Velocity and accuracy

[cdoc42]

It seems to be well understood that the highest velocity is not always synonymous with accuracy, although I would guess most of us work toward obtaining the highest velocity we can. I would suggest that is not an unreasonable goal, for having determined the highest velocity is safe, but not accurate, we can then back down, looking for accuracy without intensely worrying about safety.

That said, my thoughts wondered to the possibility that velocity in a given caliber for a given bullet weight might be where the secret lies.

For example, is it possible that the most accurate bullet in a .270 might be 130 grains going no more than 2900 fps? In 7mm Rem Mag, 160 gr at 2800? In .338, a 215gr bullet going 2600? And so on.

Now certainly one can attempt to find an accurate 150gr bullet in .270, or 175gr in 7mm, leading to the eventual conclusion that one needs only one rifle for all his/her hunting needs. But in my reloading experience I’ve experimented with 90gr, 100gr, 130gr, 150gr in .270; 139-140-160-175 in 7mm, with the thought in mind that I would get all of these as accurate as each other in my particular rifle. Of course, it didn’t happen. That led to the thought that perhaps a given caliber, twist rates notwithstanding, finds its best accuracy at just one given bullet weight and velocity.

Another issue I would toss up for thought is seating depth and the relationship to velocity due to changing pressures. If seating the bullet at the lands with “X” grains of powder “A” gives sub-MOA groups, I assume the velocity achieved is a result of the pressure generated by the charge. Is it possible to seat the bullet off the lands, then increase the charge to achieve the same velocity, likely to produce the same accuracy? If so, seating depth is not as important as pressure and velocity.

Any interest in commenting on these wandering thoughts?

[croc4]

the trouble is that there are so many factors that influence accuracy, that its like hitting a moving target. I have two pistols, same make/model caibler, one likes load A, and the other likes load B.

The actual "make up" of the rifle is what determines what the most accurate load is. let me explain, when a round is fired a pressure wave is created, this wave will vibrate the rifle, if the wave is at the harmonic resonant freq then the rifle will behave more consistently from shot to shot meaning that the bullet will exit the barrel at roughly the same point in the vibration curve and hence better accuracy. If the wave is not at a resonant freq then the bullet will exit at different (less predictable) points of the rifles vibration curve hence throwing off the bullets POI. and each gun is unique in that the metal and wood density will vary, bedding will be slightly different etc, etc. All of these factors determine how the rifle will vibrate at a given freq that is caused by the pressure wave of the round fired.

If there was a way to determine this freq for a given rifle and then a way to calculate the freq of the pressure wave at a given load then finding the best load would be far less time consuming, but baring that we just pick a point in the safe load range and either work up or down in increments until we hit the sweet spot.

[Shoney]

I agree with croc4, but would add: The brand of bullet and the brand of primer can also be a factor. In many of the cartridges I have loaded for over the years, some weapons simply didn’t like some brands of bullets, no matter the weight. Others would do well with a any brand and most weights.

In some weapons the best groups varied with the brand of primer or the strength (regular or magnum).

You just have to try different bullets, with different primers, with different powders, with different seating lengths. No magic, just time with trial and error.

[cdoc42]

I agree with your points, Croc4 and shoney, but don't miss my point - all the variables you list affect accuracy, but is it because they also affect pressure and velocity? In other words, if a 130gr .270 Speer bullet is the most accurate one reloaded, is it because it happens to be going 2900 fps rather than 2800 or the magic 3100 fps in books? Then no matter what combination of primers, cases, seating depths, etc., one came up with, as long as the load went 2900 fps would that explain the accuracy? Probably best answered by those who have played with chronographs.

[Wild Bill Bucks]

May be missing the point here, but I have been working on a compitition .308 lately and with a 110 grain Hornady V-Max bullet @ 2950fps, this round will drive tacks at 100 yards, but if I change to a 110 grain Sierra, with all other things in load being equal, the groups go to 1 1/2".

Since both rounds are loaded equally, then they should perform equally at the same velocity, but they don't. The only conclusion would be that the different bullet company would make the difference. Am I missing your point totally?

[tINY]

Friction is different - acceleration in the barrel is different - pressure profile changes - harmonics change........



-tINY

[cdoc42]

Wildbill, did you chronograph them or assume that the same load will end up driving both 110gr bullets at the same velocity?

