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Old April 13, 2002, 08:04 PM   #1
fpotter
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Stilll more comparator confusion

Okay - I've read all the posts and done most or maybe all of the stuff recommended. But I still have a specific problem figuring out what it all means.

Let me take a precise case in point (precision being what this whole thing is all about, no?). I have an Armalite AR15, shooting .223 Remington rounds. The factory length of rounds is stated to be between 2.250 and 2.260 inches. And the rifle shoots factory ammo okay - not great, but okay.

So I haul out the rifle and a bunch of variously weighted and shaped bullets, together with the Stoney Point OAL gauge and the Sinclair comparator. Here's where it gets complicated.

A Sierra Match 53 gr bullet, put into the OAL gauge, measures an average of 2.183 inches. Add that to the Sinclair comparator and I get a measurement of 2.812. How, taking all these things into consideration, do I construct a dummy round, which I can use as a template from now on? (Other weights run from 55 gr--2.147 (2.821) to 69 gr--2.227 (2.813)).

In theory, at least, the factory round may be touching the lands in my rifle when it is put into battery (or at least so it might indicate). I suppose that I could do the comparator on that round as well, but I'm not sure what that would tell me.

If I sound confused as hell, then I've described my situation perfectly.

Frank
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Old April 14, 2002, 12:25 AM   #2
Doctari
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A comparator for bullet seating depth should allow you to construct ammunition that is any OAL you desire. Take your sierra 53gr bullet. I take it that the max OAL with that bullet in your chamber is 2.183". Or in other words the bullet is seated so that it is on the lands, which means measuring from the case base to bullet ogive is 2.183".

What good does this do you? Well, you now have a reference point to construct loads of various OAL to determine what OAL the rifle likes best. Once you arrive at a given powder charge you want to use then load up cartridges that are 0.01, 0.015, and 0.02" under MAX OAL. In other word seat 5 cartridges to measure 2.173", then 5 to measure 2.168", then 5 to measure 2.163". Shoot these groups for accuracy. If your other components are the same, (case, primer, bullets) then you'll see what if any effect OAL has on accuracy.

I am a bit confused what this 2.812" measurment is.... That is way over MAX OAL for 223 rem. Your stoney point comparator should be able to give you the definative number on its own when used with dial calipers. I don't know what "add that to the sinclair comparator" means.......

Do you have the special 223rem case that stoney point makes to use with their comparator? Screw the modified 223 brass on the the comapator, advance the rod to the case neck, put the bullet in the case then insert the whole mess into the breech end of the rifle, advance the center rod to move the bullet into contact with the lands, tighten the center rod. Pull the whole mess out and take a measurement with your dial calipers and the included insert. You now have max OAL for that bullet type in your rifle. Is all of this sounding familiar?

Sorry if I have confused the issue,
Mike
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Old April 14, 2002, 08:03 AM   #3
Khornet
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OAL

usually refers to legth from case head to bullet tip, not the ogive (by which we mean the point down from the tip at which bullet diameter becomes nominal,e.g. .223 for AR 15, .308 for &.62 NATO). So you can load rounds well over specified OAL without touching the lands if the ogive is in the right place. And not all bullets, even of the same caliber and weight, have the ogive in the same place.

So if you use a seating depth gauge like the Sinclair (I've never used a Stoney Point), you are finding the distance from case head to OGIVE which will put the OGIVE in contact with the lands. In fact, the POINT never touches the lands, so measuring to it means nothing as far as accuracy goes. It DOES matter as far as whether the round will feed in your gun, because too long and it won't fit the mag or the action length.

Once you have that case head-to-ogive length, you use your comparator. Slip it over the bullet tip until it stops at the nominal diameter, which will be somewhere back of the tip, and then measure with dial caliper from case head to the flat on top of the comparator. For my 03A3 this distance is about 3.800" . So with any .308" bullet, whatever its shape, if it measures with the comparator on to 3.800, it will touch the lands. But is overall length may be greater or less than the official 3.340" OAL of the .30-'06 cartridge. I don't care about OAL as long as it will feed; I want that bullet ogive a specific distance from the lands, usually about .010 off them. So I seat to give a comparator measurement of 3.790.

