Fireforming help needed

300 Driver

Well-Known Member
Jul 16, 2015
I’ve read through the old fire forming threads and never encountered anyone admitting to this problem. I’m getting light primer strikes and misfire on half of my Fireforming loads. First my process, then my questions. Good news was the Fireforming group was showing tons of potential.

This is my first attempt at wildcatting other than a 338-06 before it was SAAMI. I rechambered a TC Venture originally in 338WM. I know some are wondering why, but this is my learning project and the entire rifle is a consumable in this project.

loading for 33SM.

- 70gr Win 780 with 300gr MKG seconds (used 340 WBY data and a similar burn rate powder, just trying to use this last lb up)
- mix of CCI 250 and Win WLRM primers
- expanded Peterson 300PRC to 338 and jammed bullets .005
- .362 bushing in Hornady match die
- the rounds that fired have sharp shoulders and measure within .001 of spec for OAL, shoulder to base, shoulder diameter, and base diameter. That makes me think the chamber is not the issue.

I am assuming the light strikes are due to the case not fully pressing against the bolt face. But I can’t figure out why I’m successful 50% of the time. Could it be the ogive on the factory seconds is not the same? Or is .005 not enough jam?

i expanded a few pieces of brass to 356 and set a false shoulder. Added primers and tried to fire primer only to test. Only 1 out of 3 CCIs fired and 3 out of 3 WLRM fired, but the primers backed out slightly. They did not pierce or show blow back. I then took a formed piece of brass and tried CCI primers. Fired every time. Is 356 not enough of a false shoulder? When setting it up my bolt was either not able to close or hard initial force in the first 1/4in of throw, but normal after. I could not find a point where the bolt crushed late in the throw.

Thoughts from those that fireform frequently? Thanks in advance.
Light strikes often indicate a primer not seated with the anvil legs touching the case. Pop out a primer (unfired) and look where the anvil is in regard to the cup. If the anvil is not flush with the cup, then it is seated too shallow.

I’ll have to take a look at that. So you’re saying that would be a factory defect?
It sounds like you should take a look at your primer seating like M.M. said, or the case has too much play in the chamber and you either need to set up a sub shoulder or jamb a bullet as Dmagna suggested. The only time I've had issue with ftf's on any sort of fireforming, it was large headspace on my 7stw using Bertram 7stw brass, which is a bit small for my chamber (that batch has a bit of a shallow belt too). A simple sub shoulder cured the issue.
It could be primers not seated deeply enough as MM says. It could also be variation in false should or light neck tension allowing firing pin to drive case or bullet deeper allowing case to move forward. Is bolt closing snugly? If not, increase your false shoulder length until it does.
The primers were all seated to the same depth. So that doesn’t explain why 50% ftf.

Is 356 enough false shoulder diameter on a 338 or should I go up to 375?
.020 jam and redo your false shoulder. If you necksize the full length ith should close easily - test just to make sure its not something else going on. Now expand neck with your .358 cal mandral. Back your bushing die top or die off about 1/8” size neck. Check for resistance chambering. If won’t close size neck another .010 and try. When it closes back off on the die or bushing stop .005 and run it.
Seat your bullets long enough so that when you close the bolt you are seating the bullet final few thousandths. That will ensure that the case head stays on bolt face. A little case lube inside the neck helps things move smoothly as well. Identify the CBTO length of your chamber with that bullet and add about 0.003” to 0.005”. Similarly, moving your false shoulder forward will also ensure the case head stays on the bolt face during ignition
Thank you all. You confirmed my plan to run false shoulder and a greater jam.

Having half form properly and the other half not fire messed with my confidence. I didn’t measure firing pin projection, but I’ll make sure to verify. The rounds that fired, the strike looks completely normal. I prime by feel with a Lee auto bench prime and will try to measure that, but every primer is seated until the stroke hits a hard stop and they are all well below the case head. Primer depth is not a measurement I’ve ever taken before.