+1 on using the Lee FCD. If you are shooting an AR-15 or any other semi-auto, a decent crimp is mandatory imho. If that bullet slams into the feed ramps, it could easily cause bullet set-back and inadvertently increase pressures, if that happens it can not only cause accuracy issues, but potentially dangerously high pressures. Of course if you are loading long heavy bullets out to a COAL beyond magazine length and single loading them like many service rifle match shooters do for the long range portion of the match, it shouldn't make a difference, and you may get better accuracy due to more consistent neck tension.
If it's for semi auto use, crimp them. I pick up rounds at the range all the time that have had the bullet pushed back into the case far enough that the powder is falling out. Probably from a jam of some kind. Thank goodness they have the since to toss them instead of trying to fire a round like that.
And, if the case just happens to be too long for the chamber, you WILL get a crimp weather you want to or not.
I usually have enough neck tension in my M1A that I don't need to crimp. I can feel it as I seat the bullets. No issues.
SeeMor, my question is how exactly do you KNOW you have that proper neck tension.
There are two cartridges that I don't crimp - (1) 45/70 because the two guns I have are single shot (2) 5.7X28 because when I shoot them through my Ruger 57 and reload them, I only shoot the reloads through a Thompson Contender
Well, you can size them and then use an expander bushing to size the neck. You can measure the expander and know exactly what the inner dimension of the neck is or you can use calipers and check the brass itself. Either way, you want the number to be .002 to .003 smaller than the nominal size of the projectile being put in it. Easy peasy.
223 Rem. for use in an AR platform I have always applied a crimp, that is if the bullet has a crimp groove/cannelure. I have used for a long time the roll crimp in the RCBS bullet seating die with unsatisfying results, (I seat the bullet with one die and apply the roll crimp with a second seating die.) but as of late, I have been using the Lee, Factory Crimp Die, (FCD) and I am more than satisfied with the results, Cases can be +/- .005” AOL, and it will not bulge the cartridge case shoulder/body junction which always is a problem with the roll crimp type die if a cartridge case is to long.
As Pawpaw40 stated that the crimp is for ammo fired in an AR platform does indeed help keep the bullet from being pushed back into the cartridge case neck. Another reason, is that I usually load 223 Rem. with ball type gunpowders such as Winchester 748 / Hodgdon, BLC-2, H-335 / Accurate, 2460 / Ramshot, TAC / etc. Extreme cold temperatures can sometimes cause erratic primer ignition of these types of gunpowders unless a Small Rifle Magnum (SRM) primer is used such as the CCI #41 primer. The caveat is that with a SRM if the load was developed in cold temperatures, it could produce an over pressure in an AR if that load is fired in warmer temperatures. I have learned that if I load 223 Rem. with these ball type gunpowders with a standard SR primer, a crimped bullet will substantially reduce and or eliminate cold weather erratic primer/gunpowder ignition.
These are 223 Rem. I loaded with Hornady 55gr FMJ, in LC, FC, Rem, Win, and Norma once fired and trimmed to 1.755” +/- .002”. The crimp was applied with a Lee FCD.