So I seated bullets in 5 sized cases, no primer and no powder.   Cases are once shot Speer nickel plated brass.  

I have a pin gage set and when I subtract the neck ID as determined by the pin gage set from .224 I come up with .003

The necks were sized with a Lyman M die.

These are Speer Gold Dot 5.56 SP bullets with no cannelure.  

Using a digital caliper I measured the OAL.  Put the cartridge in a PMAG, inserted the PMAG, dropped the BCG, then ejected the round and re-measured it.

Here's what I got for the 5 cartridges OAL before and after length:

1. 2.2350 -> 2.2395
2. 2.2280 -> 2.2330
3. 2.2325 -> 2.2365
4. 2.2365 -> 2.2390
5. 2.2285 -> 2.2360

In every case the OAL of the ejected cartridges is longer which would seem to indicate the bullets are shifting forward.  Is this something I need to be concerned with?  I would image all of these would have fired OK.  

Just to be sure the bullet was shifting I dropped the BCG again on the first cartridge which now had an OAL of 2.2395 four more times.  Here the OAL measurements after each additional BCG drop:  

2.2455
2.2530
2.2565
2.2640

It's obvious the bullet is sliding forward a little each time I dropped the BCG on it.  Is this normal?  Would a crimp stop forward slide?
 

I haven’t ever measured any that got longer.

Maybe try non-nickel brass? Two thoughts…the lubricity of the nickel allows the bullet to slip forward easier. Or the nickel is affecting neck tension. Like the bullet stretches the brass and it is slightly less elastic because of the nickel?

Wild ass guesses.

I agree, I would try some brass brass first.

With pistol, I load nickel plated stuff but it usually splits much quicker than regular brass.
That tells me its properties are different.

I don't personally run across much nickel plated 223, but when I do, I chuck them.

I'm thinking shifting forward a little may be normal with bullets that have no cannelure, crimp, and tar sealant.  Basically the case is coming to a sudden stop when the BCG slams it into the chamber but the bullet wants to keep moving forward.  Same principal as a bullet puller hammer.

Could probably fix it by dipping base of bullet in a little Elmers glue.  

Measure the outside neck diameter, before & after bullet seating.  This is the neck tension.  A .002" change should produce 35 pounds needed  to move the bullet.  The military call it bullet pull.

See "Neck Tension"   https://www.sierrabullets.com/reload-basics/reloading-for-semi-autos-and-service-rifles/

I've never really paid attention to this, but just for kicks, I did the same test with a 55gr FMJ and a random piece of brass from my ready to load bin.

After initial seating:  2.2445.  Then chambered and ejected four times with the following after each successive chambering:

2.2465
2.2475
2.249
2.251

So looks like it is getting longer by ~.002 each chambering.

I think you have three simple, no-cost options.

1.  Crimp - even without a cannelure, you can add a small crimp to help retain the bullet.

2.  Increase Neck Tension - this is done by honing the expander ball to a smaller diameter.

3.  Both - you shouldn't need to do both but there's nothing wrong with doing them together.

If that doesn't stop the slippage, something else is wrong.

I don't have a Lyman M die, but I thought they were just a mandrel die with a flare at the top to aid in bullet seating.

Do they also size cases?

Yeah, that's what I thought. How did the OP squeeze down the once fired cases, to be able to use the M die to "expand the neck"?

I must be missing somthing.

I bet the bullet slid a lot further when you pressed the trigger

As has been mentioned, try brass.  As also has been mentioned, measure the outside of the case neck before and after seating the bullet.   Put the gauge away.

Unless you routinely rechamber ammo time after time without firing, I really consider it a non issue at those numbers.

Can't speak for the OP, but people who want to set an exact neck tension remove the normal expander, and/or use a bushing die for an exact size down (undersized) , then use a mandrel for an exact expansion up.

I still have the stock expander ball in my size dies, but using just them, sometimes I was shaving copper seating bullets, so I got an M die to open up the case mouth ever so slightly to keep that from happening.

First of all I hope that the the OP has FL sized the brass with a FL sizer.  And neck sizing with the M die only?
People are making reloading way harder than needs to be.  

Where did the brass come from, did the OP shoot the brass from factory ammo.  Or the OP inherit the brass from someone else? Bringing into question is it truly once fired?
Nickel plated brass is more brittle that regular brass.  And with that being said I have some 45 ACP nickel plated brass that is wear off because it has been fired a boat load of time.  

Quit using the M Die! Not like you are loading precision long range ammo.  I have loads for several uppers that are near 0.5 MOA with heavy Berger bullets....guess what I have no problems with neck tension at all.

Reloading is about minimizing variables.  And, people just try to make things SO HARD!
A regular FL sizer will work just fine as it has for years and years and years.
Size some brass with your FL die.  Trim to length ie 1.750" Chamfer and deburr the case mouth.
Then try seat bullet and see where you are at.
Are bullets factory new? or are the pulled bullets from some other source?  Bringing into question that they could have been altered during the pull down process?

Forster neck bushing die without the expander ball in it.  But needed to use the Lyman M die because it opens the very top of the neck so that you can seat flat bottomed gold dot bullets in it.  It also opens up the rest of the neck to .003 under .224  

Brass was first resized with a full length Forster bushing die with the expander ball removed.  Then the neck was then expanded with a Lyman M Die.

Brass was purchased from a company online.  Once shot Speer 5.56 Gold Dot LE brass from a police range.

I'm reloading with gold dots.  They are flat bottomed.  You can't get the bullet started with out first using the Lyman M die to slightly flare the top of the neck.  Otherwise you can't sit the bullet on top of the neck without trying to hold and guide it as you push it up into the die.

Bullets are factory new Speer Gold Dot 55gr.   They do not have a cannulure.    

No need to trim.  All this brass is mid way between being to short and too long.  Chamfering the case mouth is problematic because it cuts the nickel plating off which then flakes, thus the M die which flares the top to avoid needing to chamfer.  

Why the taper over the fcd?

Lyman's description of the M die:



I don't even get to the expander step, I just use the bottom of the mandrel to touch the case mouth so I don't have to inside chamfer every round of blasting ammo after it's trimmed... it works well.

Ahhh, ok. I just could not get how you sized it down to begin with.

The OP didn't state how he sized them down, just that he used an M die to size them.

All uncrimped bullets will grow longer when the BCG slams them home. Usually not as much as you are experiencing.

Spin polish your expander ball so that is .001" smaller in diameter to increase neck tension, or use a light crimp.

I rarely see more than .003" OAL growth from the feeding process.