I've never submitted to a forum before, so I may very well get 'shot out the air' by a fellow forum reader. But I'm gonna try it anyway!  First off, I'm gonna set this up as a story. My best friend acquired a  used 550B press from another reloader who obviously got discouraged that he could not reload consistent loads that would function in his AR-15 & AR-10. My buddy had this press just laying around and had NEVER reloaded anything on it. I've heard about how finicky AR's could be and how difficult it is to reload ammnition that would function in them. I convinced myself I could set up the Dillon 550 and mass produce .223 ammo that WOULD function in my AR. Now, take into consideration, all I've ever owned is a RCBS Rock chucker press. Now, most know that it is a very slow  process to produce loaded rounds. Mind you, It was VERY precise--but very slow.

After taking possession of the 550, I went to this very website and downloaded the owner's manual and read it from cover to cover. After completely disassembling the whole press, powder measurer, primer assembly, I cleaned everything with a dremel tool/wire brush. I then reassembled and lubed the press according to the specs. of Dillon Manufacturing. I now know exactly how this press works.So I mounted the press on my bench and started adjusting the dies and reloading a few cartridges. I loaded about 50 or so and checked the powder charges every 10 rounds or so. The powder charge is very consistently accurate, I was throwing consistent 26 grs. of Win 748. I thought to myself, there's nothing to this. I then took the loaded rounds and the AR to the range. The first 10 in the magazine fired easily but I did notice that 2 rounds out of this batch seemed to be a little rough to chamber, Didn't think much about it, they said "bang" when I squeezed the trigger. After placing all 50 rounds through my gun, I noticed , 24 of them to be exact, didn't cycle. I never let anything conquer me so I investigated further into the matter. I said to myself, This is the problem that every fellow reloader is talking about. Well, As any reloader does, I took these loaded rounds back to my bench and got the Caliper out. Yep, as some of you are saying.  The case dimensions were out of whack somewhere. It was up to me to figure it out. The Loaded C.O.L was less than 2.260"  (2.255 avg), NOT THE PROBLEM.  The O.D. of the brass just in front of the case head was less than 0.376" (0.374 exactly), NOT A PROBLEM EITHER.  The O.A.L of the brass was 1.759-1.761. NOW we are getting close to something. I then pulled each of these bullets and trimmed these cases. (THEY were NEW to start with!!) I put the brass back through the press and reloaded them. I then tried to cycle them through my AR. Out of the 24, 4 would allow the bolt to close fully. However, the cartridge was still tight and, just didn't sound right when the action closed.
I backed up, scratched my head. I got the caliper back out and measured these cases again. All dimensions were in line with reoading guidelines of Speer Reloading manual. I had somehow missed the measurement of (base of neck-to the cartridge head). THIS WAS THE PROBLEM!!  For some reason, The Resizing die was not pushing back the neck of the cartridge enough. REMEMBER AR's are a 'bitch" to load for. I turned my attention back to the resizing die. I noticed that My Resizing die was 'just touching' my shell plate, as recommendations suggest. But for some reason, my die was not resizing my cases in the neck area to fit into the chamber of my AR. 

Since I was using a Dillon F.L. Resizing die, I called Dillon Precision.  They suggested to screw the die, that was 'just touching' the shell plate, down a quarter of a turn--maybe 1/2 a turn and this should press the neck a few more thousandths. I did just this, Now my press hit's bottom and I have about 1/2 inch to go and then the  press completely 'cams over.' The shell plate now tilts a little when the handle is cycled but it doesn't unbalance any of the other rounds that are in 'reloading row' on the shell plate. The Dillon Rep told me, that's why there is a spring under that shell plate to allow for this tilting action. Then why do they recommend the die to 'just touch' the shell plate when setting it up? 

After making this adjustment on the resizing die, These 24 rounds would NOW FINALLY allow my bolt to slam shut without a problem.  I now can reload in mass quantities of .223 ammo for my AR-15. I will admit, It has been fun trying to figure this out. 

IN SUMMARY:   Trim those cases to------------- 1.750-1.75                                                                                                   Know that you CAN screw that resizing die down 1/4-1/2 turn further than                                               "just touching" if you need to.
     IF it won't go--Don't FORCE IT!