I have once-fired LC 5.56 that is not "slipping" into, and not fitting into the Lee shell holder.

Not *every* case doesn't fit. In some lots 90% of the cases will fit, and be resized just fine... Then in another lot, it is 50-50 hit and miss.

Should I be using force to get the case into the shell holder?

It seems to be an issue only with LC brass.

I am using: "Lee Universal Shellholder #4 (17 Remington, 204 Ruger, 223 Remington)"

look at the extractor groove on the casing.

sometimes there's a burr that needs to be filed out.

Couple of possibilities:



1) Could be an undersized shell holder (check it with known good brass, are any of them hard to seat?)



2) Could be out-of-round rims on once-fired brass, in this case you'd need to either file them down a little or toss them.



3) Could be the rims are dented in some way, in which case you'll have to file down the dented or chewed up area.



PPU brass often is a bit sticky in the shell-holder as the rims are slightly larger than most brass.  A little extra pressure to shove them in and resize them has never had any ill effect for me.  They also don't fit well in the loading blocks I made which fit most other brass just fine (I guess the tolerances on the holes I drilled were a wee bit tight).

Sometimes 5.56 brass gets the rim bend back from the extractor working so hard to pull the case out of an overgassed weapons chamber.  This is most common among CARBINE gas systems.

Look closely at a really tight fitting piece of brass.  You can see the extractor mark on the inside of the rim, and there may be some bending of the rim in that area.

I remember coming across a sweet pile of once fired brass in the desert.... only to find that I had to recycle almost all of it because the rims were so bent.

Happens alot on full auto fired rounds.  The extractor leaves a dent some can be filed out some are scrap.

Lee shell holders are the only ones that have ever given me fits. Use a Dremel and polish the interior and entrance way on the shell holder. The part that actually contacts the exterior of the case rim. It will work fine after that.

If you don't have a Dremel use really fine sand paper, 600/800 grit wet/dry. Try not to touch anything other than the groove the case rim sits in.