Bullet comparators compared?? Reviews please

[eric n]

No. What's missing in this thread is declaration of WHAT comparison to discuss. There are different kinds.
The BGC rests on two datums at once.
#1 a bullet inserted to stop at a datum near land contact
#2 an indicator with a seating stem (like Wilson's) rests against the bullet nose forward of #1
The measure here is the distance between the two datums, and is showing ogive radius variance.
The value in this 'comparison' from bullet to bullet is in matching them -so that all other measures off ogives are qualified (to any matched standard).

Now with qualified ogives, it does not really matter where on bullet noses where you set datums.
The Sinclair 'nut' cursed as inadequate, is totally fine. Yes it does not represent land contact datum, but neither does any other tool unless it's cut to same throat angle as your chamber.
Do full seating testing, measure (with ANY tool) your best CBTO, and always use THAT tool to recreate the condition.

Just keep in mind that until you've qualified ogive radius, none of what you're measuring from a bullet nose is qualified. I don't know if Erik knows this, but ogive radius is the base from which to begin discussion w/resp to CBTO.

Got it now. Thank you for the response.

 

[mikecr]

Here is a graphic to show what I'm describing for stem contact high -vs- low.
HIGH (leade contact) for this 11cal tangent bullet provides only 1-3deg contact angle to press against:


Whereas a normal stem contact provides for 10-12deg contact angle to press against:



Would you want to push against, 3degs or 12degs?
If my seater stems were pushing high on the nose, I would customize them to push at a lower point. Lucky for me, they already contact low.

BTW, the 'LgDat to OthrDat' value (0.3695") in this graphic is what the BGC provides.
If you get this distance between two datums matching on all your bullets, it does not matter what datum on the nose you use to measure CBTO from.
If you have not qualified the nose shapes like this, your variances can be, and often are, from this variance. And by far most folks do not know or consider this..

 

[SGK]

See here: Reply I was encountering this just today. Thank you @mikecr for a proper explanation of the BGC tool! My Forster Datum Dial Kit is measuring 'close to leade contact' but my seating stem is contacting way out on the nose (at least for this example). You have also answered my question as to most likely why seating stems are made to push high(er) on the nose. ("Would you want to push against 3degs or 12degs?")

To your penultimate point, I guess perfection requires getting both the BBTO and the distance between the two datums consistent. The former impacts case volume; the latter jump/jam (?)

Does the BGC tool take any seating stem? I guess the tool stem doesn't have to match your seating stem but it would be better if it did. I have to say, he could do a much better job explaining the tool.

 

[mikecr]

Bob Green is a really good person. Not so sure about his line of thinking behind the tool, and I got the impression talking to him about it (making it a common cal kit) that he'd prefer not to bother with the BGC at all. Seemed very honest to me.

I moved on, building my own system for these measurements. Can't afford $250 for every cal..
And before anyone asks, I wouldn't divulge anything about my approach while Bob is still selling his.

 

[SGK]

The painful way would be to measure BBTO with a comparator like the Forster and then take your seating stem and measure BBTseating stem contact point (zeroing out the length of the stem. The difference provides the desired measurement.

 

[normmatzen]

Just read this thread and did some measurements on a small sample of 10 7mm ELD-Ms.
I have a fixture to grade bullets which is a 6"diameter 1/2" steel round plate with a magnetic mount dial indicator . I put my Hornady comparator tip up-side down on the base and grade by B-O. So, I measured my sample from one lot of bullets and found 40% were spot on 0.815", 10% 0.001" longer and 50% 0.0005-0.001" shorter.
Then I replaced the comparator with my Seating Stem for my Wilson in-line seater with a VLD stem and the variation was within +/- 0.0005" and didn't match the B-O!
My conclusion was that for this poor sample size experiment, the comparator was useless and I should be measuring with the seating stem.
This conclusion is based on my way of selecting loads. I start with 0.020" jump and do 5 groups of 5 shots per load level in round robin fashion.
I increment the loads in 1% steps with center where I think the best load will be.
I shoot my groups with MagnetoSpeed attached.
I take the Mv from these shots and evaluate Mv ES with Excel and do a 4-5th order Polynomial curve fit. The curve shows me exactly where my load should be for lowest ES. Then, I load that load and go to 1000yd and tune the tuner for best group. I have gone back many times for both my 6BRX and 284 WIN and tried these loads with different jumps to no avail. They ALWAYS shot best groups with 20 thou. jump.
So, I believe changing jump is nothing more than adjusting load by varying available case capacity including bullet.
Bottom line, I want every bullet to extend the exact dimension into the case. And, based on my poor sample, I'm better off grading by seating stem instead of comparator.
And, of course I'll do this experiment with a larger sample!

 

[SGK]

And before anyone asks, I wouldn't divulge anything about my approach while Bob is still selling his.

Out of interest, what did you use to produce the graphic?

 

[mikecr]

Then I replaced the comparator with my Seating Stem for my Wilson in-line seater with a VLD stem and the variation was within +/- 0.0005" and didn't match the B-O!
My conclusion was that for this poor sample size experiment, the comparator was useless and I should be measuring with the seating stem.

I don't wish to butcher the context of your input, but wanted to visit a couple points. B-O,, is this bearing to ogive, base to ogive?
And does it really make sense to conclude one datum measure is better than another purely due to lower variations? Ask yourself WHY readings higher on the nose are varying, and WHAT this means.
I've put some effort here into describing that these measurements can vary until ogives are qualified. That qualifying ogives means matching two different datums on bullet noses, and you may be seeing the very offender that this qualifying identifies.

I believe changing jump is nothing more than adjusting load by varying available case capacity including bullet.

What bullet seating is doing is another thread itself, but be open to the possibility that seating results are not about pressure.
I know it doesn't make sense, but this notion fails tests. You can vary pressure with powder or powder temperature easier than large seating changes, and yet this does not cause the same results as seating change. I've seen a good cartridge with good barrels that would not break 1/2moa to 300yds, even with a full gamut of powder testing spanning nearly 6gr and ~10Kpsi (per QL). Yet a 10thou seating change could take these results to ~1moa. That 10thou change, while off the lands (OTL), means less to pressure/MV than an Oehler chronograph can even see..

 

[mikecr]

Out of interest, what did you use to produce the graphic?

A spreadsheet I started years ago & handy enough so far.
One of these days I'll finish it, combining some other bullet related works.

 

[snoopy001]

Actually you have it backwards, the hole in the datum must be smaller than the caliber or major diameter for the tool to work. Since it is a comparitor it does not matter how much smaller just as long as it is large enough to stabilize the bullet in. It can be .001 smaller or .10 or .050 below. I don't have a 6 mm insert so I use the .22 caliber insert with my 6CM. For comparison purposes it works fine.

if the datum holes were the same diameter as the major diameter of the bullet they would fall through. You might want to review what a ogive is in reloading terms

oh and yes I know a datum is a number not a physical object but I speak hillbilly, English is my second language


http://www.mssblog.com/2016/06/10/bullet-basics/

if you're able to read the numbers on Bruxbarrels table you probably will recognize the bullets would not fall through