Using an OAL gauge and bullet comparator


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azar
January 26, 2008, 02:21 AM
I recently purchased the Sinclair OAL gauge (http://www.sinclairintl.com/cgi-bin/category.cgi?category=RESDTSD&item=59-4000&type=store) and bullet comparator (http://www.sinclairintl.com/cgi-bin/category.cgi?category=RESDTCO&item=09-600&type=store) which finally arrived on my door step today. Now I need help interpreting the measurements.

Just for reference the gun is a CZ 550 American in 6.5x55 SE and the first bullet tested was the Sierra 6.5mm 160g semi-point.

Following the instructions I measured the bullet base to tip (step 1) and got 1.260".
My measurement from the cartridge head to the base of the bullet (steps 2 through 9) was 1.750".
The final step says to measure the length of the bullet and add this to the measurement used in step 9 (1.750"). But... didn't I get that measurement already in step 1? :confused: Or is this a different measurement?
Anyway, by adding the two measurements I get 3.010" cartridge head to bullet tip.

Now, I know that it's best to measure a bullet on it's ogive which is what the bullet comparator is for. If I measure the the bullet in the comparator (just the bullet) I get a measurement of 1.824". This is from the flat side of the comparator to the bullet base.
The measurement (if this matters) of the comparator from the side marked "26" to it's opposite side marked "25" measured at 0.98".

So what does all this tell me? How do I put these numbers together properly to get the OAL for which the bullet touches the lands? What else do I need to do to get my optimal OAL? I know that generally a bullet should sit 0.015" to .02" from the lands so that I'll need to subtract that from my final measurement, but I'm not sure what my "final measurement is."

Sorry if this is a noob question, I feel stupid even asking it but I'd rather get this right and know how to do it properly. :o

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mrawesome22-250
January 26, 2008, 08:43 AM
Take an empty case. Cut a vertical slit in the neck, barely into the shoulder, with a hacksaw. Barely seat the bullet you want to use in this case with your hands. Chamber this round in your rifle. Eject it with your hand covering the ejection port. Take it out and measure it from base to tip with your calipers. This will give you MAX OAL (over all length) for that bullet. In other words, this bullet is touching the rifling. Now you can experiment with different seating depths. I've found that my rifle usually gives best accuracy .030" away from the rifling. But you'll just have to experiment as to what shoots the best out of your gun. This method works very well and you don't have to waste money on a Stoney Point gauge.

swiss7.5
January 26, 2008, 09:04 AM
Azar. you have the correct tools do not revert back to the old method as others suggest. i only use the comparator but im sure there is someone that uses the AOL guage here.

Khornet
January 26, 2008, 09:29 AM
I'm away from my reloading bench so I can't be positive, but I'm pretty sure you're doing it right. Now take that 3.010 round, and insert the bullet in the appropriate hole in the comparator, and with calipers measure from case head to the flat on the comparator opposite the hole the bullet is in. This number is THE number, for any bullet you use in that rifle, to load a round which will have the ogive just touching the rifling.

For my 03A3, that number is 3.800. Since I prefer a few thousanths away from touching, I load my rounds to be 3.795 or so when measured in the comparator. With different bullets, there will be different OALs measuring from case head to bullet tip since different bullets have different lengths, but all rounds measured with the comparator at 3.795 will have the bullet ogive 0.005 away from the rifling.

In fact, even in a box of one type of bullet, there will be significant variations in the bullet length, especially with lead-tip hunting bullets, since the lead deforms a bit rattling around in the box. But it doesn't matter, because the ogive (the point where, as you move down from the tip, the bullet diameter reaches nominal bullet diameter, or 0.308" in a 30 cal.) is always the same distance from the rifling.

You made a wise investment. Just review the instructions and practice a few measurements and it'll click for you. I have found some variation in repeat measurements of the same case and bullet, and that can't be helped. So I just do five measurements and take the average.

I wouldn't dream of loading a new bullet without using the comparator to get the seating depth right.

taliv
January 26, 2008, 10:06 AM
yeah, it is kind of confusing at first.

basically what you're doing is finding

a) the measurement from the base of the bullet to the case head (by using two reference points and some math as described in the instructions)
b) the measurement of the bullet from tip to base

and adding a + b to get c, the overall length from tip to base

now in order to get f, the OAL from the ogive instead of the tip,
the easiest thing to do would be to seat THAT bullet to the length and then measure the OAL from ogive with that little hexagonal nut doohickey with the holes in it. the problem with that, imho is that once i've seated that bullet, i can't easily use it to repeat the measurement later. i like to save the bullet and use the same one over and over to measure the throat erosion over time. granted, there may be an easier way i don't know about.


