Here is one for the "Brain Trust" folks.  I have a Ruger AR556 stock rifle.  I have been reloading for it for a few months.  I reloaded some .223/5.56 brass and checked them with my case gauge and the rounds seemed to me like they should feed/fire OK (these were sized/reloaded with regular RCBS dies.  When shooting I had several rounds that didn't chamber completely.  However these rounds fired just fine when I tried them a 2nd time.  I now have a set of Small Base RCBS dies to use and I have quite a few rounds that I have already sized with my regular dies & primed.  I plan to resize the brass with the SB dies.  I could remove the Decapping Rod, Pin Holder, and Decapping pin assembly to protect the primers while resizing.  My concern is that the mouth of the brass will be "squeezed down" and won't be expanded out to size to accept a bullet when seating.
Should I leave the Decapping Rod Assembly out and not worry about being able to seat 55grain FMJ bullets - or - should I remove the Decapping pin from the bottom of the Expander/Rod assembly in the die to expand the mouth of the brass to accept a bullet without problems seating.

I was going to call RCBS but they were already closed for the day.  I can try to contact them next week.

I would appreciate any feedback from you folks with lots more experience than I have.

akbluz

remove only the pin.  you want to make sure the neck is open correctly.

Yes this ^^^^^^^^^

You are fortunate enough to have RCBS dies and the pin only can be removed.

I'm not saying that your Ruger isn't a little tight but can't help to think that you are not getting 100% of what your standard die is capable of.

Keep a close watch on your brass life. If it shortens considerably using the small base dies you probably don't really need it.

Motor

I have a JP Enterprises .223 Wylde/5.56 Case Gauge which I have been using.  It's entirely possible that I didn't get my regular die adjusted correctly.

Small base dies over work the brass.  And, it is essential that you set your standard FL sizer up properly.  If the brass came from the same gun it is going into a FL die should work just fine.
Small base dies are for know extremely tight chambers.  Or for taking brass from another gun to use in a different gun.  Or like in the case of 7.62x51 coming from a sloppy chambered MG to be used in another rifle.
But, usually one a small base die is used you will not have use it to size again for the most part!

I used a Redding bump die when I ran into some similar issues.  It bumps the shoulder/resizes without messing with the neck.

Use an old 22 brush and hit the inside case mouth to remove excess carbon debris... then lube neck to allow expand ball to pass freely.. dry dirty neck will drag ball and pull front of case forward.. no amount of die setting will help with case neck pull when expander ball pull out of dry - dirty case mouth

Like was said, remove just the pin, but there must be a reason they didn't chamber, screw the size die down till it cams over if you have to and check the diameter of the neck with loaded round in place, and headspace.

Remove the pin in the RCBS sizing die then run the cases through a Lyman M-Die to set the neck tension.

Before you resize already sized brass, make sure the brass needs to be resized.  

First understand that I do not own a case gauge and have never used one.  That has not stopped me from loading thousands of rounds of .223 Remington ammunition using a standard (not small base) RCBS die over the last 38 years that has functioned flawlessly through any of two different Ruger Mini-14s and two different ARs.  

I suggest you try the following first:

* Compare (visually and with calipers/micrometer) cases that have fit in your gun with cases that have not.  Are there any differences?
* Color a case that will not chamber with a Sharpie or similar marker, try to chamber it and see where it is hanging up.
* Verify that the dies is properly adjusted to set the shoulder of the case back far enough - an additional 1/16 of a turn has cured this problem in more cases than I can count.
* Verify that the case neck was not partially collapsed (which will make the case body just below the neck widen and prevent chambering) during the bullet seating process - this cause accounts for some instances where some reloads from a batch will chamber just fine while others will not.

Once you have verified all of this, then look to the small base die.  As others have posted, just remove the decapping pin and then reinstall the expander rod and expander button.  Remember to lightly lubricate the inside of the case neck to minimize stretching and size away.

I long ago added a carbide expander assembly that Hornady was selling to eliminate the need to lubricate the case necks.  Hornady no longer makes that assembly but Forster and Lyman both do.  It will be some of the best $30 you spend.  I recommend the Forster unit because it is more rugged than the Lyman and Lyman's customer service has been totally unresponsive to my calls.

I small body size for my  .223 and .308 gas guns. In saying that I use Redding SB dies.

Had a friend buy a Ruger ar-15 , inexpensive model.  Very accurate, but would not manually eject an unfired professionally reloaded round.  This annoyed him, so advised he checked what was causing the tight fit.  The Ruger has a very tight chamber, and the case was being forced into the chamber. He now uses a RCBS sb die. Solved his problem.  RCBS sb dies allow you to bump the shoulder back further than the Redding version, if that matters. Redding body die allows you to resize loaded cases, although some advise against it.

Unless trimming brass, I do not bother to expand the 223 case necks at all, and use a Redding comp seater to ensure the boat tail bullets are seated straight.  Almost never clean the case necks, the ash acts a lubricant.  Perfectly clean brass neck may need a lubricant. Unless done with a loose expanding ball, using one can throw off the concentricity of the case neck to rest of brass.

Buy a set of calipers, and measure virgin vs fired brass from differing rifles.  Shoulder data measurement, and expansion of case diameter. Roll your loaded round across a flat surface, very easy to check concentric loaded bullet.

Lots of different ways to do things, and all rifles are different. Hard chambering is usually caused by fat base, long shoulder length or bullet jamming into rifling (kinda rare in AR)