Lapua Cener X and SK Standard Plus

[grg]

Looks like my post with measurements didn't take. After measuring a number of rounds my methods really don't show any differences. I used a micrometer and a digital caliper. I have tried numerous time to chamber other rounds and without polishing the chamber out they won't go in. Center X does chamber perfectly though and that was my goal.

I also have a picture of my 50yd target. Zeroing at 25 it was laying rounds right on top of each other. When I went to 50yds the first 4 rds went in the same hole, 5th went high. The wind was bad and I assumed that I must have timed it wrong or moved. After shooting a few more groups and wondering what happened I found the rear ring bas loose. I could snug it up some and groups closed a little but it would not tighten down good again. The rings were the only ones I had high enough for what I wanted but I knew they were pretty junky rings.

I pulled another mount and scope off a proven shooter to put on but the wind is holding steady around 15mph and on our range it swirls terribly so I'm going to have to wait. I'm hoping my 25yd and first 4rds at 50 weren't a fluke.

[NIB]

From viewing your measuring methods it still appears as though you're missing a critical one. Measuring from the top of the rim (where an LR head-spaces from) to the end or tip of the driving band or bullet band as you call it. I do see your OAL measurement and if this also means the driving band being further ahead in the SK then this is likely why the tighter fit.

[grg]

I guess I didn't include that measurement which as best I could get it was virtually the same. I would agree with you if the problem was the final push and closure of the bolt but the real struggle is right away when the case starts to go into the mouth of the chamber.

[Steve Newman]

grg, just out of curiosity, measured a round of approx. 1 year old Sk Rifle Match, and 3 year old Wolf Match Extra....confirming identical dimensions on case length and diameter at each end, bullet diameter at driving band and overall length (.980) as your Sk Standard Plus results show. The Bullets both have a tiny flat on the tip (not perfectly rounded). Also rim thickness, which you left blank in your dimensions, was difficult to measure with dial calipers, but after several readings, averages .040 on both my examples. So these factory Sk dimensions must have been standardized for quite some time. So, it appears the Sk is just slightly longer COAL than your Lapua ammunition.

[MKnarr]

I'm not thrilled getting into these "rim thickness" discussions because I've spent a lot of time and not a small amount of money measuring rim thickness and convincing myself rim thickness has nothing to do with precision. I haven't convinced anyone else yet but here goes anyway.

I bought an $85.00 Bald Eagle rim thickness gauge and measured the following high end rounds.

Note that none of the measurements come close to the lowest headspace anyone uses of .043. Also note that some of the brands were then fired through a chronograph and the velocity recorded.

Next as a range manager, I picked up a lot of live rounds and measured the rim thickness on them. These are basically bulk ammo.



Note that none of those are above .039, still below the threshold of .043. But the STD is larger than the high end stuff except the Win Xpert and I guess that is to be expected. What is surprising is these rounds are not necessarily from the same lot and as you can see in the first pic, even in high end ammo the avg varies from lot to lot.

The reason I did the velocity test in the first pic is because I did a correlation plot of the rim thickness versus velocity and found no correlation. Rim thickness does not equate to precision.

Now here is the interesting part about measuring rim thickness even with a highly regarded Bald Eagle Gauge. I took one round of each below and marked 4 quadrants on the rim and took three measurement on each of the 4 quadrants. I did this by rotating the round 1/4 turn for each measurment.


Except the Eley Match I got .001 spread on the same round. So what are you measuring.

[NIB]

Originally Posted by MKnarr

Now here is the interesting part about measuring rim thickness even with a highly regarded Bald Eagle Gauge. I took one round of each below and marked 4 quadrants on the rim and took three measurement on each of the 4 quadrants.


Except the Eley Match I got .001 spread on the same round. So what are you measuring.

A high spot or low spot?
Eley may have just replaced their dies on that .0310 measurement? Certainly some precision there.
A very interesting report.
In most cases I'd bet the numbers vary as shown in your 4 quadrants test.