AI small vs large firing pin..lots of questions, tolerances and specs plus more...

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brianf

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Apr 8, 2010
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#1
i think ive read most threads on the subject so far but i still have a few questions

what i think know so far:

small firing pin was put in place because certain calibers with factory ammo were getting pierced primers

is the piercing caused by a large firing pin (actual size) or does the original large firing pin have more clearance between the pin and the bolt body?
more clearance gives less "foundation" for the primer...

does the new small pin bolt from AI match specs with aftermarket "bushed" pins clearance or are the aftermarket pins tighter or looser fitting in the bolt face?
i believe .003 clearance is targeted but im not 100% sure on that.

im guessing these are the 3 most important specs:
Pin diameter
Firing pin hole diameter
Firing pin protrusion
firing pin tip radius

does anyone have the above actual/measured specs for all 3:
OEM large pin
OEM small pin
aftermarket bushed bolt/pin

for guys who are shooting small pins OEM or bushed:
has anyone shot military/nato ammo (usually a harder primer case), and if so has there been issues with ignition
are there certain primers that do not consistently fire because of the small firing pin (or small pin protrusion)

i have only seen the OEM AXMC 308 bolt in small pin offered

is there a actual issue with small pin "magnum cartridges" (not AI being AI) why the other large bolt faces do not have a small pin..if a small pin is better in general why have large firing pins in general?


i have more questions but i could go on forever...

thanks

brian
 
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#2
One key element you left out is the use of small primers. Nobody had any issues in AI’s until we started shooting .473 case heads with small rifle primers. 6.5x47 was the first, then wide use of the creedmoor, with Lapua brass, and now we have a host of small primer brass in Creedmoor and some other cases.

I’d also disagree that it was with factory ammo. 99% were handloads. Nobody made small primer creedmoor factory ammo(they may still not, not sure), and there was limited use of factory 6.5x47 ammo in general.

Otherwise you’ve got it understood.

If I’m understanding you correctly on your last question, the large frame AX was designed so you’d switch out the bolt body only when switching calibers. You’d always be using the same striker assembly. For magnums, which are all large primer, it doesn’t matter. And for the standard offerings of the time, the small calibers like .308, it didn’t matter either.
 
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#4
There are various theories. Mine is that the bushed pins address any profile and clearance issues, thereby solving the problem. If one simply bushed the large pin hole, and made sure the large pin profile was correct, the problem would likely be solved that way as well.
 
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#5
I'm about to learn the answer to this question.

I was piercing primers in a 6.5 PRC modest load and sent my 300 WM bolt and firing pin assembly to LRI to be bushed and the pin turned down.

I then realized I use the same firing pin assembly in .338 Lapua so sent that bolt in as well.

When I get those two back and with the 308 bolt already with a small firing pin, I will have all three with small firing pins.

My only concern is I may experience ignition problems with the 338 but doubt it.
 
#6
Hawk in WY said:
I'm about to learn the answer to this question.

I was piercing primers in a 6.5 PRC modest load and sent my 300 WM bolt and firing pin assembly to LRI to be bushed and the pin turned down.

I then realized I use the same firing pin assembly in .338 Lapua so sent that bolt in as well.

When I get those two back and with the 308 bolt already with a small firing pin, I will have all three with small firing pins.

My only concern is I may experience ignition problems with the 338 but doubt it.
Click to expand...

That small pin will work fine in the 338.
 
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#7
Hawk in WY said:
I'm about to learn the answer to this question.

I was piercing primers in a 6.5 PRC modest load and sent my 300 WM bolt and firing pin assembly to LRI to be bushed and the pin turned down.

I then realized I use the same firing pin assembly in .338 Lapua so sent that bolt in as well.

When I get those two back and with the 308 bolt already with a small firing pin, I will have all three with small firing pins.

My only concern is I may experience ignition problems with the 338 but doubt it.
Click to expand...


Waiting for results...thanks
 
#8
Supersubes said:
That small pin will work fine in the 338.
Click to expand...

If a small pin will work fine (not doubting), why are there large pins in general?

I can see a legacy action like R700 or alike where you’ll have 50 years of active actions, but why would a “new” action design with a large pin?
 
