Fire forming and raised primers = low pressure?

[thefitter]

Fire forming Lapua 22-25 into 6.5CM and have raised primers. At first I thought I was over pressure but after searching I'm wondering if its low pressure. "...not enough powder to expand the case and seal it in the chamber...."

I had calculated the load low to be on the safe side. Looks like too low?

Thanks

 

[GrocMax]

Raised but not flattened on a low powder charge fireforming load sounds like a way sloppy long headspace deal where the cartridge is not forced flush on the bolt face with a false or true shoulder.

 

[thefitter]

Maybe I'm pushing the shoulders back too much also.

Thanks

 

[bigedp51]

Your cases are headspacing on the primer, my 30-30 Winchester does this with every cartridge, the chamber pressure is not great enough to force the rear of the case against the bolt face. P.O. Ackley did an experiment where he removed the bolt locking lugs from a 30-30 and fired the rifle, the case gripped the chamber walls but did not move to the rear. Meaning the chamber pressures was high enough to for the case body to grip the chamber walls, but not high enough to cause the brass to stretch to meet the bolt face.

Below is a cartridge being fired at higher chamber pressures, the firing pin drives the case forward until the shoulder of the case contacts the shoulder of the chamber. The cartridge goes "BANG" and the primer is forced to the rear until it contacts the bolt face. As the chamber pressure builds it forces the case to stretch to meet the bolt face. If the head clearance is great enough the brass is stretched beyond its elastic limits and you will have thinning in the base web area.

This is where you get various opinions on fire forming and methods to prevent the case from gripping the chamber walls, or other methods like false shoulders or seating the bullet long to hold the case against the bolt face.

 

[thefitter]

I'd really like to hear more about the various opinions on fire forming.

This is my first go at fire forming brass, and I could use all the help I can get.

I can not seat the bullet long because this is a auto loader.

Thanks

 

[bigedp51]

The amount of head clearance or "air space" between the bolt face and the rear of the case corresponds with shoulder bump or how far you are pushing the shoulder back during resizing.

How far is the primer protruding after the first firing before resizing ?

If you look below at the blue red and green dotted lines, the blue line shows the shoulder squeezed forward during resizing. By adjusting your die upward and away from the shell holder you can make the case longer in cartridge headspace, meaning squeeze the shoulder forward and not have the resizing die touch the case shoulder. If your fired cases do not have excessive primer protrusion you could try and place a .010 feeler gauge between the die and shell holder and see if the shoulder is squeezed forward and work downward and see what happens.

You might have good luck by just increasing your load, if you have ever started at the suggested starting load and worked up you reach a point at approximately mid range where the primer becomes flush with the base of the case.

If you do not have one already I would recommend the Hornady cartridge case headspace gauge, for measuring your fired cases and your resized case for proper shoulder bump.

Also if you have an AR type rifle you can still have pressure in the barrel when the bolt starts moving to the rear and this can cause the primer to back out of the primer pocket and protrude. And a very good reason to have the Hornady gauge.

 

[BoydAllen]

How does your bolt close feel on an expanded but empty case? You have a 28 degree shoulder case in a 30 degree chamber, which means that you have line contact at the neck shoulder junction to establish cartridge fit in the chamber fore and aft, sometime called "headspace" in the vernacular. With that situation, it will help if you load your bullets with a lot of neck tension and so that they heavily engage the rifling. Even so, the case may be driven forward in the chamber, and with insufficient maximum pressure, the sides of the case body my hold the case forward, but the pressure may be insufficient to cause the case to stretch, near its head, so that the head is brought into contact with the bolt face, coming back to bring the primer flush, after it has been pushed to the bolt face. I have seen this on moderate '06 loads particularly with a rough chamber. I believe that you will be able to bring the primers to flush by gradually increasing your load untill this is accomplished. Also, if your bolt has no resistance when closing on an unloaded case you may want to try adjusting your FL die out a bit and see if you can produce some feel on closing. The problem with trying that on unfired brass is that it is probably smaller than your FL die all along its body. If the load is raised till the primers are flush, you may find that your bolt closes easily on fired cases, and if that is the case, do not set your die to bump shoulder the first time around, but rather to produce the same case "headspace" as the fired brass. All measurements of fired brass should be made with the primers removed or reseated well below flush with your hand priming tool. In closing, I cannot fail to complement Biged51's fine use of graphics. Good work!

 

[Webster]

Can anything be learned about headspace by taking a primed case, no powder, no bullet and measuring the differece in primer position after firing? Would the primer push the case forward seating it against the chamber shoulder and the primer protrude backwards indicating the space between the case head and the bolt face?

 

[fguffey]

Can anything be learned about headspace by taking a primed case, no powder, no bullet and measuring the difference in primer position after firing? Would the primer push the case forward seating it against the chamber shoulder and the primer protrude backwards indicating the space between the case head and the bolt face?

Webster, you are onto something, problem 'fire forming', there is case forming and there is fire forming. case formers would have all the dimensions worked out before pulling the trigger. The Ackley improved case has a longer neck then the parent case meaning the parent case has a shorter neck. When the parent case is chambered the neck/shoulder is sized when the bolt closes. The rest of the shoulder is formed from the case body when fired. Meaning part of the shoulder becomes part of the case body and part of the shoulder becomes part of the neck.

A reloader that understood case forming would use a different case than the 30/06 when forming 30/06 Ackely Improved cases 'because ' the 30/06 case will shorten as much as .035", true the case length increases from the case body/shoulder juncture to the case head but it shortens from the from mouth of the case to the case head.

F. Guffey

 

[bigedp51]

Can anything be learned about headspace by taking a primed case, no powder, no bullet and measuring the differece in primer position after firing? Would the primer push the case forward seating it against the chamber shoulder and the primer protrude backwards indicating the space between the case head and the bolt face?

Webster

If you start at the suggested starting load and work up you will see the primer protruding from the rear of the case until the approximate the midway point of your workup load. And this is governed the the cartridge you are firing, as I stated before the primers protrude on ALL my 30-30 cases at the NORMAL low chamber pressure of 38,000 cup or 42,000 psi.

Here is an old trick, take a resized case and measure the overall length of the case and write it down.

Now insert a fired spent primer in the primer pocket with just finger pressure and slowly chamber and close the bolt on this test case.

Now remove the case from the rifle and measure the overall length of the case again and write it down.

Now subtract the first case measurement from the second and this is the amount the primer is protruding from the rear of the case.

On a rimmed case if you add the rim thickness and primer protrusion you will have your exact rifle headspace.

Your shoulder bump is also your head clearance or the "air space" between the rear of the case and the bolt face.

And the worst type of excess headspace is created by unscrupulous, uncaring bartenders in dimly lit bars.

 

[fguffey]

I had calculated the load low to be on the safe side. Looks like too low?

Thefitter, if you are fire forming why would you choose to load on the low side? I form first then fire.

F. Guffey