Gunsmithing small and large primer firing pins

[338LM]

Does a rifle that shoots calibers that use small rifle primers have different firing pin geometry than a rifle that shoots calibers with large rifle primers?
This question is pertaining to making a switch barrel .308/6.5x47 Lapua being the .308 is using a large rifle primer and the 6.5x47 Lapua is using a small rifle primer. Any issues making a switch barrel with these 2 calibers?

 

[WRM]

Re: small and large primer firing pins

No difference, no issues....

Bill

 

[GasLight]

Re: small and large primer firing pins

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: WRM</div><div class="ubbcode-body">No difference, no issues....

Bill</div></div>

Why are guys having to bush their pins for the 6.5x47, is the hole just too big, or do they turn down the firing pin when they do that as well?

 

[Goin'Hot]

Re: small and large primer firing pins

It is a smaller FP as well. I just sent a bolt out for my 338 edge to have it bushed. My primers looked way HOT with minor loads.

 

[WRM]

Re: small and large primer firing pins

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Goin'Hot</div><div class="ubbcode-body">It is a smaller FP as well.</div></div>

Sure - that's why Gre-Tan lists two firing pin sizes, right? Oops, no it's not small and large - it's short and long (action). I repeat, there is no difference.

The reason guys bush their 6.5X47s is because they're trying to push them into 260s, and pressure's cratering their primers

. Bushing the pin only decreases the clearance, but doesn't necessarily/purposely change the pin diameter.

Cheers,

Bill

 

[Goin'Hot]

Re: small and large primer firing pins

Taken straight from the Gre'-Tan site "Bushing the firing pin hole and <span style="font-weight: bold">turning the firing pin </span>sounds small but this one operation to the bolt has so many benefits it's hard to believe."

Now he may be full of sh*t (which I doubt) but thats what it says. I doubt that he turns the firing pin "<span style="font-weight: bold">bigger</span>".

 

[338LM]

Re: small and large primer firing pins

Thanks WRM

 

[WRM]

Re: small and large primer firing pins

My pleasure.

I've never seen any action that has "different" firing pins to use with different size primers. I doubt that Greg takes any more off than necessary to true the diameter, and then he matches the bushing. Smarter men than me designed the geometry of firing pins many years ago, and since a smaller pin is weaker than a larger one, I, personally, wouldn't reduce one "on purpose". And, if Greg also "turns the FP down" for use with large primers, then there is still "no difference"?...

Cheers,

Bill

 

[Clark]

Re: small and large primer firing pins

1) The 1889 Mauser 7.65x53mm case head design, when built with a large Boxer primer and shot in a strong rifle is good for 62kpsi factory ammo and 65kpsi custom hand loads.
Pressure sign: primers fall out
Better pressure sign: primer insertion is too easy
Best pressure sign: extractor groove grows .001" anywhere around the circumference.
Examples: 22-250, 243, 6mm Rem, 250 Savage, 257 Roberts, 25-06, 260 Rem, 6.5x55 [US brass], 270, 7mm-08, 7x57mm, 280, 308, 7.62x51mm, 30-06, 8x57mm, 338F, 358, and 35W.

2) The 1950 designed .222 case head with small rifle primer is good for 75 kpsi with custom handloads.
Pressure sign: primers fall out
Better pressure sign: primer insertion is too easy
Best pressure sign: extractor groove grows .001" anywhere around the circumference.
Examples: 17 Rem, 204 Ruger, 221 Rem Fireball, .222 Win, .223 Win, 5.56x45mm, .222 Rem mag, 6x45mm.

3) The 1889 Mauser 7.65x53mm case head design, when built with a small Boxer primer and shot in a strong rifle with bushed firing pin hole is good for 85kpsi custom hand loads.
Pressure sign: primer pierces
Better pressure sign: extreme cratering of primer
Examples: 22BR, 6mmBR, 6x47mm, 6.5x47mm, 7mmBR, 30BR, Lapua small primer 308

Even after the firing pin has been bushed, the Mauser case head with small Boxer primer will still be limited by primer piercing, albeit limited to a higher pressure when the primer pierces.

 

[scotharr]

Re: small and large primer firing pins

German Salazar is a pretty knowledgable guy. Here is his input to my recent question on this subject. I'm choosing a small (.062") firing pin hole...

