Causes of extream spread

[Bart B.]

Yes. I used a Kowa TSN1 spotting scope to see bullet holes in the black

 

[Alex Wheeler]

Yes.

Sure would like to know what you consider a decent scope. If its resolving holes at 1k good enough that you are using them to shoot with I sure want to know what scope that is!

 

[Bart B.]

how is it, everything being equal and loaded to the same precision that one powder charge will produce good es and other wont?

A powder charge in a case needs to be ignited just right and have the right chemistry and charge weight to produce uniform and consistent pressure curves for the barrel's bore size and length & diameter for the bullet used. Otherwise, the pressure curve will be inconsistent and so will be the bullets barrel times and therefore the muzzle velocities.

So says the ballistics engineer at Lake City Army Ammo Plant when I queried them in 1971 about a bad lot of 7.62 NATO M118 match ammo; lot number 12064 made in 1966. 3 or 4 rounds in a 20 round box would shoot 2 or more MOA low at longer ranges and was not all that great through 300 yards. Yet its new case and primer were good enough to be stuffed with 44 grains of IMR4320 under 190's to test near 1/2 MOA at 600 yards through 7.62 Garand barrels. Its original charge weights of IMR4895 had about a 3/10ths grain spread; normal for all M118 match ammo.

 

[Bart B.]

if your stopping in your record string to look for bullet holes through a spotting scope, your not going to have much of a chance.

That depends on how much you want to catch a wind change then hold off to correct for it. As well as how adept you are to scope your shot while reloading.

 

[Alex Wheeler]

A powder charge in a case needs to be ignited just right and have the right chemistry and charge weight to produce uniform and consistent pressure curves for the barrel's bore size and length & diameter for the bullet used. Otherwise, the pressure curve will be inconsistent and so will be the bullets barrel times and therefore the muzzle velocities.

So says the ballistics engineer at Lake City Army Ammo Plant when I queried them in 1971 about a bad lot of 7.62 NATO M118 match ammo; lot number 12064 made in 1966. 3 or 4 rounds in a 20 round box would shoot 2 or more MOA low at longer ranges and was not all that great through 300 yards. Yet its new case and primer were good enough to be stuffed with 44 grains of IMR4320 under 190's to test near 1/2 MOA at 600 yards through 7.62 Garand barrels. Its original charge weights of IMR4895 had about a 3/10ths grain spread; normal for all M118 match ammo.

That doesnt explain why. In the node where everything is happy lets say you have an es of 12. Add .5 grains and go out of the node, now you have an es of 35. How does a very small change in powder, which is only slight increase in pressure now become inconsistent. I am not really asking for an answer because I doubt any of us know. But its an interesting topic.

 

[dmoran]

That depends on how much you want to catch a wind change then hold off to correct for it. As well as how adept you are to scope your shot while reloading.

Sounds conforming to high power.... but not LRBR
No disrespect intended here, but will say, I feel much of your input is not up to current times, at least with LRBR. In todays LRBR, a well tuned rifle and its tracking ability in the rest, along with the reloaded capability of the ammo, will destroy "aimers" by far more days then not.
Donovan

 

[ebb]

Joe Salt I am not talking about all the little things that one rifle may like and one not so much. I am talking about reloading. I frequent benchrest central and one day the talk centered around primer pockets. Some one was talking about all the work required to clean his primer pockets. Many posted that they didn't clean primer pockets and had tried dirty and clean side by side and had seen no change in group size. So many of the life long benchrest shooters agreed on this I took it as fact. I haven't cleaned pockets in a few years. Primer pockets are common to all but rimfire rifles, so I don't see this as being some thing that is good on one rifle and not on another. I am talking about reloading procedures. I was loaned a book called "Rifle accuracy facts" by Harold Vaughn. If some one with a scientific back ground like Harold were to test reloading procedures and say this works and this doesn't, and this I am not sure about. I could proceed with some idea that I was heading in the right direction. Just look at what Alex Wheeler has done with his scope testing!!!!!

 

[Bart B.]

I have been racking my brain. I know its a little different question than the op, but how is it, everything being equal and loaded to the same precision that one powder charge will produce good es and other wont?

How many shots per load are you shooting?

Chronographing tests are like accuracy tests. More shots means more statistical significance in the the numbers obtained.

But it's easier to get good reliable results clocking bullets than accuracy tests. Bolt the barreled action to a 2x4 clamped to the bench top in such a way it doesn't recoil. Then only the components and barrel variables as a system will be measured for repeatability. Us humans inability to shoulder rifles the same way is no longer part of the system. Then shoot at least 20 shots per load.

 

[Bart B.]

I think pressure, bullet type and charge weight are part of the cause of barrel lives. 22 caliber cartridges burning powder charge weights in the low 20's and 55K psi pressure with jacketed bullets have about 3000 rounds of life for competition. 22's burning 1 grain charge weights and 24K psi pressure with lead bullets have about 30,000 round barrel lives in the same discipline.

