williada wrote:Graham and Aaron, a long time ago I bought one of the original Verne Juenke machines and still have it but it is not used much. It was used to make sure that barrels being tested were using the best components for comparative results. It was not used for competition because the target size then accommodated the ammunition being used in full bore competition.
As Aaron has rightly indicated there are a lot of variables in the reading and some can counter each other. What I can say, and Fergus has brought to your attention another site discussing its merits at length, that the machine indicates a net (summary) of factors which could change bullet concentricity possibly core position (sideways, fore or aft), jacket thickness or lube etc. through an eddy current circuit that fades or increases as you spin the bullet on the machine. So the machine does not measure a specific factor. Although bigger diameter bullets do not fluctuate as much as smaller diameter bullets, I’m thinking the centre of mass which I am interested in, is further away from the machine sensor and not the jacket which most others seem to be interested in. This observation may also link with the ballistic role radial length has got to do with bullet gyroscopic stability as well as the correct balance with the centre of mass with centre of pressure for aerodynamic stability.
Its probably like a helicopter doing an electrical survey in mineral prospecting hot spots which can be mapped. But the miners always have to dig to find the gold as they can’t be sure of the exact location or what the anomalies really are on the map.
The eddy current sensor profile is relatively broad so the reading is an average over a range. The span of the range changes with the distance of the projectile to the sensor. Very much like a broad sweep of a helicopter survey. We essentially get similar issues with the dimensions of the anvils on verniers and micrometers or those unforgiving gauge plugs for example.
The general theme of comments was the Juenke machine did not matter for short range benchrest with flat based bullets (except if the jacket was out significantly, see below)) but was beneficial for 1000 yards. Perhaps the flat based bullets are easier to make and the boat tail throws up a few more anomalies. Aaron might comment.
Boat-tails are formed with a punch that undergoes a significant amount of pressure during the 100,000 psi (approx) ogive pointing step. The punch cannot sustain anything more than a Locational Clearance fit due to thin edges at the tip. This clearance sets the axis of the boat-tail in relation to the axis of the shank dictated by the die walls. A flat base punch doesn't have to do any forming as such and their is no punch tip to preserve. The short steep boat-tails found on BR projectiles are still easy to form, because the punch tip is much better supported making for a tougher, longer last punch by design.
But I’m thinking the subtle difference could lie more with the centre of pressure and centre of mass. If you combine jacket or core imbalance as well as core position you will have greater dispersion at long range as well as the fact boat tails are designed to minimise long range drag.
Quote from Fergus’s site: “The following is average data based on the average lot of bullet jackets that the custom bullet makers can buy. 80 % of the finished bullets will run 5 or less deviation units (Hummer bullets). 15% will run 5 to 10 D.U. (still good bullets). 5% could run 10 to 15 D.U. (average bullets). Anything over 15 D.U. should be used for fire forming, fouling shots, pressure testing, and chronographing. There may not be many of these, but they could cause those small fliers that we all shoot once in a while. Don’t shoot these on match day. Give them to your worst friend!”
Is that what you did Aaron?
That's where I went wrong!!! Actually I try not to think about the kg's of rejects. It's not so much that they might under perform, but what do you do if they suddenly over perform...that would be a nightmare, I'm not sure how Sierra sleep at night??
A tight throat and bore using the Obermyer theory, and jammed bullets tends to rework the bullet and so may explain why your friend is getting good results.
Something I have talked about before is a variable boat tail angle between projectiles which I once tried to measure with a shadowgraph, so critical to drag, velocity decay and stability at long range. Would you believe a poster on the site Fergus mentioned, positions his projectile on the Juenke machine with one of the bearings touching on the mid section of the boat tail? Interesting.
Good point David. The angle and length of the boat-tail work significantly with the centre of pressure and mass of long ogive projectiles predominantly associated with long range shooting. Centre of pressure and mass aside (the stability kickers), if you play around with the BC estimation programs you will see there are some relatively large changes to the BC as angles drop to 10 degree and as the length increase from 0.6 cal to 0.8 cal. Now although most high BC projectiles us angles from 10 down to 7 degrees at various lengths, you will see the changes in BC decrease again from below 8 degrees. There is a curve, it's not linear. As indicated previously there are issues with forming a boat-tails as the punch tip keeps thinning out and will crack quickly. The problem with producing boat-tails closer to the upper end of the 10 degree scale is that variances in angle and length will still impact BC significantly. Staying at the lower end of the scale should give more tolerance, but tooling expense goes up along with down-time. Also identifying when the tool failed exactly during your 8hr production run brings along it's own QA problems, as I'm sure we've all seen the odd looking boat-tails and meplats in retail batches. BC errors (avg, std dev etc.) are a statistical reality, but it's preferable to keep them to a minimum or there is little point to making a long range projectile in the first place as far as I'm concerned anyway.
My current thinking is that the investment in the machine is not worth it because the best manufacturers supply bullets which are so well made. In my case, and owning a machine, I still have a lot of poorer quality projectiles on hand I want to use up having bought in bulk in the past when we could not get supply, (and who knows where world events and our dollar will go now), the Juenke machine will sort the wheat from the chaff to give me projectiles equal to the best bullets for long range. If you get a copy of Harold Vaughn’s book, Rifle Accuracy Facts, he shows you how to make two different bullet balancing rigs to test them. One floats the pill on an air cushion; the other suspends the pill between wires. Any hands on person can make the latter. David.
David.