I bought a Redding neck sizer and when I sizing my fired cases, the case length grows about .005".

I believe it is because of the expander.

Should I remove the decapper/expander and just use a universal decapper to remove primers?

I lube the case necks/mouth really well but they cases still grow some length.

Are you measuring each case before you neck size? Is it a "neck size only" die? I always use a decapper first, then adjust neck tension through other means.

You will have too much neck tension if you don't expand the interior in some manner. You could buy K&M's, Sinclair Internationals or another manufacturer's expander mandrels. The apply the expanding pressure in a downward motion as opposed to pulling it up through the case neck. They also apply the pressure to the full length of the neck, as opposed to drawing a small ball through it.

I use the "NT" or neck turn mandrels. They are .002" smaller than bullet diameter, this usually gives .003" neck tension when the brass springs back. More than enough to hold a bullet without a crimp.

Lubing the interior of the case necks should be helping, the major problem is most dies (except bushing style) oversize the crap out of the necks. You can measure the diameter of the expander buuton, .307" to .306" is usually a good measurement. If you lube the interior of the case necks, tumble them clean again before priming.

I'm using a Redding neck sizer.

did you brush and lube the necks? try buying reddings carbide expander button.

Yes. I brushed necks and lubed. I use Imperial Sizing wax.

It doesn't do it everytime. Out of 20 cases, maybe 3 or 4 stretch.

I just watched a video on polishing the EB.

I'll will be doing that today.

The guy in the video used 240, 1200, and fine steel wool to finish.

I just used 1500 to polish. No metal was removed and if there was it was very, very little.

It did work though. Much easier when bringing case back over the EB.

You can measure the case neck on a finished round with your brand of brass and redding will hone the neck of the die for you. Making the expander ball smaller will only work to a
point. There just is to much variance in brass thickness to have one die that is perfect for every brass manufacturer. Ideally the ball should be .002-.003 smaller than the bullet + 2 x the
brass thickness and that should be the ideal bore diameter of the die neck. But if in the future you buy some thinner brass the die won't give you any bullet tension. And that is why they
now make bushing dies.

I am not making the EB smaller. That is not my intention. That is why I used the 1500.

I am polishing it to make it smoother. I have maybe removed .0005" of material.

There is still resistance as case first goes over the EB. But when bringing the case back down, it is much smoother. Less resistance.

Be sure you are not measuring the on a extractor burr on the base of those cases.

Even with neck bushing sizing dies you sometimes need an expander ball in case the neck is bent some, the sizing bushing doesn't straighten the neck out. The expander ball straightens it out as you pull the case back out of the die. I had this trouble with my Hornady Competition FL bushing sizing die that didn't have a sizing button, the necks would still be bent(on a few that were bent) after sizing because their was no expander to straighten the case neck back out.

I finally give up on the bushing die and polished my expander button on my Custom Grade New Dimension dies down from .222 down to .221 to give a little more neck tension and reduce drag on the expander button were it is smooth now and gives me just the right amount of neck tension.

The bushing dies are OK but like mentioned above neck tension will be very different if your brass has different case neck thickness, the only way to get a true ID sizing is with an expander ball or a collet neck sizing die like the Lee that sizes the case neck ID to the size of the mandrel inside for an exact case neck ID every time regardless of case neck thickness.

EWP

PS: I think

Are you checking the headspace on these stretched neck brass? because if it's getting longer because of the expander ball it is pulling the case shoulder back out also not just making the necks longer.

I have no way of checking headspace. I wish I did though.

You need this to tell what is really getting streched, the case neck or the shoulder.

Headspace kit

I still use the expander with my bushing dies. They allow you to pick the proper bushing for your brass. If you have only one batch of brass, all the same, you can customize the neck of the die
and drop the expander completely.
  If you have set your die to just bump the shoulder enough to allow chambering or no shoulder bump at all, just chamber the case and you'll  know if it stretched the neck or shoulder. If it chambers
fine it really doesn't matter. "Donuts" inside the case neck just above the shoulder, hardened brass, thicker necks from brass flow, can all cause what you are experiencing and it is very common.
An inside neck turner and annealing the necks will cure any of these problems. The donuts in particular are hard on accuracy.

I polished the expander balls to almost a mirror finnish. That cut down on a lot of my neck stretching also. Just don't go crazy with it and take off too much of the ball material. If you're putting lube inside the case neck, there's a chance you could get lube in the powder which could effect performance.

I also decap with the Lee Universal Decapper before putting cases into the tumbler. It does a good job of cleaning out the primer pockets.