primer cratering 7mm ultra mag

[den]

new rifle using hornadys data loaded 3 @ start up to max 2 grain increments same result on primers all the way through no sticky bolt & no extractor marks but almost every primer had cratering that you could hang your finger nail on a gunsmith told me it was probably the primer were soft
or somrthing with firiing pin these are cci 250 mag primers i got at a sale they are
probably from 70s 80s as they are in smaller boxes per hundred I will get some different ones & try them but I love how it is shooting now W/ a maxes @ 3263 W/ 90.3 rl 25 will have to go back to start & work up with the different primers any sudgestions on a good primer for 7mm ultra mag & other advice appreciated

 

[Fiftydriver]

Take your bolt out of your rifle, let the cocking piece on the back of the bolt turn to lower the firing pin and then with your calipers measure the diameter of the firing pin sticking out the hole in the bolt face.

I am about 95% sure your problem is simply that the firing pin is oversized. Firing pins for modern, high pressure magnums should in reality be no larger then 0.062" in diameter. Why there are no rifle makers that seem to understand this is beyond me as they are still using firing pins that would be more at home on a black powder or lever action from 100 years ago where chamber pressures are very low in comparision.

What happens, most factory firing pins will run from 75 to even 80 thou in diameter, this is simply way to large in diameter. When the pressure builds after the powder in the case ignites, it pushes back on the bolt face and also the primer cup. The primer cup is relatively thin and weak. It can not support the pressure over a large firing pin diameter area. Because of this, this portion of the primer cup is forced back by the chamber pressure which pushed the firing pin nose back into the bolt face.

The primer cup material follows the firing pin into the firing pin hole, as such, you get primer cratering.

so what do you do about it, well, unless you plug the firing pin hole, refinish the bolt face surface and then redrill the firing pin hole to roughly 64 thou diameter and fit a 62 thou diameter firing pin, thats about the only cure to prevent this.

Thats the bad news, the good news, as long as the cratered cup on the primer has rounded edges and not sharp you are not in any trouble. If this crater starts getting sharp on the edges it means the primer pocket material is actually shearing under the pressure and primer piercing is close at hand.

If the edges are rounded, your well under the level where you will see piercing.

Simply put, its really nothing to worry about, just a problem with the rifle makers making the firing pin to large but no real problem will result from it as long as you do not overload your ammo.

Winchesters are famous for this. Savages can be nearly as bad. Remingtons are generally not as bad but you see it often still.

I would be nearly positive this is your problem so meaure your firing pin diameter and that will tell you alot. If the firing pin is not oversized but the firing pin hole is, you still have the same problem.

Kirby Allen(50)

 

[den]

thanks kirby , I measured it right at .75 they seem to be rounded .
I wonder how long before they are not ,I will check with a gunsmith as I
am thinking of having it trued ,bedded ,&trigger worked it is a remington
bdl and I dont want to worry about wheather W /wood stock.
also had this in one load for 250 sav mdl 70 cstm @ max load I think 40 gr h 414 I wasnt expecting it because i was told the 250 data was for mdl 99 @ lower pressures I will have a look @ its pin to

 

[Fiftydriver]

If the crater edges are round and there are no other signs of high pressure, increased bolt lift force, Sticky extraction, ejector mark on case head, flattened primer edges, etc.... I would say this is your problem. 75 thou is just to large but its not dangerously large by any means.

Best thing to do is shoot over a chrono to get an idea what velocity your getting.

As mentioned, its certainly possible to correct this but in your case, it may be more surgery then you really want to pay for. Your call however.

Alot of guys will say these cratered primers are from high pressure. If there are no other signs of high pressure, that is generally not the case at all and its an oversized firing pin hole issue more then anything else.

At times, a heavy duty striker spring can reduce this to some degree but it will not solve the problem, just a band aid and at times, it will not make much difference at all except to reduce lock time which is never a bad thing.

Kirby Allen(50)

 

[davewilson]

Kirby, just to clarify.not sure if everyone understands your post. it's the gap between the firing pin and hole in the bolt that counts. not necessarily the size. too much clearance is what lets the metal flow. and yes, a weak firing pin spring is a problem with some guns, and they don't have to be that old either. i'd put a 28 lb spring in it no matter what. just in case it gets brought up. i wouldn't use a lighter pin either.

