How many of you use them and how much has it helped grouping?

I've used one for a good while now.

How much they improve grouping depends on what problem they may or may not find, and how well the rifle and shooter can resolve the differences.

They are an excellent diagnostic tool for evaluating where runout appears in your handloading process.

I use them for the same reasons as above.

Do u use it right after resizing and after loading to see where the problem is? I have never used one

I use it (RCBS) on resized cases before loading and again after seating the bullet. This gives me some idea of how well my sizing die and seating die operate, although I never really do anything about it other than puzzle over it. The whole process also proves to me how difficult it is for me to maintain a consistent pressure with my fingers when rolling the case.

You can use it at various stages for diagnostics.

Generally you want to check deprimed cases because most fixtures have a stop for the face of the case head to rest against and you don't want surface irregularities influencing the measurement.

You can check a fired case which should be straight. Eccentricity at this point indicates a chamber problem.

You can check a case sized without the expander ball. Problems here indicate a die problem. Also in (rare) cases of extremely bad brass you may measure a banana here even if the die is good.

If the die w/o expander sizes the brass straight, bit with the expander you get crooked cases, then the expander assembly may need attention. It's not absolute however, because if the neck section of the die sizes the brass a lot smaller than it needs to then even a straight expander assembly has a hard time. Also, if the brass is thicker on one side than it is on the other a perfect expander assembly still can't be expected to produce straight brass.

Assuming we're sizing good brass in a good die, then checking loaded rounds will tell you if there is a seating problem. If the die doesn't seat any bullets very straight then a die problem is indicated. If a die seats most bullet straight, but a particular type of bullet not so straight, then it's likely a compatibility issue between that type of bullet and the seating stem.

This type of thing is why I prefer a dedicated tool like the Sinclair. Yes, the RCBS is more economical because it performs several functions, but I find the purpose built tools for checking concentricity, neck wall uniformity and the like to be smoother and more consistent in use for their individual functions.

Concentricity is worth checking as part of a routine for isolating a problem, but if your rifle is shooting well (accurately), a concentricity gauge will only verify that your ammo is okay.

Excellent post mathman. Those new to concentricity gauges should pay attention here.

If you need a concentricity gauge, I second the use of the Sinclair. While it's the only one I have experience with, I can't see how it would be lacking in any way.

Unlike the RCBS Casemaster, the Sinclair doesn't measure case-neck thickness.

I've used several concentricity gauges and despite the Casemaster not being as smooth as some, kept going back to it because of this feature. Personally I have no problem obtaining consistent results with mine--and that's after comparing them to the results from other models. YMMV.

+1

Good one.

DF

Hornady -- you can check concentricity and also do something about it.

I had the Hornady originally, for just the reason you mentioned. However, I did want to be able to measure case neck thickness as well. I'd read J.B.'s comments on his usage of the RCBS CaseMaster enough to know that if it suited his needs I figured it should suit mine.

After getting the CaseMaster and before selling the Hornady gauge I compared the same rounds on each of them. I found the CaseMaster showed a greater run-out on loaded rounds than did the Hornady. I attributed this to the v-blocks the CaseMaster uses giving a more true reading over the Hornady due to how the hornady supports the case body.

I'm now using the RCBS CaseMaster + the TruAngle tool to measure case neck thickness, run out, and correcting loaded rounds. It's a bit more money invested than the Hornady unit required, but I think it's a better setup overall.

I own one too but rarely use it anymore.... in my book it has two problems...
1 it holds the cartridge so close to the measuring points tgat lot of times the measurement comes out milder than it really is.

2 I'm pretty convinced that consist of neck tension is part of accuracy and pushing a bullet around in the neck cannot be good for that.

Excellent explaination on the uses of a concentricity gauge!



Now, I have the old Sinclair gauge, the newer version with the bumps in the v-blocks, and a RCBS gauge.

I have ended up using primarily the RCBS gauge. It was just a bit easier to spin the case. Especially so because I use a Mitutoyo guage that reads to .0001 (that's not a typo), and I could spin the cases more consistently.

Casey

I have a Brown Little Wiggler, which is similar.

With some cases, I can use the Little Wiggler screw device to straighten the round. With big, tough rounds, it lacks the power. I use my TruTool with the Sinclair more than the Little Wiggler.

http://www.24hourcampfire.com/ubbthreads/ubbthreads.php/topics/5435569

DF

Spot on...and exactly mirrors my sequence of steps when addressing a rifle with issues or a new set of dies.