From: German Salazar
Sent: Tuesday, March 08, 2011 8:36 PM
To: Scott
Subject: Re: Firing Pin Question

Hi Scott,

Oncein a while you'll run into an action with a regular firing pin that works OK with the small primer, I have a 40X like that. More often, they will crater like crazy and can blank the primer. It is well worth while to get the 0.062" firing pin, not only for the small primer cases, but it actually can improve ignition with large primers also.

German

----------------------------------------------------------------------------------
On Tue, Mar 8, 2011 at 7:10 PM, Scott Harris wrote::
Hi German,
As you may know, just about every feature on the Defiance action can be customized and my question for you pertains to the firing pin tip diameter. Is there any compelling reason to select a small firing pin diameter (.062”)? I don’t use any small rifle primer cartridges presently, but would like to have that option in the future. My research to date indicates a large diameter firing pin works fine with small rifle primers and my three current actions use large diameter pins. I prefer standardization if it’s not important. What do you suggest?

 

[RADcustom]

Re: small and large primer firing pins

It depends on the action, more specifically, the bolt.

Some have problems, others don't. Finish your build, if you find you have a problem, address it then.

 

[DAVETOOLEY]

Re: small and large primer firing pins

One of the main reasons for pursueing smaller diameter firing pin tips was to mangage pressure. This started with the short range BR guys many years ago. Leaving the discussion about which size is more accurate for each size primer on the sidelines for now it's all about physics. Pressure is measured in Pounds Per Square Inch. Primer cups for the most part do not change thickness to maintain sensitivity. So as pressures increase the only way to control the PUCKER FACTOR is to reduce the surface area exposed to the firing pin hole in the bolt face. The smaller the diameter the less surface area, the less primer cup is pushed back into the firing pin hole with the same chamber pressures. The smallest I've heard being tried were under .060". Some think .0625" works bests with small primers. I run .068" in all of my bolts. It works for either large or small primers. The discussion about the 6.5X47 case again relates to the benchrest guys. Good brass with small primers strenghtens the primer pocket area. That's what happened with the 6PPC case, good Lapua brass. Reloaders will take advantage of that and load to higher pressures as long as primer pockets last a reasonable number of firings. The pressure limit many times is dictated by the strength of the primer cups not the toughness of the brass. My experiences with the 6.5X47 is that it is hard to read the standard pressure indicators when working up loads, squaring of the corners of the primer and the pucker from the firing pin hole. I find it easy to use too much powder and lose accuracy.

So smaller is better to a point. That's why many of the custom actions are a better choice when considering a new build.

 

[colchester]

Re: small and large primer firing pins

great replies Tag

 

[buffybuster]

Re: small and large primer firing pins

Running a larger diameter FP (.075") with Small primers results in more primer cratering/puckering than using a small diameter FP (~.062").

If I intended on having a switch barrel rifle using both Large and Small primers, I would run a bushed FP diameter of around .067". You can run a larger diameter FP, but you'll see cratering.

 

[DAVETOOLEY]

Re: small and large primer firing pins

I've got a .373" bolt face five digit serial number Remington I just measured the FP hole and it's .076". If I was going to use that for high pressure loads I would bush the bolt. In all likelyhood it's going to end up a dedicated 300 AAC sub gun so I'm not to worried about it but will probably do it any way when I true up the action.

I've seen several Remingtons 223's that had a factory chamfer on the FP hole, go figure.

Another thing that the average shooter doesn't need to be concerned about but most <span style="text-decoration: underline"> </span> smith's watch is the shape of the firing pin tip. A full radius is not what you want. You want a flat with the corners rounded off. Under high pressures a full radius allows the primer to extrude further down the sides of the firing pin before it gets supported increasing the chance of pierced primer.

 

[Casey Simpson]

Re: small and large primer firing pins

338LM, I've been up and down this road. RAD got it. Try it first.

The matter, in my experience, isn't necessarily determined by small versus larger primer. The matter is determined by the amount of pressure you try to run in the small primer round. I made a switch barrel from 308 to x47 and had the result you fear, but rather than bush the bolt and pin I used CCI #41 primers and solved the problem.

Later I built a large chassis AR x47 but to run high pressure rounds had to bush because DPMS pin holes are too stupid big. Madesha, Tubb and those guys have been doing it for years. Madesha did mine and solved the problem. My bolt was a DPMS. Incidentally, DPMS Lr pin hole is .078". An Armalite big chassis pin hole is .065". Imagine why that would be? It matters!

Do it. Just don't run it too hot and use OCW and RL 17 with 123 Lapua's and that'll get you the least pressure you can get in my experience with 6.5x47.