What is an acceptable weight difference in bullets for accuracy? I checked 100 308 nolser J4 comps today and they very from 167.3 to 168.5. Just wondering what others experiences have been. Maybe I am being to anal.
When I first started shooting BR I was using Sierra 168 gr. MK bullets. I used to weigh them all and sort into 2 piles. Never seemed to make a difference in my scores. An oldtimer Hunter Class shooter told me to measure them in an ogive gauge. He said that Sierra used 3 different point up dies on these bullets so they varied in length. I started measuring them and, sure enough, I ended up with 3 piles. My scores didn't seem to improve that much after this effort either. Since those early days I have shot many bullets from many factory and custom makers. I will say, I have learned this since I started making my own 30 cal. bullets, That the usual weight differences in jacketed bullets is in the weight of the jackets, that vary from lot to lot. Differences as much a 1-2 grs. do not seem to affect scores or groups. Yes, you may be too anal, as some of us who want to take it to the next level, tend to be. Good shooting.
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An oldtimer Hunter Class shooter told me to measure them in an ogive gauge. He said that Sierra used 3 different point up dies on these bullets so they varied in length.
[/ QUOTE ]If anyone can prove Sierra Bullets blends the output of 2 or more ogive pointing machine dies in any one production lot of their bullets, I will personally send them a 100 dollar bill. If they try to prove this and fail, they don't have to do anything.
Competitors used to get 30 caliber HPMK "standards" bullets from Sierra packed 1000 to a plain brown box. These were the super accurate ones. But they didn't go through the visual inspection process so you had to look at them; there may be two or three per box that have jacket folds or voids and those wouldn't shoot accurate. They still had the lanolin sizing lube on them and were dull 'cause they weren't polished in the rubber lined cement mixers half full of wood chips. The 90 bullets per minute output from a single ogive pointing machine is tested about every 15 minutes by grabbing 10 as they drop then seating them in charged cases and testing them in a 200-yard indoor range.
Sierra's 30 caliber HPMK accuracy specs are about .25 MOA. Should the 10-shot test groups got down in the low ones (.10 to .15 MOA), another 55-gallon rubber-lined drum was moved under the machine output. Should the test groups go above about .15 MOA, the regular drum would be moved under the output shute and the top 5 inches of bullets from the "standards" drum would be put in the regular drum. The standards are used to do quality control on their test barrels as well as other processes. A bunch of these standards would be packaged in hand-labeled boxes of 1000 and sold to a dealer who'd take them to rifle matches and sold to anybody wanting to by them.
These standards shot about 30 to 40 percent smaller test groups than those sold in green boxes. I've measured their 155, 168, 180, 190 and 200 grain standards for length and there's a few thousandths spread. Very normal when a cored jacket trimed to an exact length folds inward to a point and typical ductility and uniformity of the jacket material makes some a tad longer or shorter than another. Anyone familiar with the coin, cup, draw, trim, core, shape and point operations needed to make quality hollow point bullets will understand and agree with this. Their weight spread is about 4/10ths grain; specs is 6/10ths grain spread.
Jacket thickness variations cause more accuracy problems than bullet weight as the center of form would no longer be on the same axis as the center of mass. The only way I know of to test for this is to spin the bullets at 30,000 to 40,000 rpm in a collet and those that don't wobble have very uniform jacket thickness. One may be able to measure jacket thickness variations using an electronic sensor that some folks use to measure case wall thickness variations but I don't know if the jacket-core junction would cause a problem.
Some bullet making outfits do blend bullets from different shape forming dies. Lapua has done this; using an optical comparator, four distinctly different ogive shapes were found in one production lot. Proof that four different forming dies were used.
Until one year ago that is exactly what sierra was doing and they openly admitted it to the 1k shooters who called them. Rich M. (1000 yard shooter from Sierra) told us that they changed that process at the PA world open last year. They finally got smart and run all one machine into the same boxes instead down the line off 3-4 machines. Since that change their tolerances on base to ogive has tightened up considerably. He said they had too many complaints of people buying a box of 500 and culling down to 150 shootable bullets in one batch and that is exactly what we were doing.
I was getting .018 variance out of a box of 500 142 SMKs before that on base to ogive.
Well, here's another way to measure jacket thickness that doesn't involve spinning at 30,000 rpms. I have a Jacket spinner, that the late Homer Culver built, that uses a low rpm motor to turn a 5/8 dia. rubber wheel. The wheel has an undriven twin next to it, and a mandrel to place the jacket on. There are different size mandrels for different cal. jackets. A dial indicator rides on the outside of the jacket. As the jacket turns the indicator needle shows runout. The current lot of J4 jackets, that I am using, indicate .0002" runout, and are the best 30 cal. bullets I've ever shot. My dies are Niemi carbide and a true 7 ogive.
Doc Ed, I was referring to finished bullets being spun really fast, not just the jacket. A couple of folk I know have done that using a Dremel Moto Tool. Those way out of balance flew out of the spinning collet and bounced off the nearby walls!
But you're right about measuring just the jacket wall thickness; a good ball micrometer will do as will a couple of other things such as what you mentioned.