"Can I load .223 on a progressive press?" 

I have seen that question presented numerous times, rather on on-line forums, websites or asked in person or through email.  The answer is, of course, yes.  However, there are three decent methods of doing so but the best solution I have found is the use of an RCBS X-Die.

Most people that ask this question are usually either new to reloading or are just trying to get into the hobby and am not sure what kind of press to buy. Also, some that learned to reload on a progressive press set up for pistol ask this as they too may be unfamiliar with the differences between reloading for rifle  and pistol.  Though both are considered metallic cartridge reloading, they can be quite different.  This is the main reason I advocate the use of a single-stage press for learning to reload.  Before you can understand how an X-Die works, we need to have a fundamental look at how regular dies are used in the reloading process.

The difference in reloading vary between pistol and rifle for two main reasons: 1. Rifle cases stretch when fired and resized while pistol cases are less affected by firing. 2. The dies themselves differ requiring rifles to be lubricated during the reloading process while it is not necessary with pistol cases and modern dies.

Most pistol cases are straight walled and headspace either on the rim of the cartridge (most, but not all revolvers) or the case mouth (most but not all pistols). They are usually of lower pressure than rifle and as such, the case does not change very much when fired. Rifle cases, on the other hand, are headspaced on the rim. When fired, the brass expands to fill the chamber.

Rifle sizing dies return the case to the original specification. They need to move the brass a lot during this process and require that the cases be lubricated.  Modern pistol cartridge dies have a carbide ring that allows the straight walled cases to enter and exit without any lubrication.

Since pistol cases do not need a lot of preparation, they are ideal for use in a progressive press.  However, since rifle cases stretch when re-sized, they can become over limit sometimes even from one firing and resizing and need to be measured after each resizing. Often they also need to be trimmed before continuing with the reloading process. As such, rifle cases do not lend themselves very well to use in a progressive press.

Now comes the X-Die from RCBS.

The introduction of the x-die has changed all of this where rifle cases and progressive presses are concerned.  The x-die has a mandrel at the top of the stem that retards the flow of brass. The result, when the cases are resized, they do not stretch beyond the maximum limit, eliminating the extra steps of case measuring and trimming with each resizing.  This allows the case to advance from the resizing die to the priming station without the need to remove the case, measure, trim, debur and return it to the progressive line.
A cut-away showing the x-die from the Protips portion of the Shooting USA website.

The way you use the X-Die is to first prepare all of your cases using a standard resizing die. They should all then be measured and trimmed to the same length recommended in the x-die instructions.  After this has been done once, you install the x-die in place of the standard resizing station on your progressive and reload through the rest of the progressive stages.  Now every time you fire these cartridges, run the cases through the progressive using the x-die. As a bonus, since all of your cases are of the same uniform length, crimping the bullet in a cannalure on the progressive is also a breeze.  You still need to inspect your cases before reloading but you have eliminated the need to measure and trim after each subsequent resizing.

I mentioned that there are two other methods of loading a rifle cartridge through a progressive press but without the use of the x-die, you really are reducing your progressive press to that of a big indexing single stage.  You could instead choose to measure and trim after the resizing step. To do this, you must remove the case from the press after it is resized and return it to the press before the next stage and pull of the handle. This is not easily done on most presses but the Hornady L-N-L AP will allow it quite handily.  The last method would be to only have the resizing die installed, run all of your brass through, measure, trim if necessary, then set up the remaining stages and continue. Again, A Hornady L-N-L AP will make this easier and soon perhaps Lee will add their new Breechlock system to their progressive which will offer the same quick setup now offered only by Hornady in a progressive.

I'll stick to using the X-Die and let the progressive run progressively.