In fact, 'ogive' may not be quite the right term; I think it specifically means the shape of the tapering from nominal diameter to bullet tip. A match 168 gr BTHP has a different ogive than a 200 gr round nose bullet. As the ogive increases in diameter as you go down from the tip toward the base, it eventually reaches nominal diameter, .308 for my .30-'06. It's at that point that a comparator measures. From there back to the base (or the beginning of the boat tail) the diameter willbe a constant .308. But where the bullet touches the lands willbe where it first reaches .308 (or whatever the land-to-land diameter of the bore is).
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Old April 14, 2002, 09:07 AM   #4
fpotter
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Clearing up, or perhaps enhancing the confusion: I have the Stoney Point OAL gauge, which measures to the bullet tip from the base. And I have the Sinclair comparator, which measures, in effect, the distance from the base to the ogive.

I'm pretty sure that the OAL numbers that I set down are correct, although they are significantly shorter than the standard 2.250-.260 in the manuals. That's the first point of confusion.

The Sinclair comparator is a hexagonal nut, with six different bullet sizes. You take the round as it came out of the OAL gauge, stick it into the right hole in the comparator and measure the whole thing. That gives you the second number.

What I am still having trouble with is the next step. Do I then take the same bullet, create a dummy round that is 2.250, put it into the comparator and then adjust the die so that the bullet gives me the second (comparator) reading?

There really is going to be a light at the end of this tunnel, isn't there?

slow but educable Frank
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Old April 14, 2002, 09:49 AM   #5
Steve Smith
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"I have the Stoney Point OAL gauge, which measures to the bullet tip from the base. "

No you do not. The Stoney Point OAL gauge measures absolutely nothing. Your calipers, TOGETHER with the comparator, measure the OAL gauge.


Are you putting calipers, sans comparator, onto the gauge and measuring the Stone Point cartridge when you pull it from the rifle? If so, stop doing that. Only use the comparator. Tip measurements will only confuse you.
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Old April 14, 2002, 10:38 AM   #6
Doctari
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I agree, stoney point gauge, used in conjunction with your calipers should measure from cartridge base to ogive. NOT cartridge base to tip of bullet which is a relatively meaningless measurement except as to whether your reload will fit in the magazine.

In my previous post I refer to OAL a lot but in general i mean base to ogive not base to tip. I havent yet seen an abbreviation yet for base to ogive. Sorry if I added to the confusion.....

Frank, forget about the 2.250" that is nominal OAL. You are trying to take a measurement of your rifles chamber. The stoney point gauge should give you that measurement with dial calipers, for your particular bullet. Once you have that measurement you can then set your seating die to duplicate it. Leave your comparator on the calipers, then keep a adjusting the seating die down and checking the cartrdge in the gauge until it reads the number you want. Write the number on the side of the case with marker. Presto now you have a dummy round.

I could show you this in 2 minutes but it is taking a week to explain<g>


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Old April 14, 2002, 10:38 AM   #7
Steve Smith
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Going back, looking at what you said, there are a lot of questions. first of all, you measured the base to tip of some factory rounds: "The factory length of rounds is stated to be between 2.250 and 2.260 inches." That a base to tip. 2.260" is max OAL from a book somewhere. Do you know why that is? That's so it'll work in the magazine and feed. Your chamber should be at least that long so that factory rounds will work...of course, that's using factory bullets that they consider to have a "normal" ogive, whatever that is. Anyway, there is a good possibility that your rifling begins well after the point where the ogive on a factory round would end. To find that spot, you do as the instructions, others, and myself have said and stick your bullet in the SP OAL gauge, stick that in your rifle, push the bullet 'till it stops, lock the nut, and pull the stuff out. Now, you measure that using calipers WITH the comparator in place. Do this several times 'till you can get a consistent measurement. Subtract the length of the comparator. (you know, just close the calipers with the comparator in place and see what the measurement is) Write the measurement of the cartridges minus comparator length down. With that bullet type, THAT is where the bullet ogive will touch the rifling. Now you must do this with all the bullet types you plan on trying out.

