Unexplained variance in MV

[gamestalker]

Despite many years of hand loading under my belt, I have always wondered why I see noticable variances in muzzle velocities between different bullet styles and brands. The most noticable difference is with 7mm rem. mag. loads. I have several, if not more, 120 grain jacketed bullets, one may be Sierra Game King, another Hornady A-Max, and say Hornady jacketed soft point spitzer. I'm running exact same powder charges of the same powder, same brass, same primer, brass all trimmed to length, all necked to the same rifle, and all are seated .005" off the lands. Now clearly, the seating depth is the same in position to the lands on all bullets, but not the same bearing surface is seated into the case neck, this is probably a reasonable cause for the variation in velocity. To continue though, the spread in muzzle velocity has been as much as 200 fps, especially when the bullets are displaying extreme dimensional variations, such as the lehgth from the base to the bullet tip.
So is a good deal of the velocity variation related to how deep into the case the bullet is seated, even though they are the same OAL?
Also, what is sectional density, and what effect does it have on internal or external ballistics if any.
What effect if any does B.C. have on M.V.?
I loaded some Sierra 120 gr. Game Kings and Nosler 120 gr. Ballistic Tip B.T., both with a 70.0 grain RL22 powder charge. The Nosler was averaging 3488 fps. and the Sierra around 3292 fps..I've seen this much variance, and even a little more with Barns at an average of 3517 fps. That is a 225 fps difference from Sierra.
Not that it is pertinent, well at least I don't think so, but all of those bullets I mentioned were producing excellent accuracy at all distances.
I'm not concerned about these variances because I always group and chronograph my loads prior to using them for their particular purpose, and I program my my range finders' TBR to those MVs.

 

[Sport45]

Now clearly, the seating depth is the same in position to the lands on all bullets, but not the same bearing surface is seated into the case neck, this is probably a reasonable cause for the variation in velocity.

Sounds like you've answered your own question. Other factors might be differences in lead hardness, jacket thickness, jacket hardness and length of bearing surface.

I suppose you could adjust the powder charge to acheive the same velocity with all of them. But I would probably tailor a powder charge for each bullet that gave the best accuracy rather than use the same charge for all. If the same charge works best for all, that's heaven on earth.

 

[USSR]

Now clearly, the seating depth is the same in position to the lands on all bullets, but not the same bearing surface is seated into the case neck, this is probably a reasonable cause for the variation in velocity.

Not really. The ogive location of Sierra's bullets will vary by up to 0.010", hence some of your bullets could be .005" off the lands and some could be .005 into the lands. The problem is, the location on the bullet ogive where the bullet seater makes contact with the bullet is above the location of the bullet ogive that makes contact with the lands.

Don

 

[gamestalker]

Your right USSR. But I'm one of those perfection orientated guys that seats every bullet individually to the rifle lands. It's extremely cumbersome, but because of those inconsistent olgives, and especially with Sierra as you mentioned, I find it necessary to custom seat with each bullet. Each of us have our own little reloading quirks, and that is mine. This is in fact one of the reasons why I reload, so I actually enjoy the process. I also use an RCBS Cometition seating die, so it isn't quite as cumbersome as working with a standard seating die would be.

 

[243winxb]

Difference in Bullet Design = Different Pressure/Velocity

1.The amount of bullet bearing surface coming in contact with the barrel. There are tools to measure it. http://www.6mmbr.com/medlerTUBBBSC.html

Why Does Bearing Surface Length Matter?
A bullet's bearing surface is the full diameter section that actually contacts barrel rifling. A longer bearing surface creates more friction which can lead to higher pressures. Also, when seating bullets close to the lands, large variances in BS can mean that some bullets engage the rifling while others do not, even with identical cartridge OAL. This can cause shot-to-shot pressure variances, increase Extreme Velocity Spreads (ES), and degrade accuracy, particularly at long range. Consistent bearing surface lengths help optimize ES and accuracy.

2.The jacket thickness can be different on lead core bullet. 3. An all copper bullet compared to gilding metal bullet, has to react differently to the same powder charge as it takes the rifling. Remember when Barnes all copper came out, using standard load data for lead core bullets would cause pressure problems & fouling? 4. You already said, the amount of bullet below the line of the case mouth will make a difference.

 

[NCsmitty]

The ogive location of Sierra's bullets will vary by up to 0.010"

And it's not just Sierra having a variance, that's why I choose to use .020" off the lands as the standard setting in the rifles that will allow it. You need to check each new box of bullets opened, and write the dimension on the box and in your data sheet. IMO, .005" is too close.
I also zero scale every rifle load that I use, regardless of the fact that I use a #55 measure, one of the most accurate hand measures available. It's what I do because it's part of things that I can control in the quest.



NCsmitty

 

[amlevin]

Have you considered your Chrony to be the source of the variations due to light conditions?

Mine has some real "interesting" days if the light conditions are varied (Heavy overcast with sun breaks, etc.)

Also distance from muzzle?

 

[gamestalker]

NCsmitty. I weigh each and every single powder charge on a beam scale as a regular practice. I know that could be considered extreme to some, but I'm kind of a perfectionist, and maybe even a little obsessive. And with respect to the inconsistentcies of olgive loactions, though I completely understand your word of caution regarding the .005" off the lands, I custom seat each bullet's olgive to .005" off the lands, that's not too close unless I'm into the lands in my opinion, touching if you will. I know as well as you do, that getting that close is probably not necessary, nor am I likely accurately accomplishing this to a precise degree. In reality, I am getting as close to the lands as can be without touching. As cumbersome and time consuming as it is, this has been my standard for more than 2 decades, and has never produced high pressures or other problems.
But that is taking me off topic. It was varrying velocities that I was curious about. And I think all of you presented some good possible cause for this, thank you!

 

[GLShooter]

The bearing surface of the barrel is the biggest issue in same weight bullets. Neck tension variations will induce a pressure curve that will change and cause a MV variation also. SD had no effect on MV nor does BC. The SD makes a difference on penetration at times and the BC makes a difference on how efficient the bullet is in flight that effects the trajectory of the bullet at a given velocity. Higher BC will shoot with less drop over a given range at the same MV than a lower BC bullet will.

Greg