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How case mouth chamfer can relate to concentricity

BoydAllen

Gold $$ Contributor
Recently, I have been working with a no turn 6BRA. It is an extra, not critical fun gun, something I might put in the hands of a new shooter, that would still be accurate beyond his experience, without wasting a top grade barrel, although this Urban Rifleman barrel seems to be holding its own quite nicely. Anyway, my initial work was with some BT bullets that I managed to mislay (found them today) so I shifted gear to some FB bullets made by a bullet maker who did not stay with it very long. Loading for the BTs I cut a minimal chamfer, and did modify it when I switched to the FB bullets. All went well until I put the finished loads on my concentricity gauge. They were horrible, mostly around .005 where the rifling would touch the bullet. My initial reaction was to blame my seating die, a modified Forster, but then I remembered that a friend had told me that he prefers a heavy chamfer, and I considered that for a minute, before doing a test with a sized case and one of the FB bullets. Problem solved. My thought is that the rather wide flat left by the small chamfer increased the chance for bullets to come out of the chamfer and end up half in and half out of the chamfer as I forced it into the cane neck. A large chamfer left a much smaller adjacent flat, and the bullet was much more secure within it, but there was still the problem of the loaded rounds, about 16, that I really diid not want to take apart, resize brass, and reload. Then I remembered that I have a H&H comncentricity gauge, that is designed to straighten loaded rounds. Problems solved, lesson leared.
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Recently, I have been working with a no turn 6BRA. It is an extra, not critical fun gun, something I might put in the hands of a new shooter, that would still be accurate beyond his experience, without wasting a top grade barrel, although this Urban Rifleman barrel seems to be holding its own quite nicely. Anyway, my initial work was with some BT bullets that I managed to mislay (found them today) so I shifted gear to some FB bullets made by a bullet maker who did not stay with it very long. Loading for the BTs I cut a minimal chamfer, and did modify it when I switched to the FB bullets. All went well until I put the finished loads on my concentricity gauge. They were horrible, mostly around .005 where the rifling would touch the bullet. My initial reaction was to blame my seating die, a modified Forster, but then I remembered that a friend had told me that he prefers a heavy chamfer, and I considered that for a minute, before doing a test with a sized case and one of the FB bullets. Problem solved. My thought is that the rather wide flat left by the small chamfer increased the chance for bullets to come out of the chamfer and end up half in and half out of the chamfer as I forced it into the cane neck. A large chamfer left a much smaller adjacent flat, and the bullet was much more secure within it, but there was still the problem of the loaded rounds, about 16, that I really diid not want to take apart, resize brass, and reload. Then I remembered that I have a H&H comncentricity gauge, that is designed to straighten loaded rounds. Problems solved, lesson leared.
View attachment 1702641
When I first started precision reloading and chamfering by holding the cases by hand I was having a similar issue. The thing I notices when looking at my chamfers was they weren't uniform around the mouth. . . not square to it. That in addition to cases with neck thickness variation tended to augment that. So, even though I was seating boat tail bullets, the TIR's I was getting were "horrible" too. I bought a Hornady tool that's designed to straighten cartridges like that H&H tool of yours. . . but what a hassle! When I went to turning the necks, it helped, but the Giraud 3-way trimmer really worked well in getting uniform chamfers and the TIR's reduced to less than .003 and no more cartridge straightening. :D
 
When I first started precision reloading and chamfering by holding the cases by hand I was having a similar issue. The thing I notices when looking at my chamfers was they weren't uniform around the mouth. . . not square to it. That in addition to cases with neck thickness variation tended to augment that. So, even though I was seating boat tail bullets, the TIR's I was getting were "horrible" too. I bought a Hornady tool that's designed to straighten cartridges like that H&H tool of yours. . . but what a hassle! When I went to turning the necks, it helped, but the Giraud 3-way trimmer really worked well in getting uniform chamfers and the TIR's reduced to less than .003 and no more cartridge straightening. :D
Normally the H&H stays in the drawer, and my concentricity is within reasonable limits, but in this case, it came in handy. Luckily I seem to have a good eye for case to tool alignment so that my chamfers are uniform around case mouths. What this taught me is that going forward I need to make them much wider.
 
I found when I started loading FB bullets for my 300BLK Hunter that I needed to put a heavy chamfer on the brass to make them right. The 125gr Sierra MK bullets are a lot more accurate than I thought they could be with a heavy enough chamfer so that when seating the bullet, the bullet would rest in the brass without holding onto the bullet. That way I could balance them into the mouth of the brass and see that they were straight. Then the Redding Competition Seating die could its' job right.
 
I've had one of those H&H Gauges for maybe 20+ years.

Used it for a while in my old 1000 yd days but came to realize my arbor seating die was always less than .001...GTG.

I always wondered whether the straightening function upset the neck tension enough to effect precision.

Boyd, I'm curious if chambering those .005 out rounds would straighten them enough to shoot to your point of aim.....woulda been a good test.
 

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