i just bought a 257 ai and im new to fire forming im getting case seperation and miss fires what is the best way to fire form cases

I always take the standard cartridges (in this case, the 257 Roberts) and either use factory ammo or will load new cases to the minimum recommendations for the selected bullet with the slowest powder I have that is listed for that bullet and fire away. Never had a badly formed case or misfire.

Be sure to use new brass for reforming.

When you say "misfire", do you mean the primer is not being struck sufficiently, or that the primer fires, but the powder doesn't ignite?

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Welcome to ShootersForum, Soulrunner22.

It sounds like your AI was built w/o setting the barrel back and may have a chamber that is just large enough to create a headspace problem, when shooting the parent case. If you're shooting factory ammo, not your own handloads, there isn't much you can do, short of pulling the bullets and seating them longer.

When fire-forming a wildcat that doesn't have a rim to help with headspace issues, the key is to have the bullet seated at, or slightly into, the lands. With this seating depth, a middling charge of a faster powder works best, especially if you are handloading and using a heavy-for-caliber bullet. The idea is to keep the bullet in the case mouth long enough to hold the case head back in the chamber, allowing pressure to build in the case, such that it pushes the shoulder forward. With slow powders and/or light bullets (seated short) the firing pin pushes the case as far forward as it can go in the chamber, where it seals against the walls. This causes much of the stretching to occur at the web, instead of simply reforming the brass around the neck.

If this isn't as clear as I tried to make it, please send me a PM and I'll elaborate some more. Mid-weight charge, fast powder, heavy bullet, at or near lands. No head separations and well-formed cases.

use a 270 neck die,then use a 257 neck die and neck down about 90% of the neck length and load up and you will get slight crush fit

I used the step shoulder until I got a nice crush fit as mentioned then I used a 13 gr load of unique corn meal and a little Cisco to hold everything together
It worked well for me.
But it was time to give it to my son so I set it back and reamed it to the proper headspace

im using new brass and i pulled bullets out to the lands that helped with the miss fires
i dont know what powder i should use?
im using IMR 4350 and they were not light loads they where medium loads from the nosler book useing ct bullets 85g and 48g of powder i was told by a old timer that was to much powder and the wrong kind he has the gun now getting half of a casing stuck in the chamber

oops un stuck from the chamber

For future reference, you can go back and edit your own posts, so you don't have to add a new one if you want to fix a "typo" in a previous post.

I'm not sure what the old-timer was referring to, but I wouldn't use a bullet that is light-for-caliber, when fireforming. Ideally, you would use heavy-for-caliber bullets, like 117 or 120gr, and a somewhat faster powder than 4350. You might use something like 37 grains of IMR4320 under a heavy bullet, seated at, or slightly into, the lands.

The classic prescription for fireforming is a moderate to light load of a medium speed powder, like 4895.

when i get the gun back ill try the lighter loads and a heavy bullet thanks for the information if i have more problems ill get back on here hopfully i dont thanks again

I can not help, I form first then fire, to form first the reloader/wildcatter must be able to determine the length of the chamber first, my dies are adjustable, all of my presses are adjustable, when it comes to 257 Roberts (being as short as the 7mm57) I start with a 30/06 case And I check head space three different ways meaning I know where to form the shoulder before firing, if the Improved chamber is the length of the 30/06, I use the 280 Remington case, the 280 Remington is .051 thousands ahead of the 30/06.

Fire formers moves the shoulder forward by firing, that is some scary stuff (as Seaweed yusta say_) and I say a reloader is not required to know anything, chamber the round, pull the trigger and now? you are a fire former.

Just to take up space because no one understands it, Hatcher pulled the trigger on cases he chambered in rifles with the shoulder moved forward .080 thousands and nothing, he became a fire former, he formed cases for his 30/06 Hatcher Modified +.080 chamber, his case did not stretch between the head of the case and case body. AND THEN, a friend built 5 magnificent 7mm modified/improved chambers using 5 magnificent 03A3 actions, the first attempt at forming cases he had 5 case head separations out of 10, and the difference between the length of his chamber and Ackleys chamber was less .080 thousands but not as long as my chamber, my chamber was/is .047 thousands longer than Hatchers. Me? being a case former before firing does not know why these things keep happening and the answers are always the same, Does the rifle being used have any influence on the outcome, I do not believe it could be laveled 'fatal flaws. I do know there are difference in action designs that fire formers do not understand, those that form first then fire know why it is necessary, then there is seating the bullet against the lands and false shoulder, seating agaainst the lands is cute, the false shoulder is necessary if the reloader knows where to form the shoulder, if not it becomes busy work.

And remember, the shoulder of the case does not move when firing or forming. And I am sure their is a twilight zone where bump ends and forming begins.

F. Guffey