Nomercy448

Flying at 30,000ft here, it sounds like you need to look at your die set up.

It sounds like your full-length sizing die is set too deep, and your bullet seating die and FCD are not set deep enough. Sounds like you might be bumping the shoulder too far and expanding the neck too much, and not putting enough taper crimp back on the neck.

The die set-up instructions provided with dies tend to give good ballpark case dimensions that will be functional and safe, but not necessarily ideal dimensions to match up to pure SAAMI specs, let alone with your individual firearm's chamber.

Not necessarily. The case expands like a balloon due to the internal pressure, however, that doesn't necessarily mean it will 'extrude' more length into the neck, lengthening the case, especially after just one firing.

Alternatively, if your case shoulder is a bumped back too short for your chamber (i.e. your FL resizing die is bumping too much), when the shoulder fire-forms to the chamber, it will have to "suck" brass out of the neck, shortening your case. That might be what's happening for you.

It's not surprising that you're getting less "crimp jump" for the rounds ran through the FCD - you don't have enough neck tension, nor enough crimp, but the FCD is adding grip that the taper crimped rounds didn't have, so they slip less. I think of the "grip" that the neck has on the bullet like this - neck tension holds onto the bullet at the shoulder/neck junction, then crimp holds the bullet at the case mouth. This could easily go hand in hand with your die being set up too deep - if your FL sizing die is set too deep, the decapping pin and expander will run too deep and open your necks too much, killing your neck tension (base of the bullet isn't held tightly). Then if your seating die is set too shallow, it won't replace enough taper crimp to properly hold the bullet. Equally, if the FCD isn't seated deep enough, it also will not add enough crimp to make up for the lack of tension.

Is it easy to set the bullets into the case before seating? How far does the bullet tail slide freely into the case by hand?

As I mentioned before - I start out setting my neck tension by measuring the diameter of the bullet and the thickness of the case neck, bullet dia + 2 x neck thickness - 0.004 = neck diameter of loaded round. (0.003 to 0.004). That generally holds just fine.

You shouldn't need the FCD to hold 6.8 bullets in place, a taper crimp in your seating die should treat you just fine. The FCD is simply a "feel good" extra measure. You do not have enough crimp or neck tension right now.

The bullets are most likely getting pulled by recoil. You can confirm this by this process: Load a full magazine, shoot all but the last 2 rounds. Drop the magazine (should have ONE round left) and eject the round from the chamber and mic them both. If the one in the mag is longer than when you put it in the mag, then it got pulled by recoil. If the one in the mag is NOT longer, but the one in the chamber is, then it will likely have been pulled by the rifling, so you're seating to jam length, which isn't advisable for AR's, or even really hunting rifles. I HIGHLY doubt that's your problem - more likely that it's the recoil pulling them.

The case mouth SHOULD shrink when you size it. It had to expand to release the bullet, and the chamber has to be larger than the loaded round to let it fit to feed, so yes, that's normal that the FL die shrank the case mouth.

With a careful hand and eye, you can measure your fired brass to determine the length from the base to the 'corner' where the shoulder meets the tapered body, but that's not a perfect system. There are headspace gauges, precision mic's, and other kits out there that will let you measure your fired cases to help you set your dies appropriately. The idea here is that you want to minimize how much you're working your cases, and minimize how much obturation (stretching) your cases have to do to fill the chamber when fired. Too much of this and you'll get premature case splits at the shoulder. You should be setting your FL sizing die just deep enough to bump the shoulder so the rounds will chamber easily and your bolt with close without resistance. If you run your chambers clean, you can usually get away with a 2-3 thousandths bump, but a good rule of thumb for AR's is 3-5thou.

So with NO other tools, you measure the distance with calipers from the base to the corner of the shoulder and the case body. Twist your sizing die out so it won't touch, and run your ram all the way up. Spin the die back down to make contact, and lock it down. When you cam over the handle, it usually gives a very slight bump. Measure the case again. With a 7/8-14 die (standard), 1/16 of 1 turn is a touch over 4 thou, so you can't turn them much otherwise you'll bump it a LONG way.

You can use the depth spindle on your caliper/mic to determine how far to turn the die to gain your desired bump. I've done this in the past, add a washer or a quarter on top of the shell holder to let you press the die up against the press as if it were sizing a case as you measure.

A good investment is an RCBS precision mic, or at least a hornady headspace gauge. This will give you very repeatable results and eliminate the guess work in the measurement method I described above. You'll be able to VERY precisely set your dies to your desired shoulder bump.

So again, I'd start looking at your once fired brass and your die set up on your press and make sure everything is ticked and tied appropriately.