Now, when you're loading rounds, you have an end-point that you should not go over. In fact, don't go within .005" of your max with that bullet for safety concerns untill you learn more about this stuff. When setting up your seating die, seat a bullet and measure it using the comparator. You will know what the max with that bullet is because you wrote it down, so now you can compare this to that and see where you need to move the seater die.
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Old April 14, 2002, 11:37 AM   #8
fpotter
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I think the penny just dropped. Finally.

The part that I hadn't understood clearly enough is that the ONLY function of the SP OAL gauge in this context is to hold the bullet in the comparator.

Where the rabbit went into the hat was the fact that, having found the measurement of the case, bullet (to the ogive) and comparator, you then SUBTRACT the width of the comparator. What you are left with is the info you were trying to come up with from the beginning.

Do I finally have it?

Thanks for all your patience, guys. This stuff is not intuitive.

Frank
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Old April 14, 2002, 01:06 PM   #9
Steve Smith
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Yes, except that you subtract the length of the comparator, not the width. The width is inconsequential to the result. I think I know what you meant, though.

You're welcome.
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Old April 14, 2002, 06:36 PM   #10
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Here are a couple of points concerning AR15 and ammo OAL.

1. Fpotter, you said that your rifles chamber measured out to 2.8XX. Yep you are in the ball park for a NATO standard chamber, so your good on this.

2. 99.9% of the reloading books were written for hunting/SAAMI standards chambers. These have shorter chamber throats.

3. NATO and SAMMI standards are different. The chamber throat of an M16/AR15 are much longer than what you will find on your Remington or Savage hunting rifle. If you run your cartridge OAL out until the ogive "just hits the lands", over 2.260, you will not get them to work properly in the magazine. If you are using a heavy bullet, upper limit for a 1:9 twist is 69/70 grains, seat the bullet until it just clears the magazine.

The only way to really know what your chamber is all about is to make a casting of it using cerrosafe. If your rifle is a stock and trade AR15 from Bushmaster, Armalight or any of the clone makers, you have a NATO chamber. You may have paid extra for a "match barrel", but your chamber is still NATO standard with the longer throat.

My suggestion is keep the comparator for use on your varmint guns and do some exprerimenting with COALs and find the barrels sweet spot by shooting some groups.
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Old April 15, 2002, 03:07 PM   #11
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Yeah, I gave up trying to hit the lands on my AR. I just load to 2.26" and think no more of it.
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Old May 11, 2006, 12:28 PM   #12
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I hate to revitalize an ancient thread, but I just have to say that after scratching my head for the past day about this exact same issue, and feeling just identical to fpotter's exact details on this issue, this thread and Steve Smiths explanation made everything 'click' for me, just as it did for fpotter.

So, just a huge "thank you" to Mr. Steve Smith (hopefully he'll read this sometime in the future as I can see his last active post was way back in December of 2002), as well as TFL and the 'search function' pulling up truly useful pre-discussed information!

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Old May 11, 2006, 09:35 PM   #13
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fpotter
What twist is your barrel? Your twist rate (along with throat length) will determine if your bullets (or what bullets) can be loaded longer than what will fit in your mag (to reach the lands). I believe only the heavier bullets can be loaded long enough to reach the lands. For a Armalite AR15 I would load to fit in the mag unless your rifle is a varmint barreled rifle and you want to shoot long distance. I've got an RRA varminter with a 1/8 twist and it does real well with 69gr SMK's loaded to fit in the mag at least to 300 yards (farthest I've shot these).

Here are a couple links you might find interesting -

http://www.benchrest.com/forums/showthread.php?t=28867

http://www.benchrest.com/forums/show...=seating+depth
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