#9
I’d imagine AI would give an answer that has to do with robustness of the pin itself, mixed with some insurance that the large faced pin will strike the anvil of an off center primer(fat chamber/smallish cartridge, off center primer pocket, etc). Large pins have been used for a really long time in virtually all centerfire guns, and very successfully. It’s the target crowd that started with the small pins, and those guys are almost always hotrodding their cartridges, and do so with minimal tolerances all around.
 
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#13
Supersubes said:
I only have a rough idea of size .062 for small, .080 for large. The protrusions spec ange is .040-.060. None of those numbers are exact.

So, when are you buying your AI?
Click to expand...

Already have a AXSA and a AXMC, but neither are small firing pin

started using my google fu, and started reading benchrest and f-class threads.

and as usual with the internet i couldnt fine any 100% data to go by
 
#14
Supersubes said:
One key element you left out is the use of small primers. Nobody had any issues in AI’s until we started shooting .473 case heads with small rifle primers. 6.5x47 was the first, then wide use of the creedmoor, with Lapua brass, and now we have a host of small primer brass in Creedmoor and some other cases.

I’d also disagree that it was with factory ammo. 99% were handloads. Nobody made small primer creedmoor factory ammo(they may still not, not sure), and there was limited use of factory 6.5x47 ammo in general.

Otherwise you’ve got it understood.

If I’m understanding you correctly on your last question, the large frame AX was designed so you’d switch out the bolt body only when switching calibers. You’d always be using the same striker assembly. For magnums, which are all large primer, it doesn’t matter. And for the standard offerings of the time, the small calibers like .308, it didn’t matter either.
Click to expand...

My 308 bolt blew factory 6.5 creed Large primers...hell it was close to blowing mid range 308 loads
 
#16
Huskydriver said:
My 308 bolt blew factory 6.5 creed Large primers...hell it was close to blowing mid range 308 loads
Click to expand...
Which era rifle is that?
brianf said:
Already have a AXSA and a AXMC, but neither are small firing pin

started using my google fu, and started reading benchrest and f-class threads.

and as usual with the internet i couldnt fine any 100% data to go by
Click to expand...
Are you having problems? What cartridges are you shooting?
 
#19
Supersubes said:
Which era rifle is that?

Are you having problems? What cartridges are you shooting?
Click to expand...

Currently shooting only factory (ffm, Lapua,RWS,Hornady)and copper creek through both.

308, 260,6br,300wm,338 lm.

I have not seen any issues across the board.

So I figure there must be something to this, if I’m not seeing it.
 
#23
brianf said:
Currently shooting only factory (ffm, Lapua,RWS,Hornady)and copper creek through both.

308, 260,6br,300wm,338 lm.

I have not seen any issues across the board.

So I figure there must be something to this, if I’m not seeing it.
Click to expand...
My AW had the crappiest shape firing pin I’d ever seen. Large radius at the tip and a sharp edge where the radius met the shank of the pin. I never had issues for the most part. I reduced the protrusion and reshaped the tip. Many thousands of 6.5x47 through it. I’ve only shot 700 or so rounds through my 15 AXMC, but only super magnum cartridges. It’s got a proper tip shape, and no issues. Husky’s experience above is definitely unfortunate, but not the norm. He’s not reporting any issues with the small pin on large cartridges, so that’s good.

If you’re not having issues, rest easy. If you do pierce a primer, stop shooting. The erosion to the edge of the hole in the bolt face and pin tip happens fast(think plasma torch).
 
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#24
Supersubes said:
I only have a rough idea of size .062 for small, .080 for large. The protrusions spec ange is .040-.060. None of those numbers are exact.

So, when are you buying your AI?
Click to expand...

AI firing pin diameters
LFP 0.078”
SFP 0.063"
All SNs starting with a 15 (AX and AT) are SFP
I have not measured my AXMC which was purchased as a 338 LM
 
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#25
Supersubes said:
My AW had the crappiest shape firing pin I’d ever seen. Large radius at the tip and a sharp edge where the radius met the shank of the pin. I never had issues for the most part. I reduced the protrusion and reshaped the tip. Many thousands of 6.5x47 through it. I’ve only shot 700 or so rounds through my 15 AXMC, but only super magnum cartridges. It’s got a proper tip shape, and no issues. Husky’s experience above is definitely unfortunate, but not the norm. He’s not reporting any issues with the small pin on large cartridges, so that’s good.

