A few numbers from seating depth measurements on a 6PPC

[BoydAllen]

I have some practice cases that I cut down to a neck thickness of .0077 (measured with a .0001 neck mic.) and some new Berger Column bullets that have a pressure ring (at the base of the bullet) that measures .2437. The bushing that I sized with was a carbide .256 that is what it says it is. After measuring the length to which the bullet was pushed back to with this specific brass neck tension, barrel and bullet, I adjusted my seater so that the same bullet gave me a loaded round that was .030 shorter (than jam). At that lenghth I did a little shooting an powder tuning. (I had gotten the seating depth from a couple of excellent shooters so I thought that I would give it a try.) A couple of days later, I decided to see where that much off jam put me off of touch, as measured with my Hornady seatiing depth tool. As it turned out. I was about .023 shorter than hard thumb pressure touch. Technically the "neck tension" was .003, but with the thin necks (cut to try more chamber clearance) this is not as much bullet pull as thicker necks would have provided. Adding .030 to my seated length, and comparing it to my length when touching, the bullets had been pushed back to a seating depth that was .007 longer than the aforementioned hard thumb pressure touch. I thought that these real world numbers might give new shooters some feel for the subject. This bullet has a double radius ogive, and is based on a .790 length jacket.

 

[lpreddick]

neck tension in quotes...exactly. we refer to neck tension as the measured EXPANSION of the neck after bullet seating. this obviously doesn't take into account the neck thickness. i suspect neck expansion of .003 of a neck that is .007 thick is not the same "bullet grip" as a neck that is .015 thick. these two loads have the same "neck tension", but the amount of pressure to seat the bullet deeper is different as will be the internal case pressure to move the bullet down the barrel. a precise measurement of the force needed to move a bullet into the case as it relates to neck thickness would be interesting. perhaps a mathematical wizard could develop a formula whereby neck wall thickness and neck expansion would give us a number that we could relate to load development, etc. forget it, you'd have to factor in coated, naked, pressure ring or not, short vs long bearing surface, etc.

 

[BoydAllen]

I have a very imprecise system that involves various bushings, the feel when seating bullets, and the biggest indicator of all, the size of groups. We have this marvelous computer between our ears that allows us to sing a note without thinking about the number of cycles per second, throw a pitch without plotting coordinates, and work up loads with little else but a target to evaluate our progress, and a sense of feel to understand the differences in bullet pull. I like gadgets and numbers, but a fellow can get carried away.

 

[skeetlee]

Boyd
Were did the bullets shoot best for your barrel? I also tried the .030 off jam and the colums shot pretty well. However they shot better .007 off jam in my barrel.
I dont think i measured the pressure rings on my colums. .2437 is pretty fat. I wonder how they will perform in a .236 barrel? Probably just fine, but the tune may take a little less powder???? Lee

 

[FergusBailey]

Lee

I have been trying a lot of different handmade 6mm bullets lately, and .2434 through .2437 seems pretty common. In fact I tried one recently which was so fat, I had to stop because my cases did not have any clearance on the neck. And I usually run around 3 thou clearance, so I had plenty of leeway to begin with.

For what its worth, I have had 2 different .236 cut rifled barrels and neither would shoot competitively for me in a 6PPC. I will only use .237 barrels now. Intuitivly, I am uncomfortable that one thou seems to have had such an impact on performance. But the barrel will dictate what it likes, and for me, I never found what a .236 barrel liked.