 

[Macropod]

I've always been intrigued when with a 308 you can have a load which groups in the teens at 100m then find that the same load is struggling to hold 1 to 1 and a 1/2 moa vertical at a thousand.

Carefully prepped loaded cartridges, weighed charges to 0.02gns and the ES is terrible yet does excellent groups short range. Finding that magic group and having an extremely low ES is one of the most frustrating things about this sport. Still keep coming back though, as when you do crack the code, it's hard not to be happy as a clam.

 

[Alex Wheeler]

I've always been intrigued when with a 308 you can have a load which groups in the teens at 100m then find that the same load is struggling to hold 1 to 1 and a 1/2 moa vertical at a thousand.

Carefully prepped loaded cartridges, weighed charges to 0.02gns and the ES is terrible yet does excellent groups short range. Finding that magic group and having an extremely low ES is one of the most frustrating things about this sport. Still keep coming back though, as when you do crack the code, it's hard not to be happy as a clam.

If you want a good 1k load, tune at 1k and ignore the es.

 

[Jimbo47]

Precise charge weight is critical to precision and repeatability. Lets assume
1) brass is properly prepped. Flash hole primer pocket clean. Neck chamfered AND clean INSIDE.
2) mike the necks top and bottom and around before and after bullet seating. What the neck tension is is not as important as it is the same round to round.
3) mike the OACL for your jump. Don’t chase 0.002 but do chase .010. Unless you are on the lands or have a slight jam.
4) keep the longitudinal runnout under 0.002. Hornady talks about this.
5) powder charge weight is critical. Hope I don’t loose you here. At lower minimum
weights the incremental velocity to additional charge will remain fairly linear. As charges are increased to maximum pressure there will be an inflection point in the curve of velocity change to weight change. It will move up and sometimes sharply. It is at these loads that small changes in charge weight yield big changes in velocity. Your chrono data should be used to calculate the velocity change relative to charge compared to lower charge level ratioed to total velocity. When the ratio of average gross velocity to charge weight to incremental velocity to charge weight starts getting above 1.0 heading to 1.3+ your pressures are rising. It is here that tenths of a grain may not be good enough precision. Some powders ( like IMR 4350) which are also temperature sensitive are not easily metered to .005 grains.
6) As others have said shooting and repeatable recoil management is important. Ambient temperature rises during your shooting and cartridge heat soak must be managed. Release the shot in 5 seconds and get the spent brass out. Do not wait. Just a 10 F temp change can add or subtract 15fps with many powders ( except Retumbo and a few others). Hot magnums need cooling often after just two shots.

 

[riverwolf1]

Density Altitude, Temperatures, Humidity. All things that effect velocity of the bullet, considering that the loaded rounds all all to the same specifications. Like VV-133 cool mornings works fantastic, but as daytime temps increase the loads go south, extreme heat air density gets thicker hence velocity decreases. Same reason an aircraft needs more runway in hot summer days thicker air. Look at LT-32 folks back east have a tough time keeping it in tune as humidity increases, out west performace much improved because the heat is there but not the humidity. Just my observations and your milage may differ!

 

[dcali]

Anything like this is a summation of uncertainties (tolerances in this case). If everything is *exactly the same, you're going to get exactly the same velocity. But that's never hte case. Listing all the things I can think of.

Powder: If you've ever looked closely at powder, the kernels are not all exactly the same size and color. There will be slight variations in burn rate as a result of the normal manufacturing tolerances and coatings. There is also the obvious variation in charge weight. Heating barrels will heat the powder slightly.

Case: Case volume varies, and case sizing varies. Case volume impacts pressure. Also case heat capacity varies because case weight varies. Case hardness varies. All of the above impact the amount of energy absorbed by the case. case neck consistency impacts starting force.

Barrel: Each shot down the bore changes the dimensions of the chamber and rifling. Each shot also increases the temperature, and the barrel may distort, and will certainly grow slightly after each shot. This alters friction, which impacts velocity. Temperature also impacts the stiffness of the bore, and how it responds to pressure. Powder residue left behind will alter friction and possibly heat.

Primer: Priming compound is not exactly the same from primer to primer, and neither is the cup/anvil geometry. They will vary in energy content. Firing spring hysteresis, lubrication, grit, dirt, and temperature changes will impact the force of impact on the primer.

Bullet: jacket thickness and hardness vary, as does weight, volume, and hardness. Same with the core. Seating depth impacts case volume and start pressure.

Some of this we can control, but some of it we can't. It's also tough to tell which ones really matter the most. It seems like powder charge, case volume, consistent sizing are good places to start, as is using high quality bullets. I don't want to get into the primer weighing thing, but some study there might be illuminating. Heavy, long barrels with large heat capacities would be helpful. And it wouldn't surprise me if some powders are better than others due to the sort of mess they leave behind.

One thing to look at is how the velocity trend changes as the barrel heats. The true variance is often a little less than we think if you account for the rising mean velocity. In other words, if you know your barrel walks a 1/4 minute up over 20 shots, then you can deal with that. The random part, you can't.