Neck size esoterica / 308

[offshore44]

I'm back playing with paper patching the 308.

The question is how much neck tension do I need and what can I get away with.

If the latest batch of PP boolits is seated in an unsized case, I can't easily push them in with my thumb, but they do go in easily with the press. The brass is mil-surp 7.62 and fired from this rifle. If I seat long and let the bolt close to finish seating, it closes easily. I'm guessing that I should be OK, but then guessing with something that goes bang that close to my face is probably not a good idea.

It occurred to me that setting the COL a couple of thousandths long to let the bolt finish seating for consistency in boolit position in the throat might be a good idea. What does everyone think?

[Digital Dan]

Thinkin' you're ok for single shot use. Seating long won't hurt, might help. Taper crimp if you want to use the magazine. .001-.0015 neck tension is good for smokeless PP.

[offshore44]

Thinkin' you're ok for single shot use. Seating long won't hurt, might help. Taper crimp if you want to use the magazine. .001-.0015 neck tension is good for smokeless PP.

Nice! If the calipers aren't lying to me, that's right where I'm at! Well, 0.000 - 0.001"+ a hair anyway. Of course, measuring the inside of a case neck with uncalibrated calipers is iffy at best, but it's at least in the ball park. Measuring the cases and the boolits with the same calipers yields an ID of 0.309 - 0.310" on the cases and a very consistent boolit OD of 0.310". Love it when a plan works out... (measuring with the micrometer yields 0.3100 - 0.3105" for boolit OD over the patch.)

Edited to add: It seems a little case sorting may be in order. I've got a metric boat-load of cases for this rifle, so a little judicious selection may be in order for "the perfect" neck tension. This just keeps getting better and better.

[RickinTN]

Calipers won't measure the inside accurately anyway. Measure the OD of the case before seating a bullet then the OD after seating a bullet and this will give a more accurate measurement of neck tension.
Good luck with your loads,
Rick

[offshore44]

Calipers won't measure the inside accurately anyway. Measure the OD of the case before seating a bullet then the OD after seating a bullet and this will give a more accurate measurement of neck tension.
Good luck with your loads,
Rick

Excellent tip Rick! Thanks. Why didn't I think of that?

Edited to add: Just did couple...and it looks like 0.000 to 0.001" neck tension. Still golden. Couldn't pull any of then with my fingers.

[303Guy]

I find that a boolit seated as you describe won't move in the neck in the magazine on firing. At least if I understand you correctly. I can start the seating with a firm twist but have to finish seating in the press. I lube the patch prior to seating by rolling the boolit on my lube pat with STP in it. That surface lube soaks away leaving an essentially dry patch to grip the neck. I can only measure the neck expansion using an electronic micrometer with 0.001mm resolution. That's 0.00004 inches. One could call that 0.000". The alloy I was using was fairly soft so could not tolerate much pressure so I needed the paper only to compress. A harder alloy would no doubt be different. In fact, I seated one boolit into a case and it wouldn't chamber so that one definitely expanded and apparently the boolit was not compressed.

[offshore44]

0.001mm resolution!? Suppressors AND electronic mics? 303guy, I'm jealous, a bit anyway.

True about the neck tension being enough to hold the COL with 308. It's about the same tension as the 458, maybe a little less, and the last round takes 5 recoils of pretty stiff charges (405 grn pp at about 2250 fps with a case full of H4895) The 308 is much tamer.

Seating the boolit takes a pretty firm twist into the case, and then it gets finished by the press. I can put about 2.5 - 3mm into the case by hand, and that's it. There is no way that I can pull these with just my fingers. It takes about two or three good whacks with an impact bullet puller to unseat those things.

The nice thing is that the bullet actually hasn't changed diameter on any of the rounds tested so far. My standard alloy is 16:1, so it's not really soft, but it's not harder than Lyman #2. The last couple of batches that I have tested are right in the 11.0 to 11.5 BHN range. I use bullets that I dig out of berms in popular shooting spots, combined with wine bottle capsules (tins) or pewter. It's not the greatest alloy, but it's what I have.

The sized castings are 0.309+" sized to 0.3015", wrapped with cotton vellum to a final size of 0.311". I then run them through a 0.309" sizing die with BAC. They come out at 0.310" or so.

Next step is loading these things up and giving them a run, I think.

[CJR]

Offshore44,

On standard 308 Win chambers, the diameter of the cylindrical section before the forcing cone is 0.310"D. Therefore with PPCB at 0.310"D or greater, the PP can be cut during round chambering. A cut PP can lead to bore fouling/inaccuracy(i.e. gas blow-by). If that becomes a problem, you might want to try sizing to 0.3095"D or even 0.3085"D.

Another thing you might want to do is lay some of your sized PPCB aside for a week and remeasure them for PP diameter growth. They may or may not grow larger than 0.310"D. One never knows about different papers; i.e. sizing compounds in paper, chemical additives in paper, etc., and the effect of the bullet lube on that paper.

Best regards,

CJR

[offshore44]

CJR, that could explain some of the issues that I as having last fall before the monsoons set in. The patch was probably getting cut on the way into the chamber / throat. The two dummy rounds that I have been playing with have not displayed that issue yet...but there's time for that to develop.

On another note, I changed out the sizer die for a different one and the PP boolits seem to be settling in at around 0.3095" give or take a very small fraction.

Fiddled around with some cases and it appears that the Lake City '06 cases are the only ones that consistently exhibit the correct neck tension. The LC 07's are loose and the LC 05's are pretty hit or miss. Mostly miss.

Man, this is fun stuff!

[CJR]

Offshore44,

I have thought much on neck tension/grip with PPCB. The are as many shooters who advocate some specific neck tension, for accuracy, as there are shooters who use a slip-fit CB.

In my view, a PPCB is typically seated against the forcing cone. The neck tension, relative to peak pressures of 50,000 psi, appears to me to be a less significant problem than the large gas pressure leakage past a torn PPCB . If the PP is torn, the building pressure vents/jets through the PP tear as the PPCB starts to move. Result? A large significant variation of remaining pressure applied to the base of the PPCB as it accelerates. Large pressure variations equals large FPS variations equals poor accuracy. Maintaining a good pressure seal, with the PP, in the barrel is important for top accuracy. The word "fun" is an understatement once the one-hole groups appear at hi-vel!

Best regards,

CJR

[offshore44]

You're cracking me up CJR... One hole groups? I have NEVER shot a one hole group in my entire life! I'm pretty happy with consistent 1 - 2" groups. The 458 will shoot a one hole group on occasion, but with a boolit just shy of 1/2" in diameter, that's still about a 1" group. It's just one big ragged hole is all. :D :wink:

Been reading BruceB's thread on the M1a...and I have an FAL that needs food. That's the next project on the list after this one is pretty much in the bag.

Oh, and I have to add: The wife did shoot a three shot, one hole group with her 7x57 with a load I worked up for her. You had to look at the back side of the target backer to see that all three shots went through the same hole.

Thanks again for the insights.