Bullet design, even though the weights are the same, will influence friction as stated above, and therefore, velocity. If the ogive is different the bullet will engage the rifling at a different point on the bullet, so that brand has more mass on one end of the ogive than another. If the friction is greater the velocity is higher because the pressure behind the bullet builds to a greater extent. But if you load with a chronograph and get them both going relatively the same speed (low ES, e.g.), I wonder if they would then be equally accurate.

And would not barrel harmonics be the same if the pressure (and velocity) is the same from shot to shot? So again, pressure/velocity in a given rifle may be the accuracy factor..............no?

[castnblast]

There is another factor that affects this. Barrel harmonics. Different powders burn at different rates, thus creating diffent harmonics in the barrel (in addition to the other factors listed) That is why some powders work better with some weights of bullets, and lousy with others. I've done a lot of reloading for the 7mm rem mag in the past, and found that it was very finacky on the lighter bullets, and the solution turned out to be as simple as changing the powder. My 300 win was the same way, but not so with the .270. I think barrel length was the issue and not the caliber. Both the mags had long 26" barrels which are inherently prone to harmonic issues, where the 270 was a 22". That's my theory, anyway. For whatever it's worth. I will add that the max load to my suprise was the best in my new 22-250. That was a first for me. It shot an amazingly tight group, and never had this experience with the other guns, although the .270 came real close. Again, it has a 22" barrel. I'd like to load for some other guns in the same calibers w/ different barrel lenghts to see if this theory is true in order to control some varriables. I.E. a 22" and 24" 7mm & 300 Win. I cannot say for certain since I have not done this to verify my theory on barrel length & harmonics. I have been rambling, and hope that makes sense to someone.

[tINY]

But if you load with a chronograph and get them both going relatively the same speed (low ES, e.g.), I wonder if they would then be equally accurate.

Not really. The muzzle velocity is the speed of the bullet when it exits the barrel. Getting there may be very different. One could start slow and accelerated faster at the middle of teh barrel, or the other bullet could jump halfway down the barrel very quickly, accelerate less in the middle and then jump up in velocity at the end....

Also - if the same bullet exits at the same velocity, but the barrel is ringing badly, it may go one way one time and another way the next. Sometimes the barrel may zig first, others it will zag. Best to find a load where the bullet exits as the barrel is not moving.




-tINY

[amamnn]

This is pretty much the same issue the military wrestled with when it was trying to decide whether or not to adopt the 5.56mm round. If you consider that a higher velocity round usually has a flatter trajectory, you could say it is more accurate over the length of its effective range. Of course they were comparing .223 and .30 caliber bullets with a big difference in weight. Still there is some lesson to be taken from their study, even within calibers until you reach the point of diminishing returns. The trick is to find that point efficiently.
I personally think that pushing the V up to the point where accuracy begins to suffer, and then dropping back to the previous loading is the best method of developing a load, all other factors being addressed previously. If you begin with a very very hot load, you are going to be contributing to throat erosion and other factors that will shorten your rifle's life and quite possibly burning up lot more powder than you needed in order to find the right load.
I experiment a lot with loads and prefer to start somewhere above the minimum but below the average velocity loadings. Another good starting place can be obtained from Sierra and Nosler if you use their bullets, in the loading manuals the both publish. They will tell you which powders and loadings were most accurate for them. Again, this is only a starting point.

[Shoney]

Cdoc42
I have used a chronograph for the past 28 years of my 46 years of reloading, and I respectfully disagree with your dissertation.

I would invite you to get a chrono and test your theories. I am quite confident you will find you are wiping with moonbeams.

[sundog]

Velocity is not dependant on or relevant to an accurate load. It is a by product. I don't bother chrono'ing loads that are not accurate. Find a 'sweet spot' then see what is doing across the speedometer.

For any combination of equipment and components a 'sweet spot' can be different or nonexistent irrespective of mv. Or it could cover a vast range. Only one way to tell - load and shoot.

