So I'm ready to start dropping powder and seating bullets.

Loading Speer Gold Dot 55 gr .223 SP bullets (no cannelure) in once shot Speer Gold Dot nickel plated brass...

Need to set my seating die for OAL.  I decided to take a box of factory Speer Gold Dot 55 gr .223 SP and measured the OAL of 20 rounds and
was going to use the average when came out to be 2.2325"

If it matters, I'm using a Forster Co-AX and a Forster micrometer seating die.

So I back off the bullet seating depth on the  seating die, pick a random bullet of the box, and slowly increase the depth of the seating die until the OAL measures exactly 2.2325"

I then pick another bullet out of the box and without changing anything, seat the bullet and get a different cartridge OAL.  I seat a few more and the OAL for every cartridge comes out slightly different.  

I finally measure the length of a dozen random bullets in the box and find they vary from .7250 to .7395.  That's difference of .0145

So much for consistent OAL and that's just a random sampling of a dozen dozen bullets.  I'm sure there's some shorter and longer than that in the box.

So. I think I know what's going on.  The soft points get banged around during manufacture, and being soft, the ends of some are blunted a little more than others.

So how do I set my seater die?  If I just pick a random bullet, it might be the shortest bullet in the box, and if I set my OAL using it, then the OAL of all the others will be off from the average factory OAL I was trying to match.

Is there a standard way of dealing with this?  Do I need to also compute the avg. length of the bullets and pick a bullet close to the average length, and set the seater die to get the OAL I want using this "average" bullet?  I would think if I do this, then the OAL of some cartridges would end up shorter, and some longer, but the average should be close to the factory average?

Pick a tool that measures at a point on the ogive not the tip.

Hornady type is likely the most available that goes on a 6” micrometer.

I use a Sinclair “nut” which was the tool for that in the eighties.   It’s a six sided nut that has a hole in each face that corresponds with a different diameter, ie iirc mine has .224, .243, .260, .277, .284 and .308.  This way you’re not measuring off the deformed or inconsistent tip. Back then match bullet hollow points were a bit random at the tip for length.

This?  

https://www.brownells.com/reloading/measuring-tools/bullet-comparators/sinclair-hex-style-bullet-comparators/

Had no idea such a thing existed.  When you say "goes on a 6" micrometer, goes "on it how"?  It literally attaches to a micrometer somehow?  
Or do you just hold it on top of the bullet and measure both it and the bullet / cartridge togather?

This is what you want. Base of the tool cinches down on your micrometer. Insert the appropriate insert into the base for your bullet size. The ogive rests on the insert to help with your measurement.

https://www.brownells.com/reloading/measuring-tools/bullet-comparators/lock-n-load-bullet-comparator-basic-set/

Seating bullets down to 4 digits is nearly impossible to hold.  

Two/three digits is more realistic.

Soft points will be tough to hold exact oal.

Measure a new box of factory loads and you will see variance.

using the term Micrometer when they should be saying Digital Caliper.  I have the Hornady comparator set and it works well, don't worry about the tips they are not what you are measuring for distance to the lands of the chamber.

This is the easiest way. Measuring on the bullet ogive will give you more consistent results.

Ignore the oal", it varies with hollow point and soft point bullets quite a bit. Simply seat a bullet that appears to have a perfectly pointed tip to the overall length you want, then leave the die set there for the entire production run. Every bullet will have the same jump to the rifling, even though their OAL's vary when you measure them.

I shoot a lot of 68 through 77 grain match grade hollow points, very few will have perfect noses. I select one that does and set my die to seat them at 2.250" OAL. This allows .010" clearance inside the magazines to accommodate the bullets with small burrs on the tips. If I don't leave room for this variation, I can get ammo that drags along the inside the magazine's body.

Yep, that thing.   Yes, you have to hold it and your cartridge.  I would sooner buy the comparator tool that goes on the 6” mic/caliper and secures with a thumb screw.  I have a tool that measures headspace on cartridges and only need the inserts that measure bullets iirc.  

They didn’t exist when I bought the hex nut in the ‘80’s.

as far as 6” mic versus 6” caliper, I admit to using both terms.  My Mitutoyo box says 6” caliper on it.  I get corrected that it is a micrometer by some.  To me a caliper has inner and outer jaw surfaces for measuring and a mic has just two and is generally a c shaped more accurate tool.   But I often say mic as in my mind dial......is a dial mic.  It’s actually a dial caliper.

I am sloppy on my use of the terms.

Guilty!  Albeit dial caliper.  My micrometers are all on the barrel vernier, Brown and Sharpe, and the tubing ball mic is a Mitutoyo.   Digital wouldn’t hurt my feelings as I get older.