 

[speedbump]

Kirby is right on the money about the size of the firing pin hole in a boltface. One of the 1st pressure signs to look for with the 7mmRUM is noticably sharper recoil, a shiny case head, and increased bolt lift. Next is ejector stamping, very stiff bolt lift, and more pronounced primer flattening / cratering. That should be MORE than enough clues to stop, and back the heck off.

When working up loads, try using moly grease or the old G.I. grease on the locking lugs. It really helps prevent galling and eases bolt lift.

 

[Fiftydriver]

To clearify my post a bit. I have found that the fit of the firing pin to the firing pin hole does not have much effect on the level of cratering. I have tested a small diameter pin in a large diameter hole and a tightly fit large diameter pin to a larger diameter hole. In both cases, there was excessive cratering.

That said, I have tested with small diameter firing pin holes and just for testing sake, I turned down the small firing pin even smaller to 50 thou in diameter.

There was hardly any cratering with either pin diameter with the small diameter firing pin hole.

I asked for the firing pin to be measured because that is much easier to do then measuring the hole diameter for most shooters.

If a pin of 75 thou will fit in the hole, we know the firing pin hole diameter is pretty large.

Hope that clears things up some on my thoughts on the subject.

Kirby Allen(50)

 

[Fiftydriver]

Just to add a bit to Speedbumps post, ALWAYS use a quality moly grease on the bolt lugs. Should be applied with every regular cleaning, every 35 to 50 rounds or so. If shooting in dirty or dusty conditions, every 20 rounds to clean the lugs and relube is a good idea.

Kirby Allen(50)

 

[davewilson]

i always thought it was the clearance between the 2 that caused cratering. sorry for the foot in mouth disease.

 

[Fiftydriver]

The comparitiively weak force holding the firing pin forward by the spring is no match for the pressure generated by the chamber pressure of the round.

As such, its simply a matter of surface area over the unsupported firing pin hole that will result in cratering, that is unless there is a legitimate over pressure situation but that will be seen along with other high pressure signs as well.

Its to bad that the industry does not set a standard pin diameter of 0.062" and a firing pin hole diameter to be no larger then 0.065" in diameter. Then there would be none of these cratering problems unless you really were running the pressures to high.

Kirby Allen(50)

 

[davewilson]

i understand what you're saying about the size of the hole and always felt that way until this summer when my gun started to not go off once in a while. also during this time the primers were not only heavily cratered, but some were completely pierced. this seemed strange since my loads were very conservative. all of this went away with a firing pin spring change.i don't know what the weak spring measured, but it was close to 20. the new one measured around 28. as you say either doesn't compare to the pressure on the other side but seemed to make quite a difference.

 

[Fiftydriver]

I had a customer bring me a rifle that he had just sent off to get hot caustic blued. The rifle shot great before sending it off and shot great after but it was now piercing every primer when shot with the same load that had worked perfectly before.

What was the cause, the smith blued the entire striker assembly as a unit, including spring. Talk about weakening the spring!!!

after 5 minutes and a replaced striker spring the rifle was performance as it has previously.

Yes, a very weak stiker spring can cause pierced primers but it is not common to see and a heavier firing pin spring will generally not solve the cratering problem in a bolt with a large firing pin hole that is cratering.

Kirby Allen(50)

 

[adam]

A little quick math.
.080" hole (.00503 sq inches) x60,000 PSI = 301.6 lbs

.065" hole (.00332 sq inches) x60,000 psi = 199.1 lbs

Depending on the year of manufacture and which company produced them, most primer cups measure between .019" and .027" thick.

Visualize 300 lbs balanced on a firing pin, pushing on a metal cup roughly the thickness of 2 buisness cards. If you have a combination where you have a thin cup (take away one buisness card thickness) and an .080" hole, it is easy to see where you could get cratering without the other signs of overpressure.

 

[adam]

Here is a link to an article discussing cup thickness.Primers And Pressure