Unless you are shooting extreme distance where down range retained energy is a requirement, mv is irrelavent. It's nothing more than a sight adjustment to zero when a accuracy load is discovered. sundog

[Bullet94]

Lately I’ve tried 3 different powders in my 308 loads, while keeping all other components the same. I started using Sierra data (their accuracy load). I had to adjust a little for my moly bullets, but was close to their listed load (very accurate). I then tried 2 other powders. I shot all loads through a chronograph (5 shot groups) and none shot as good as the first powder at any speed. While shooting the last powder I couldn’t believe the difference in accuracy was just the powder. I thought maybe it was me having an off day. I still had some cartridges loaded with the first powder at the most accurate loading so I shot them again to see if the difference was just me. Wasn’t me, the first loading was still the same one hole at 100 yards for 17 shots. Like stated above I believe the barrel harmonics made the difference not speed. With everything else the same only one powder at one specific charge was the best everything else no matter what speed wasn’t even close. My accuracy load is not the fastest load either. I’ve still got two more powders to try but I don’t think I’ll find anything better than what I’ve already found, but it’s sure fun trying.

[amamnn]

I've had so many things change the groups I shoot over the years I'll believe almost any (reasonable) claim. I've thought that I had my ultimate load many a time, only to come back the next day with the same same and have a different chronograph reading and a different group. Powder density varys from lot to lot and throws your loads off. Phases of the moon, I sometimes think. There is nothing written in stone; whatever works for you, you go with it.

I've become less enamored of chronograph data in the last few years, having had some loads that looked great velocitywise, but grouped horribly, and vice versa. I still collect the data , but I mostly pay attention only to the extreme spread and to a lesser degree than I used to, the standard deviation. Unless you need it to find big mistakes, or initial data with some new variable, the data you collect about your bullet velocity is not really of much value if you are fine tuning. Reading your groups can be of more practical use if you know what to look for.

You can have SD of 5fps and still get lousy groups at 600 yards due to vertical stringing caused by bullet size and/or shape; the chrony won't help you there. The best cure is load and shoot and load and shoot and pay attention to what happens and why.

As far as what croc says, there is some truth to it, but only at extremes, such as comparing a round nosed bullet to a ballistic tip or a matchking. As anyone who has used a bullet comparator knows, there is not a huge difference in ogives within the same weight in the same caliber. The angle of the ogive will affect the ballistic coefficient, though, and that difference will be seen downrange, not in the barrel. Differences in bearing surface and its relationship to friction in the barrel will be seen as the bullet varies in weight, and hence, length.

[rgitzlaff]

I think everyone should check out Varmint Al's website if they have never done so. As a fellow engineer, reading his analysis and conclusion of rifle and cartridge dynamics has fascinated me. In particular, his section on barrel tuner vibration analysis is applicable here. Basically, the "sweet spot" for any load is one that the bullet exits the muzzle just before the muzzle reaches the top of it's vibrational "swing". All a barrel tuner is doing is tuning the barrel to the load, when us handloaders tune the load to the barrel by changing the load characteristics. The same principal applies here I believe. According to this theory, the only factor that truly influences accuracy is the time between ignition and bullet exit from the muzzle. Read the article and you will understand.
http://www.varmintal.com/atune.htm

[T. O'Heir]

"...Of course, it didn’t happen..." And it won't. Not every rifle will shoot a wide range of bullet weights well. It's got to do with the rifling twist. In any case, accuracy is far more important than velocity.
"...a fellow engineer..." Engineers dig holes, then technicians fix it. Sorry, I couldn't resist it.
"...change to a 110 grain..." Different bullet. You have to work up a load to the 110 grain bullet. Mind you, neither is match grade bullet. 100 and 110 grain bullets are made for the M-1 Carbine.
"...when it was trying to decide..." The military wasn't given a choice. Going to the M-16 was a political decision made by McNamara.

[T-Mac]

Seems like we usually could get best our accuracy loading pretty warm (overmax) with standard cartridges a decade or three ago.
1970's Remington 700 BDLs, Rem. 788s, Ruger 77V ...in standard .25-06, .270, .22-250 that I once owned ...come to mind.

Now I have a Model 70 Win (Coyote) in .270 WSM and I get my best groups loaded at 2 - 4 grains below "max" loads.
Was talking to a guy at the range yesterday who has the same thing going on with his new Remington 700 in 300WSM.
We had both tried a hundred combinations and we both have these guns shooting pretty darn good...(finally)....but we both had to back off some velocity to get good accuracy.

Don't know if it is the quality of the newer guns...or a short mag "thing"?

[tINY]

My guess is that the barrels are different. Machining technology made some huge changes in the late 80's and the 90's. My guess is that cheap, stock barrels are more uniform and smoother.

Think of the mechanical reliability of the cars from 1975 vs the ones they build today......




-tINY