, same issue.  WCs and 168 Sierras are GYROSCOPICALLY stable (Sg), and DYNAMICALLY unstable (Sd).    When Sd exceeds Sg, precision suffers.  The solution is to either increase velocity (more recoil, need to redevelop load), OR use a different bullet (158 grain SWCs, 190 grain Sierras),
Mine was a tongue-in-cheek comment about how my fellow highpower shooters say the 168gr SMK cannot be used beyond 800 yards due to its less-than-stellar performance.  While this is true for M852 or FGMM168, it isn't concrete across the board.  The USMC G4 load was a 168SMK that produced excellent service rifle scores at 1000.
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Post by Asa Yam Thu Aug 18, 2022 1:17 am

NukeMMC wrote:The USMC G4 load was a 168SMK that produced excellent service rifle scores at 1000.
NOT a good example.  The G4 load was reportedly a HEAVY load, well above NMC load levels. (NMC loads are typically 41.5 to 42 grains of IMR-4895, which is right at max load levels.  G4 reportedly used 44+ grains of 4895.)  Ever wonder why early lots of M-118 LR were spec'ed as "not for use in M-14 rifles"?  PORT PRESSURE on this load was too high - damage to the M-14 gas system and action was inevitablle if 118 LR was used on a regular basis.

Remember what I stated earlier:  Use more powder (redevelop load), or use a different (heavier) bullet.  Here is how the 7.62 NATO loads listed above apply those principles:
  1. G4:  More powder.  2 to 3 grains more than near-maximum loads for NMC shooting;
  2. Early M-118 LR:  175 grain Sierra on top of a charge of a noncommercial variant of WW-748.  748 is a BALL powder, meaning the burn rate is different (slower burn rate and higher port pressure) compared to IMR-4895;
  3. Later M-118 LR:  Same bullet, powder changed to RE-15 due to change of LC ammo contract from Winchester to Federal Ammunition.  NOTE:  The load specifications for M-118 LR were changed so MV (and port pressure) became safe for use in M-14 rifles.


Believe M-118 LR was one of the ammo types whose designations changed after the powder was changed to a ball-type.  IIRC, one of the issues with RE-15 is temperature sensitivity, a bad thing for ammo expected to see use in the Middle East or Southwest Asia.

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Post by Asa Yam Thu Aug 18, 2022 1:38 am

WSR32 wrote:Is there a resource for Sd that I am missing?
Believe the late Dr. Robert L. McCoy discussed Sg and Sd in an article on the behavior of the long range ballistics of the Sierra 168 grain, 30 caliber bullet.  A copy of this article was reprinted in a "Precision Shooting" annual edition, will look into the two copies I own for the article title and location in the edition.  Hopefully, will post a response tomorrow afternoon.

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Post by BE Mike Thu Aug 18, 2022 9:49 am

For a good answer you'd have to be shooting a pistol that has proven accurate at 50 yards with proven accurate ammo at the same distance.
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Post by Asa Yam Thu Aug 18, 2022 6:55 pm

Asa Yam wrote:Hopefully, will post a response tomorrow afternoon.
Found the article.  Publication is the "1996 Precision Shooting Annual", document is titled " Long Range Flight Characteristics of the .308", 168 Grain Sierra International Bullet", article runs from page 153 to 193.  The section of interest starts at page 163, and is titled "Definition and Discussion of the Dynamic Stability Factor".  The formula for SD is:

SD = 2[CLa+ (m d^2/Ix)CMpa]/[CLa-CD-(md^2/Iy)(CMq+CMa)]

Terms:
  • m = projectile mass (0.0240 lb);
  • Ix = Projectile axial (polar) moment of inertia, (0.000001713 lb-ft^2);
  • Iy = Projectile transverse moment of inertia (0.00001277 lb-ft^2);
  • d = Projectile diameter (0.02567 ft);
  • CD = Drag force coefficient;
  • CLa = Lift force coefficient;
  • CMpa = Magnus moment coefficient;
  • (CMq + CMa) = Pitch damping moment coefficient;
  • SD = Dynamic stability factor.


