Gunsmithing FN SPR firing pin bushing.

[ken226]

I've had an ongoing problem with primers blanking. Edge of the primers were still very round, no flattening but the primer cup brass was extruding into the firing pin hole, pushing the firing pin back in.

The stock firing pin in the SPR is .079 and the hole is .084. The extrusions were .084 in diameter, but not flowing around the firing pin tip, they were pushing the firing pin back in. So, the issue wasn't the .004 clearance.

The issue was the area of the firing pin hole.

P=F/A where P is pressure, F is force and A is area. So:

If i set P to the saami chamber pressure (62,000) and A to: (.084/2)^2 (pi), then calculate for F, it get a force of 343.58lbs focused the .027 thick layer of brass over the firing pin hole.

Now, if i calculate the area of the brass in shear: (.027 x .084 x pi), i get .00712 inches^2 in shear. So, the area in shear is under 48255 psi, exceeding the shear strength of the primer cup.

The sheer strength for work hardened cartridge brass is around 45000 psi. So, the primer brass extrudes into the firing pin hole.

Now, if i turn the firing pin to .062, and bush the hole to .0635 and recalculate for a .0635 hole, i get:

62,000 = F/(.0635/2)^2(pi) i get a force of 186 lbs focused on the .027 thick brass layer above the hole. This puts the area in shear (.027x.0635xpi) at .00506"^2.

So, with the .0635" hole, the area in shear is under 36758psi.

With a .0635 firing pin hole and a .062" firing pin, the shear strength of the brass is not exceeded and does not extrude into the firing pin hole.

All values above rounded to several decimal places.


So, i turned a bushing from 4130 chromoly, machined the bolt face and with a .002 press fit, installed the bushing.

Before and after: the 7 cases in the right two columns are after the bushing install. The rest are before.

The load was the same for all, 41.6 grains of h4350, hornady brass, remington 9-1/2 LR primers, 140 amax @ 2710 fps from a benchmark 5 land chromoly 23" barrel.



I wonder why FN chose to use a .079 firing pin? It was designed for a .308, which has about the same chamber pressure.

 

[Undrgrndprdcts]

Looks like nice work. Those were pretty bad before. None of my m70s were that bad.

 

[jetmd]

Ken226, nice work!

Would you please give a little more detail on your history with this issue?

Did you purchase the rifle new?

How long has this condition been present?

Approximate round count?

Thank you for your detailed explanation and photos!

Respectfully submitted,

Chet

 

[ken226]

I purchased the receiver new. Chambered and installed a 1:8 Benchmark chromoly barrel and bedded it in a Manners T4.

It has about 500 rounds through it.

Its always cratered primers. It was tolerable with CCI LR primers. It just cratered but never really peirced or blanked until i switched to Remington 9-1/2 primers. Thats when it started blowing through.

Its common on FN rifles as the firing pin hole is .01 bigger than other brands.

The problem is commonly solved by bushing the firing pin.

A google search for creedmoor fn primer peircing or 6.5x47 primer peircing returns a plethora of similar results, with the firing pin bushing the near always recommended cure.

 

[ken4570tc in WY]

I purchased the receiver new. Chambered and installed a 1:8 Benchmark chromoly barrel and bedded it in a Manners T4.

It has about 500 rounds through it.

Its always cratered primers. It was tolerable with CCI LR primers. It just cratered but never really peirced or blanked until i switched to Remington 9-1/2 primers. Thats when it started blowing through.

Its common on FN rifles as the firing pin hole is .01 bigger than other brands.

The problem is commonly solved by bushing the firing pin.

A google search for creedmoor fn primer peircing or 6.5x47 primer peircing returns a plethora of similar results, with the firing pin bushing the near always recommended cure.

Ken226,
Nice work and report!
What's the reason for choosing the 4130 chromoly instead of bronze?
Is this a tease to let us know you're in the market to do bushing work?
If so, pricing and projected turn around time?

 

[ken226]

Nope, last time i started taking on gunsmith work i ended up with more work than i could do with 2 days per week of available time.

I used 4130 because its more similar to the bolts steel in strength and hardness, and will better resist gas erosion from any peirced primers. Bronze would likely erode faster, should any more primer problems occur. Also, i have plenty of 4130 on hand.

I still have the licenses, but have switched over to manufacturing non-firearms related parts. Its way easier as i dont have to deal with the ITAR nonesense.

I just work on my own stuff, and occasionally do free work for friends.

My Engineering Technology, Mechanical Design degree better lends itself to manufacturing and drafting anyway.

But in 8 years when i retire from my current job, ill have time to take on gundmith work full-time, which may make the ITAR fees worthwhile.

 

[ken226]

For anyone interested, i consolidated the various expressions used above into a single formula which you can use to determine the minimum firing pin hole size to prevent primer cratering:

C=chamber pressure
X=firing pin hole size
Y=primer cup material thickness
D=shear pressure on cup material
Z=ultimate shear strength of primer cup material

((X/2) ^2 × C )/ (X × Y) = D, which must be equal to or less than Z

For my purposes, Z is the 45000 psi shear strength of the cartridge brass used to make primer cups. This may vary a little depending on brand.

Y, the primer cup material thickness, can also vary by brand and primer size. I beleive small rifle primers may have a thinner cup.

Using the above formula, i get a maximum hole diameter of .078 to prevent primer cratering with a chamber pressure of 62000psi.

At .079 diameter, with a chamber pressure of 62000psi, a cup thickness of .027" and a primer brass ultimate shear strength of 45000psi, there will be primer cratering.

Forgive for the ugly equation. It looks like fidos butt typed on a keyboard.

 

[ajdemar215]

Older thread but my SPR craters like a MFer too. Been considering getting it bushed myself, thanks for the post.

 

[ken226]

No prob. If it helps, mine didnt crater as bad with CCI primers. Probably have harder cups.

But, the only real fix is to bush it. GreTan has a great reputation for this.

 

[ajdemar215]

I’ll try the CCI primers before I send it out, I’m a pretty decent tig welder and have a lathe in the garage, just not sure that’s a task I’d want to tackle quite yet

 

[ken226]

For the price gretan charges, you should just send it in. It took me about 1 to 1-1/2 hours, with no gaurantees or recourse if i screwed up.

For $80 you get peace of mind and gauranteed results. No risk. These days 80$ is Dinner for 1 at a kinda-ok steakhouse.

It didnt just make a little difference. It made a huge difference. It went from completely blowing holes in the primers to absolutely zero cratering. None at all!

Go .062 for the firing pin.