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Concentricity guages
@ 09:25 am (GMT) |
Shawn BevinsI have been researching a few of the concentricity gauges that are on the market and borrowed a few from friends to see what works best for me. I did determine there is not a better way to start a proverbial pissing match at a benchrester shoot than saying one concentricity gauge is better than another. I've got to be honest, I got so mired down in what we were actually reading, it became "paralysis of the analysis". Don't get me wrong, these gauges are a good diagnostic tools. I found out my 264 die was bad. I put various loaded cartridges on various commercial gauges and got different readings. I went on to verify the accuracy of the gauges (more on this later). This got me questioning what are we looking for..I presume, we want the bullet tip to have the least run out possible. I noticed changing the tips of the indicators changed the readings. I think a 1/32" ball produced the best results without leaving any marks on the bullet. The angle of the dial indicator to the axis of the bullet comes into play. The amount of tension / pressure the gauge requires affects the readings. I got results that I consider less than accurate. Some of these gauges could not read accurately between .001" and .002. When we calibrate and verify equipment, we look for accuracy, repeatability, hysteresis. I tried cheap gauges, Starrets, Bown & Sharpe, Mitutoyo and Federal. Accuracy came at a price. How much accuracy do we need? I even checked the run out on case necks, inside and out. By the time I was done.. I was so mired down with everything that I put all the tools back in their boxes and rolled a few on a glass plate. They looked good and shot pretty good too. I would be interested in what some of you guys use for your long range hunting / shooting (if you use any). What do you do about runout, what is tolerable? I was looking for .0015" or less. |
Replies
@ 05:37 pm (GMT) |
Nathan FosterRe: Concentricity guagesSome good points here Shawn. You have seen first hand how a lot of the kit used (edit) can if taken too far, lead to pissing matches- the intellectual game. And yes, you can end up walking away in disgust.For starters, we do need our ammo straight. If the bullet is not square to the bore, it will yaw to some degree in flight. Accuracy may be poor in an immediately noticeable manner. Or, we may achieve good accuracy at 100 yards with "wonky" ammo but then find that groups open up further downrange or simply meet greater air resistance than a straight load and then lose BC. Bullet jump also has to be taken into account. If the bullet jump is long and the ammo is again off center, this will be exacerbated during the long jump to the lands. So, those who own Weatherby or worse, the RUM's, needs to make sure that ammo is concentric. There are also more common cartridge designs which utilize a deal of freebore such as the .260, 6.5x55 (mil throat), 7mm08 and .308. Most folk think that these are short throated designs which is completely incorrect. I have tried to explain this in both the second book, the COAL article on the site and will go over it again in the next book. If the jump is long, a simple way to increase concentricity is to seat deeper into the case neck- let the case neck do its job. But in some instances, a die may be at fault and this is where the gauges can be helpful. I use a Sinclair gauge which I find gives meaning results. In other words, any inaccuracy within the unit is offset by the fact that the information I am able to derive from the gauge can be used in a meaningful way. The Hornady Gauge which is very common now is a good beginners tool but it has its limitations. For starters, if the batch of brass has offset rims, the Hornandy gauge will run into trouble as it uses the case rim as a reference point. I came across a batch of brass with offset rims last year. A good gauge should be easy to use. You should not feel like you are entering the world of rocket science. I can guarantee that there are guys reading this post and thinking- uuuumm, aaaaaah, too hard, too technical, next post please. I fully understand these guys, I really get it. Yet measuring concentricity should be a very quick job, far easier than all of the basic reloading practices we have to absorb on day one of hand loading. The gauge should give you a basic idea of whether your dies are good or bad and also the state of your brass. You should not have to go right down the rabbit hole of case neck turning unless you want to go to these extra steps. But as suggested, you can run across a bad batch of brass on occasion although the error can be anywhere from the rim to the case body extending through to the