If you’re not having issues, rest easy. If you do pierce a primer, stop shooting. The erosion to the edge of the hole in the bolt face and pin tip happens fast(think plasma torch).
Click to expand...

My 338 bolt was probably fine honestly if I just shot 338LM..with 300 norma however was insanely bad with mild 300 norma loads so I suspect the pressure curve for 300 norma is more aggressive than 338LM....300 win was borderline too and to keep everything almost fully interchangeable I had LRI just do everything. I did a write up on it. My turned down LFP are usable in all my bolts including my factory SFP bolt. So I have 3 firing pins cuz 1 is none and 2 is one that work across the board and 1 factory SFP that only works with the factory SFP bolt.

I run all of mine with minimum protrusion at .050ish

Pics of a 300 Norma before bushing after they were bushed the primers looked perfect with the same load

20190918_220314.jpg
20190918_220357.jpg
 
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#27
Huskydriver said:
My 338 bolt was probably fine honestly if I just shot 338LM..with 300 norma however was insanely bad with mild 300 norma loads so I suspect the pressure curve for 300 norma is more aggressive than 338LM....300 win was borderline too and to keep everything almost fully interchangeable I had LRI just do everything. I did a write up on it. My turned down LFP are usable in all my bolts including my factory SFP bolt. So I have 3 firing pins cuz 1 is none and 2 is one that work across the board and 1 factory SFP that only works with the factory SFP bolt.

I run all of mine with minimum protrusion at .050ish

Pics of a 300 Norma before bushing after they were bushed the primers looked perfect with the same load

View attachment 7302255View attachment 7302256
Click to expand...
68F8927A-590E-46AE-A793-C67F6207F51D.gif
 
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#28
Just another post saying LRI is good to go for bushing AI bolts. They bushed 1 AE bolt and 1 AXMC bolt for me as well as turning down spare firing pins I supplied. The work was impeccable and turned around in 2 days. Headspace grew about 0.001" from the clean-up pass on the face of the bolt.

It's hit and miss how the AI large firing pin bolts play with non-308 calibers. At one point I had 2 AE's; one had no problem with 6.5 CM, but another cratered with the same load. Decreasing the firing pin protrusion to about 0.038" helped on the one rifle that cratered a bit, and the decreased firing pin protrustion didn't affect ignition reliability.

I ended up bushing one of the AE bolts... no more cratering after that even on warmer loads. Later I converted that rifle to 6 Dasher and also didn't have any issues even with warmer loads with the bushed firing pin. I also decided to bush the bolt in the AXMC as soon as I received the rifle-- didn't want to roll the dice with the large firing pin. Had the bolt bushed first and then used the new receiver to bolt face measurement to have a 300 PRC barrel spun up. Works great and primers look great. If and when I get a 338 bolt for that rifle I'll also have it bushed.

Both my AX308's have factory SFP bolts and I don't have any primer issues with those, 1 is 6.5CM and the other is 6 Dasher. Primers look great.

Keep in mind if you decide to have the bolt bushed I'd highly recommend picking up a spare firing pin from Mile High or Euro Optic and sending it in with the bolt to have it turned down at the same time so you have a spare on hand. Sometimes when AI runs out of spare parts you can wait months for the backorder, and it would suck to damage your only firing pin and then find out AI is backordered on firing pins...

One thing I'm wondering... What diameter is the firing pin on the new ASR and AXSR? Reason I'm wondering is since 2015 AI now ships the AT and AX308 exclusively with the SFP, 2014 and earlier they were LFP only. They offer a SFP .308 bolt for the AXMC a separate item, but the AXMC 300WM/338 bolts still have the LFP. Be interesting to see what the firing pin diameter is on the ASR and AXSR...
 
