Fireforming

Can someone explain this to me?

The brass does more than hold the cartridge together, it's also part of the firing mechanism/process. Basically brass expands to fill your chamber when you pull the trigger and acts as a gas seal. That's why you will sometimes see soot on fired brass from underpowered loads, they don't generate the pressure to expand the brass and form the seal. Over pressure is a whole different issue. It then slightly retracts but it now has the same profile of your chamber. So if your chamber is slightly different than "standard" (maybe the shoulder is slightly short or long or something else), "fire forming" gives you an accuracy advantage assuming you only neck size after having formed the brass (that's the theory). Anyhow, that's how I understand it.

Also used to form brass for wildcat rounds etc that don't have factory brass offerings.

I'd been told that fire forming is used when converting to most of the Ackley Improved calibers. a light projectile with a relatively light powder charge is use in an original (for example) 30/06. Once fired in an Ackley chambered rifle, the brass will form itself to the new dimensions, because the exploding propellant will expand the case to fill the new space.

Fire forming is simply firing a round in your chamber, and the fired brass case is now formed to your chamber.

Accuracy wise, I have seen no difference from neck sizing fire formed to full length sizing. And I have played with both in multiple rifles. What you do get with FL sizing is reliable feeding.

When neck only sizing, it does not size the body of the case or shoulder at all. The neck is sized only to hold tension and hold the bullet in place. Even after neck sizing, you can have rounds that will not fully chamber and not allow the bolt to close. This creates a major problem for hunters or competition shooters who need the ammo to chamber each and every time.

I have guys who bring me fired bras in their rifle, in say a 270 Win, and say that this brass will be more accurate since it's fire formed in my rifle. Well, I'm going to full length size the brass, which sizes it back to spec, and the fire formed part is no longer valid. I don't even own a neck sizing die for a 270 Win. And I do not recommend neck sizing brass for a hunting rifle. You WILL have chambering issues, which I have seen in some customers rifles that bought the neck sizing die for me to use. They were dead set on doing it this way because "they read it on the internet". Once they go shoot and some of the rounds won't chamber, and they don't like it, I explain the process to them. They ask why would you want to do it this way then, and I end up full length sizing everything from then on.

If you are shooting strictly off a bench or prone with a rifle that can tell the difference between a .05" group or a .08" group, then maybe you can neck size. But for hunting, FL is the way to go. (Or correctly doing a partial FL size).

Ive got about 500 rounds of neck sized rounds through my 300 wby and its never had chambering issues of any kind. I fire 3 times and neck size then FL size and start again.

3 firings is pretty common before fl and bumping. What chad is referring to is just nk sizing and doing that at some point you will have chambering issues.

^^This^^

I have measured virgin 7 Rem Mag brass to the shoulder. Fired it, and measured again. It grows .014" from virgin to once fired. The next time it is sized, the shoulders get pushed back .002", and that is the way it is always sized from then on. This is what I do with every rifle I have. They always feed, they always eject, and they shoot very small. Consistency and reliablity is what I like. Brass of various lengths case head to shoulder, can yield a wider ES. Not always, but some of the cartridges I've loaded for shot more consistenly on partial FL sized brass, since they are all the same measurement. Another reason I like second loaded brass the best. It's two thousandths shorter than my chamber, it will yield more velocity than virgin brass, and it hasn't work hardened enough to make me feel like it needs annealing yet.

I'm almost regretting buying 500 pieces of 6.5 Creedmoor, because I am still shooting on virgin brass.

once again the above is why I am on this forum, for all the info that's comes up.

I've fireformed brass for a long time converting brass from .223 to 7mm TCU for a 14" Contender handgun.
Also fireformed brass from 30-30 to 7-30 Waters.

Been using 7.62x39 brass for my Grendel... Might have to try this guy's technique!!?..
https://www.youtube.com/watch?v=mQgIFscVOE8