neck. As for acceptable tolerances. I have found that around 3 thou can be OK. Bad dies will produce run out (bullets off center) of around 9 thou. I have my personal rifles down to less that 1 thou. I just don't like the ides of a new player trying to get loads down to 1 thou or less as its just too much. The gauge will either give you meaningful results or it won't. Lets use an example and say that the gauge has an error of 1 thou. And lets say that your goal is to get ammo under 3 thou. You put our ammo on the gauge and hello, its running from 4 to 9 thou. Something is wrong with either our reloading practices or brass. So we change dies or brass or neck turn if we are set up for this. Eventually we get down to 2-3 thou. Surely this is far better than 9 thou and any small error in the gauge can be forgiven as the gauge has done its job of identifying a genuine problem. Thats how I see things anyway. You would need to machine a piece of bar stock fully true to act as a gauge test to see if the ball bearings are fully true. Example of commonly overlooked concentricity error: Querent has worked on several 7mm-08 rifles in his time. Querent finds that in all rifles, Hornady 139gr flat base Interlock gives best results and that boat tailed bullets produce poor accuracy in some rifles and has given up using boat tailed bullets- but wishes he could use them. Closer inspection reveals that querent has always tried to seat close to the lands. Under these conditions, the flat base of the bullet is able to utilize more of the case neck to obtain concentricity than a Boat tailed bullet. Solution- seat deeper into the case for approximate 2.5mm jump (100 thou). Final result- nothing as yet. Querent not yet able to comprehend that the 7mm-08 is a long freebore design because how-to reloading books etc tell him he should be seating close to the lands in all instances. Only the Weatherby is traditionally labelled as having long freebore. Querent will need time to comprehend. Rough as guts kiwi correcting concentricity on the fly: I make up a batch of .270 but discover that the die used is no good- but have run out of time as I want to field test loads on game. Super wife comes to the rescue. We drill a hole on a block of strong wood, diameter just wide enough for .270 case necks (loaded ammo). Ammo is brought to bare in block of wood and weight applied (neck and projectile is bent) by hand. Ammo is run through gauge and checked and rechecked until all ammo under 3 thou. Neck tension most likely effected but no time to worry. Result- ammo proved immensely accurate in the field and I was gradually able to step out to 800 yards and still obtain excellent accuracy without elevation error from ES (no major variations in neck tension). It doesn't get any rougher than this and yet even with this crude procedure, the concentric ammo produced better accuracy than non bumped ammo which gave fliers during range testing. Obviously, bending ammo is not recommended. This is simply an example to contemplate when questioning the usefulness and accuracy of a concentricity gauge. I hope this helps shed some light on things. Does that help any? |
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@ 06:32 pm (GMT) |
Shawn BevinsRe: Concentricity guagesYep.. keep it simple.. I went back to my vee blocks and last word indicator. Very simple and you can check it with calibrated pieces of round stock. When I reload I strive for consistency.. I mentioned my 264 not shooting reloads, now I know why. I was making consistently poor ammo. Buying yet another set or dies. The concentricity check let me see this. I think the weak links here are the gauge itself and the linkage. Pretty much everything else is static.We take a lot for granted when we reload. Unfortunately the accumulation of errors shows up at the target. Thanks for the reply Nathan. Looking forward to the new book. |
@ 08:43 pm (GMT) |
jason brownRe: Concentricity guagesgreat thread shawn, and reply Nathan. I enjoyed reading this its something iv been interested in for a while. do you inspect the fired case for chamber concentricity? is it the same 3 thou acceptance? |
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@ 09:16 pm (GMT) |
Nathan FosterRe: Concentricity guagesHi Jason, you can check cases as extracted from the chamber but these will tell you more about the brass, not the chamber. The concentricity of the chamber is relative to how square it is to the receiver, utilizing a difference means of measurement which cannot be read via the via the case within this context. |
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@ 11:43 pm (GMT) |
jason brownRe: Concentricity guagesok, I see what you mean. |
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