NOTE:  on Page 169, there is a comment that "the pitching and yawing moment of any projectile is dynamically stable, if and only if the value of SD lies within the range, 0 to +2." (sic)

There is more in the article about the long range ballistics of the 168 Sierra, far more than I wish to type.

Hope this helps.

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Post by Jon Eulette Thu Aug 18, 2022 7:57 pm

From longtime bullseye gunsmithing perspective and as former top level competitor, I have always approached the pistol and ammunition perspective at 50 yds only. Why? I was taught that way; 50 yards is the true test of a bullseye pistol. For the heck of it I tested at 25 yds and found that it was a waste of time because good guns and good ammunition would shoot almost a single hole. 25 yards is not as badly affected from user error with machine rest as well. 
When I was a team shooter I always tested my yearly supply of ammunition. It was typical to get a 5 shot string at 50 yds that was sub1” and all shots clustered tightly. Getting 10 shots to shoot like that was difficult. Typical flyers opened up groups to 1.25-1.5” using .45 Federal Match. Also I wasn’t testing to share my best group ever, it was just to verify that load/lot was getting along with the pistol. Sub standard lots were used for 25 yd ammunition.
I recently posted pics of sub 2” groups with unsorted lead bullets and mixed old brass. Again verifying that that load grouped at 50 yds. 
Were groups expected to be this good? Hell yes! Premium built bullseye pistol. Premium guns will shoot excellent ammunition tight and still group within the 10 ring with nothing special hand loads. Carefully loaded hand loads will do even better. 
Taking an unknown built bullseye pistol, an old wadgun or a RO and wishing it shot sub 1.5” at 50 yds is a pipe dream. I have shot and worked on those pistols a lot, and they never ever group. Recently saw groups from RR testing that barely held the black at 50 yds on a pistol of yesteryear. Old school build style had minimal lockup to start with, and then adding mileage just makes it worse. One of my customers had Dave Sams HEG rest his ballgun. It shot 1” at 50 yds and had over 10,000 rds through it. It was built to lockup and stay locked up. You can build to compensate for it. 
So bullet this and bullet that is important, but it starts with the pistol. Was the pistol built to perform at 50 yds? If not, it probably won’t. You might get one good target, but let’s not lie to ourselves.
Jon
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Post by WSR32 Fri Aug 19, 2022 10:16 am

Asa Yam wrote:
Asa Yam wrote:Hopefully, will post a response tomorrow afternoon.
Found the article.  Publication is the "1996 Precision Shooting Annual", document is titled " Long Range Flight Characteristics of the .308", 168 Grain Sierra International Bullet", article runs from page 153 to 193.  The section of interest starts at page 163, and is titled "Definition and Discussion of the Dynamic Stability Factor".  The formula for SD is:

SD = 2[CLa+ (m d^2/Ix)CMpa]/[CLa-CD-(md^2/Iy)(CMq+CMa)]

Terms:
  • m = projectile mass (0.0240 lb);
  • Ix = Projectile axial (polar) moment of inertia, (0.000001713 lb-ft^2);
  • Iy = Projectile transverse moment of inertia (0.00001277 lb-ft^2);
  • d = Projectile diameter (0.02567 ft);
  • CD = Drag force coefficient;
  • CLa = Lift force coefficient;
  • CMpa = Magnus moment coefficient;
  • (CMq + CMa) = Pitch damping moment coefficient;
  • SD = Dynamic stability factor.


NOTE:  on Page 169, there is a comment that "the pitching and yawing moment of any projectile is dynamically stable, if and only if the value of SD lies within the range, 0 to +2." (sic)

There is more in the article about the long range ballistics of the 168 Sierra, far more than I wish to type.

Hope this helps.

Spent a little time turning this into a usable spreadsheet: bullet stability

Not sure if that link will work for you all, should be public.

Yellow cells are the ones that you would input specific bullet info into. 

At the moment the bullet shape is estimated with a cylinder....I'll have to break out my old engineering text books and maybe go back to school to get a closer approximation Wink

red cells still need to be worked on....

Fun little exercise but Ill probably stop for the day and make the time to get to the range Very Happy

M

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Post by PMcfall Fri Aug 19, 2022 12:29 pm

Jon said it all!
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