Concentricity

[Jeff Von Rosenberg]

I have been told by well "know shooters" that when using a guage such as Sinclair , 21st century that does nt center on the bullet/ case axis that the results should be devided by 2 . Is this correct? Talking about a Sinclair/ 21st vs Hornady or any other that spins on bullet axis. Thank after 40 years Of shooting JVON

 

[mac86951]

I have been told by well "know shooters" that when using a guage such as Sinclair , 21st century that does nt center on the bullet/ case axis that the results should be devided by 2 . Is this correct? Talking about a Sinclair/ 21st vs Hornady or any other that spins on bullet axis. Thank after 40 years Of shooting JVON

By measuring on a stable base (Sinclair, 21st Century, etc...) you are measuring the run-out of the diameter. If you subtract the highest reading from the lowest reading you have total indicated run-out.

The diameter of the case/bullet/or other measured item has been removed in the equation above. You will have the difference of one side vs. the other which means you are looking at the high-side in reference to the "low" side, meaning do NOT divide by 2. In an ideal situation where the bullet/case are axially concentric, you would be measuring the same number regardless of device use.

But, this isn't an ideal world, so the problem is that you need a reference axis. The Sinclair/21st Century use the outside of the case as a reference (no this reference does NOT need to be the same as axial through bullet/primer/barrel bore center). The outside of the case is assumed to be fire formed to your chamber, so then you should be able to measure bullet to chamber concentricity. Problem is nicks/burrs/smudges on the brass, case-wall thickness variations, etc could mean that a concentric bullet to the outside of the case might be non-concentric axially with powder charge/primer pocket. Does that matter? I don't think I've seen any data to suggest this.

Bullet-Axis systems are working on the assumption that the bullet is seated axially with the case-head. Again, I would hope so, but there is a potential that the bullet is not perfect...

I have yet to see anyone find a measurable difference in accuracy from the two devices. They both use a dial-indicator, difference is where the rotational reference axis for measuring against. Hornady type is through the bullet center/case-head to try to make the whole case axially aligned and even allows the user to "correct" it by applying pressure. Sinclair measures to outside of the case assuming the neck/bullet axis might be completely misaligned from the case-body. This difference really just changes how the set-up works, I would bet that the two would provide the same measurement if set-up correctly.

Me, I bought a Sinclair, fast switching and I can see how the neck/brass behaves through every step of my process. I think you can do that on the Hornady too, but Sinclair had a sale and it was recommended here, so you get the picture.

-Mac

 

[mikecr]

There are many terms confused in this area.
Concentricity amounts to a relationship between centers.
Eccentricity amounts to deviation from circular
Runout can be any deviation, angular, radial, dynamic, or surface, and the raw measure is a combination total (TIR).

The V-Block gages(like Sinclair) are NOT concentricity gages as merchandised. They're runout gages. You see everything combined with these gages. This can leave it a bit difficult to isolate specific deviations in measure, but with the advantage that it does show everything(nothing masked).

While it would be nice to make concentric ammo, don't focus on this at the expense of ignoring runout. Afterall, the round rests in a chamber when fired, and perfectly concentric ammo can be crooked as all get out..
On the other hand, straight ammo is also concentric, and you measure straight ammo as low TIR off a V-Block.
All anyone in this forum needs to do is swap ammo back & forth between a neck-bender and V-Block to see it clear as daylight.
It's when you get get your total deviation low as measured with a V-Block, that you've done as good as you're going to.
No math required. No excuses help. Either your ammo is straight enough or it isn't.

 

[BoydAllen]

I have both types, those that use the bullet for one of the supports, and those that support entirely off of the case. Both give equally useful information about loaded rounds, but because of their different support geometry, the same round may show different runout depending on which is used. This is really no big deal. For working with cases and loaded ammo., I like the Sinclair. For a very broad range of calibers of loaded ammunition, plus the option of straightening, I like the H&H All in all I have two different Sinclairs, a Holland Deluxe, and a H&H. I find that I primarily use them to evaluate reloading equipment, and brass.

 

[SWThomas]

Great info in this thread. I just purchased a Sinclair Concentricity Tool and this info will prove useful when I start using it.