OK... Just ordered the Hornady Comparator.  So just to verify.  I use this Hornady Comparator to measure a new factory cartridge and hopefully they will give a more more consistent measurement from one factory cartridge to the next.   I then seat a bullet deliberately shallow and starting adjusting the seating die to seat the bullet deeper and deeper until the Hornady Gadget measurement matches the measurement of the factory cartridge.

I then check and make sure that the OAL falls within spec (it should) and I'm good I think.

There's a lot more to this than buying a press and a die.

Last week it was a pin gage set, bushing die, and Lyman M die, just to get my neck tension the way I wanted...

The tip of SP's can vary, seating die should cup the ogive to eliminate the tip from the equation.

Most 55's data calls for 2.20"ish OAL, or even less, so you have plenty to work with.

Pretty much but depending on the brand of bullet and how they handle the production stream you may still see variance measuring on the ogive but less so.  Some places especially for a common bullet like a 55 fmj may have multiples of machines making the same bullet, only each machine has a different die and perhaps a skosh of die set up variance that stamps them out differently so the ogives may not mirror each other.

Don’t stress over it, just accept it.

For now, copy what the loading manual says, measure with your new tooling and see what the consistency is.  Copying factory ammo dimensions is a good place to start.  Only thing I would say is don’t push the case shoulders as far back as factory rounds.  That is excessive.  0.004” headspace is a good target between the chamber shoulder and the case shoulder.

You got it.

For blasting ammo from a semi auto, I just seat the bullet to comfortably fit into a magazine. I don't fuss over neck tension. You will see some variability in oal but once your seating die is averaging a good measurement, leave it alone and start loading.

Sounds like with your bullet and brass choice you may be loading defense rounds. Some choose to put more effort into the various steps to add consistency.... You can choose to do that if you think it's valuable. But it definitely adds time and analysis paralysis.

YES!  

I used to be that guy.  I have toned it down for my training practice load.

I still use the best 55 fmj I can get, Hornady.  
LC cases, trimmed, primer pockets swaged
Ball powder, one of the few places I like ball powder in rifles, because it throws accurately and consistently in a progressive
I seat to the cannalure and give a slight taper crimp.

It’s under 2 moa easily, it’ll hold the x and 10 ring on a SR or SR1 better than I can.   Off brand projectiles wouldn’t do as well. Or even brand names like Winchester FMJs which are floor sweeping garbage, (spit.)

If you want to mimic the factory loaded OAL, you don't have to measure ANYTHING!
Back off the seating stem on your seating die 3- 4 full turns.
Take a piece of factory loaded ammo, put it in the press, raise ram fully so cartridge is fully into the seating die.
Dial down the seating stem until it makes contact with the bullet. Then turn it down about a hair more so it's tight on the bullet.
Lower ram and remove factory cartridge.

You are now done.

You are welcome.

-ZA

Yep... just set the die with a factory round to match the factory oal and you're done.

What you do NOT want to do is vary seating depth to make sure each bullet tip is at exactly the same OAL.  Each bullet tip is different.
The comparator will be a good tool to have if you need to do actual length measurement that means something, but is not necessary to accomplish your goal here.

Most bullets I use are hollow points, so the bullet OAL can vary quite a bit due to variations at the tip (the meplat).  

I always allow about 0.010" extra bullet insertion depth to ensure even the longest bullet does not drag inside the magazine.  This is in addition to a couple of other features that I use to develop an overall set back below the SAAMI max OAL.  

You have to check a few from each lot of bullets, too, as there are lot-to-lot variations in base-to-ogive and base-to meplat.  Within a lot, the variability is lower.

I typically end up with a 2.235" nominal OAL.

Like they said if you have a factory round use that.

If not the tool you ordered is the way to go.

Once I bought mine...I was pissed I didn't buy it earlier.

And I bought it because these guys told me I needed it...lol

Crimping is a not an answer for POOR reloading habits.  I have shot thousands of rounds through all kinds of semi auto rifles and did not crimp!
You want to see a violent gun...check out a HK 91.  The reason is that with brass that is not overworked and normal reloading procedures
you will not need it.  
I do crimp some of my rifle ammo with a light taper crimp.

Crimping only works it if you have uniform case length brass and you set the die up correctly.
To me crimping is a separate operation!

OP, you need to verify before seating all bullets that the bullet you chose to set OAL with is one of the longest in the box as far as ogive to tip length otherwise if you are close to max OAL that the magazine can hold you maybe too long if you figure OAL with a short bullet tip to ogive length and then load a longer bullet to that same ogive depth..It won't matter if you are well short on OAL for what your magazines will hold...In other words if you lined bullets up side by side with the ogive being precisely lined up, you want to set your OAL length based on the tip that sticks out further than the rest...