I got a new toy! I’d been waffling over getting the Datum Dial for months, and I finally broke down and got one. First impressions are excellent. It is easy to mount on calipers, easy to switch between dials. Ive been measuring random things and the readings are very consistent. I think this will definitely be a useful addition to the bench. A definite keeper.
JTD
Last edited by JeffreyDeGraff; 10-12-2020 at 03:50 AM.
Ive never had a headspace comparator before, so this has been very eye opening for me.
WW 30-06 range brass I got from Wes, varied up to .015”
Hornady 6mm Remington brass that I fire formed to 257 Roberts, is .005” shorter than FC 257 Roberts brass fired from the same gun.
S&B 7x57 brass loaded with powder puff loads were .010” shorter than same brass fired at standard pressures.
New Hornady and Nosler 280 Ackley brass were .002” longer than fireformed RP 280 brass, but twice fired RP brass was .005” longer that the Hornady and Nosler.
I haven’t used the bullet comparator yet, but I’ve been using the Sinclair hex bullet comparator for years.
JTD
Jeffery, the one thing I know about forming 6mm and 7mm TCU brass is that I have to leave them long for fire-forming or they will end up short. Kinda makes sense when you think about it. (Thinking specifically about the Roberts forming) I am very curious about the 7X57 powder puff loads ending up shorter? That just doesn't compute, guess I"ve still got plenty to learn. Thanks for sharing, this is very interesting.Originally Posted by JeffreyDeGraff
"Long range shooting, It's like golf, but for men"
I don’t think I explained myself very well. It made sense to me while I was typing but after rereading, it wasn’t the best. My measurements were all base to datum point on the shoulder on fired brass.
The 30-06 range brass varied .015” at the shoulder. I surely didn’t think chambers varied that much. I would suspect some of the rifles it was fired out of may have headspace issues, but then maybe they were fired out of semi autos and stretched a little more.
The 6mm brass that necked up and fired in my 257 Roberts, didn’t fireform all the way on the first firing. It is still .005” shorter at the shoulder than the actual 257 Roberts brass fired in the same rifle. The 6mm brass will need one more firing before it is fully fireformed to the chamber.
The 7x57 brass was range brass I had gotten from Wes. I full length sized and trimmed it all. My rifle is on the upper edge of acceptable headspace, it would be considered borderline excessive headspace. I loaded some super light loads with 4227, to let my kid get used to the rifle, I also loaded some with standard powder charges ( on the lighter side). The brass that was loaded with the super light charge was .010” shorter at the shoulder than the brass that was loaded with the standard powder charges. The lighter loads didn’t expand the brass enough to fill the chamber so they are substantially shorter at the shoulder than the other brass.
The most confusing of all is the 280 Ackley brass. I fireformed some standard 280 brass in my 280 Ackley chamber. Half of it has been shot a second time. The measurement difference, to the shoulder, between the once fired (fireformed) brass and the twice fired, was .007”. I didn’t even get close to a full fireform on the first firing. My new Hornady and nosler 280 Ackley brass was closer to my chamber dimensions than my fireformed brass.
When brass is fireformed, shoulders blown out etc, the overall length always shrinks. The issue I was having with short was at the shoulder. I really thought I was getting a better fireform than I actually was.
JTD
Yep, guilty of not understanding. Should have looked into the Datum-Dial thing a little closer. Thank you for your explanation, I have a few (a lot of) things to learn yet and so enjoy these conversations.
"Long range shooting, It's like golf, but for men"
No worries. I’m just glad someone actually reads my ramblings.
JTD
New discovery.
I measured my 257 Roberts brass with the datum dial, screwed my FL sizing die into my rock chucker until it touched the shell holder and sized 4 pieces of brass.
I measured the brass again after sizing, and it grew .004” at the shoulder.
I screwed the die in approximately 1/8 turn (minimal cam over) and the shoulder was .003” shorter than the original unsized measurement.
So if your FL sizing die is not screwed in until it reaches the shoulder, your shoulder length will grow more than if you’re just neck sizing.
JTD
Man I can't say enough about having that datum dial. Or even the Hornady one would be good. It is so eye opening to see exactly what the dies are doing to the brass. The last 3 calibers I have sized have been very different experiences. The 243 Hornady die wasn't sizing enough but the lee did, and I had to have maximum amount of cam over. 338-06 I had to have the sizing die backed off away from the shell holder or else it was sizing way too much. The 35 whelen I had to turn the die about a 16th of a turn past the shell holder for barely any cam over. I'm going to say, after using it, that it is a must have piece of equipment on the loading bench.
JTD
JTD, I finally got the time to really look into this tool this morning, and it looks like a great addition to a precision reloading bench. The term that was foreign to me was "datum" so I didn't quite get the function here. I have the Hornady LnL headspace comparator on my bench for many years already and I use it extensively for both headspace and cbto measurements. It has inserts for both headspace and bullet ogive diameters, so both tools do the same thing, but I do admit, if I didn't already have the LnL, the Forster one is certainly a nicer and more compact "made in USA" kit. Thank you for spurring me on to a little more knowledge in this great hobby.
"Long range shooting, It's like golf, but for men"
Ive been using the datum dial on everything since I got it. Ive measured the crap out of everything. I’m still in love with it, can’t say enough good about it. Very consistent and easy to use.
JTD
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