what i do is some more measurements and math to avoid seating my reference bullet. so once i have that a+b, i USE THE SAME BULLET and the little hexagonal nut with the holes in it to measure d, the bullet length from the ogive to base. (by measuring the bullet in the hole plus the diameter of the nut, then subtracting the diameter of the nut)

once I know that, i also know the ogive to tip measurement e, which is d - b

so now i subtract the ogive to tip from the OAL, and get my new OAL measured by ogive, f = c - e, which i can use to seat other bullets

taliv
January 26, 2008, 10:14 AM
also, just in case you don't know, if CZ is anything like remington, it's very likely that the OAL length you get from the lands is going to be a much larger number than you're expecting, even possibly larger than the total case length plus total bullet length, meaning, it's not physically possible to seat the bullet at the lands.

lawyer-proof throats are a sad fact of life today and rebarrelling that dude is your only real option. hopefully, CZ isn't so bad

USSR
January 26, 2008, 11:30 AM
azar,

I have the very same bullet comparator that you have, and the only measurement that you are concerned with is the one shown in Sinclair's photo showing their bullet comparator. I use the Hornady O.A.L Gauge to determine the maximum cartridge base to bullet ogive length of the chamber; it appears to be much easier to use and understand than the Sinclair model.

Don

azar
January 26, 2008, 02:26 PM
Guys, thanks a bunch for your help. I'm sure I'll asking a few more questions before I've got this down. In fact, I have a couple right now... ;)

So it's normal to use the OAL length gauge with the same bullet multiple times and get different readings each time? I was getting rather frustrated that I was doing something wrong... So far today, I've measured 1.750", 1.747", and 1.750" with the same bullet. Although last night my variances seemed to be slightly greater (1.750", 1.745", 1.752" iirc).

taliv said:
also, just in case you don't know, if CZ is anything like remington, it's very likely that the OAL length you get from the lands is going to be a much larger number than you're expecting, even possibly larger than the total case length plus total bullet length, meaning, it's not physically possible to seat the bullet at the lands.

taliv, It's sounds like the method I was using is exactly what you are describing. So that is very reassuring to me. However, I had quite the opposite find from what you suggest I might. My OAL seems much shorter than I expected with this semi-round nose bullet. The SAMMI max for this cartridge is 3.150". Sierra's C.O.A.L for this load was 3.050". 3.050" wouldn't chamber in my gun so I moved it back to 3.025" which has been patterning fairly well. My measurement last night says I'm hitting the lands at 3.010". Does that mean I'm .015" into the lands?! I understand that tip deformity can change the readings so I should be using the ogive measurement to get my final C.O.A.L. but with the tip measurement this bullet C.O.A.L. should be around 2.995". Right? That's a big difference from Sierra's and not quite what I was expecting... :scrutiny:

Or am I missing something?

docgary
January 26, 2008, 05:06 PM
azar

I'm having the same problem understanding the applications of the findings
using the Hornaday OAL gauge and comparator with my AR15 - .223 cal.

In addition, feeling for the lands and simply placing the bullet/machined case
combo until it stops in the chamber appear to be one and the same.

If you get it figured out, please drop me a post!

thanks
docgary

docgary
January 26, 2008, 05:07 PM
azar

I'm having the same problem understanding the applications of the findings
using the Hornaday OAL gauge and comparator with my AR15 - .223 cal.

In addition, feeling for the lands and simply placing the bullet/machined case
combo until it stops in the chamber appear to be one and the same.

If you get it figured out, please drop me a post!

thanks
docgary

azar
January 27, 2008, 12:55 PM
docgary,

I believe I'm starting to "get it". Reread taliv's post up above and see if it helps at all. It might not since he's referencing the Sinclair tool instead of the Hornady and having never used the Hornady tool I don't know if they work the exact same.

Why don't you post your questions on what you are confused about here? This would probably be as good of a place as any. Don't feel too bad. I know that I'm a fairly bright person (I do software development for a living) but I wasn't quite sure I was doing it right or understanding my results. Plus, it had been a long week and it was late when I started trying out the new gear so my brain was half shut down anyway. I really just wanted to climb into bed but I didn't want my new toys to get the better of me. ;)

williebyte
December 16, 2008, 12:04 AM
Khornet, if you could , would you send me a copy of the instructions on how to use the bullet comparator. I didn't get any instructions with mine. I am so confused. Need help bad. THANKS
Robert

Khornet
December 16, 2008, 07:06 AM
Will check when I get home.

williebyte
December 16, 2008, 11:48 AM
Knornet
I appericiate any help you can give me. I thank you for your time.
Robert

30Cal
December 16, 2008, 02:03 PM
I seat a bullet super long in a sized case and stuff it into the chamber with my thumb. Bump the bullet back a little at a time until the cartridge drops free of the chamber from gravity alone. Then measure with the comparitor.

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