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#30
Kiba said:
AI now ships AX308's exclusively with the SFP. They offer a SFP .308 bolt for the AXMC, but the AXMC 300WM/338 bolts still have the LFP. Be interesting to see what the firing pin diameter is on the ASR and AXSR...
Click to expand...

What I was told by AI is that all AX and AT rifles since 2015 were shipped with the SFP. I did not own an AXMC at the time. Now I own an AXMC in 338 LM that was built in 2016. You are saying that it is an LFP assembly. I also purchased the 308 bolt body and was told by MHS that I would use the firing pin/shroud assembly from my 338 LM with that bolt body. But you are saying that the AXMC 308 bolt body is SFP.

Are you sure that the 338 LM bolt body uses a LFP?
 
#31
Huskydriver said:
My 338 bolt was probably fine honestly if I just shot 338LM..with 300 norma however was insanely bad with mild 300 norma loads so I suspect the pressure curve for 300 norma is more aggressive than 338LM....300 win was borderline too and to keep everything almost fully interchangeable I had LRI just do everything. I did a write up on it. My turned down LFP are usable in all my bolts including my factory SFP bolt. So I have 3 firing pins cuz 1 is none and 2 is one that work across the board and 1 factory SFP that only works with the factory SFP bolt.

I run all of mine with minimum protrusion at .050ish

Pics of a 300 Norma before bushing after they were bushed the primers looked perfect with the same load

View attachment 7302255View attachment 7302256
Click to expand...

Yep, that's what some of mine looked like in a factory 6.5creed ae mk3. I could get 2800 with 139scenar in horn large primer brass. I know several guys that had factory 260 barreled mk3 that couldn't get more than 2700fps with 140gr bullets and they'd blank large primers. I've had 3 ae's over the years, fp measured .080 and bolt face apertures were .093-.099. The 2 ae's I have now have been bushed by lri. They went from on ragged edge of primer cratering/blanking, to looking perfect, with large or small primer brass.
 
#32
brianf said:
Question...

What does the pin protrusion (if properly sized and bushed) have to do with cratering or pressure in general?
Click to expand...
It wouldn't effect pressure, but if protrusion is too long it will dent primer that much more. The excess stretching of primer cup makes the cup thinner and more likely to blank. Also extra pin fall will effect bolt lift, obviously that is very slight and almost theoretical.
 
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#33
Nik H said:
What I was told by AI is that all AX and AT rifles since 2015 were shipped with the SFP. I did not own an AXMC at the time. Now I own an AXMC in 338 LM that was built in 2016. You are saying that it is an LFP assembly. I also purchased the 308 bolt body and was told by MHS that I would use the firing pin/shroud assembly from my 338 LM with that bolt body. But you are saying that the AXMC 308 bolt body is SFP.

Are you sure that the 338 LM bolt body uses a LFP?
Click to expand...

Correct.

For the AX308 (short action) and AT, AI switched to the SFP exclusively for 2015+. 2014 and earlier models were LFP only.

For the AXMC (long action), the 300WM and 338 AXMC bolts are still LFP only unless AI very recently changed things. The brand new AXMC in 300WM that I picked up middle of last year had the LFP. Measure your firing pin tip to be sure. Factory AI large firing pin is 2mm (0.078") while small is 1.6mm (0.063".)

It was posted a while back that AI said they would not consider putting the SFP in the 300WM and 338 bolt faces as they didn't want people pushing the magnum loads to stupid-hot levels if people were no longer worried about cratered or pierced primers.

AI used to make the AXMC 308 bolt bodies in LFP only (# AI-20012), but after lots of complaints about primer cratering (especially on calibers with small rifle primers) they began to offer a *complete* SFP AXMC 308 bolt assembly (#AI-28340.) The SFP bolt tail and firing pin assembly from the SFP AXMC 308 bolt will not work in the 300WM and 338 bolts as those bolts are LFP and need a dedicated LFP bolt tail and LFP assembly. AI still makes a LFP 308 bolt for the AXMC, part number AI-20012 (which is the one they've offered ever since the 6-lug AXMC was released) and the standard large firing pin bolt tail assembly will swap between the LFP 308, 300WM, and 338 bolt bodies for true caliber interchangeability with the same bolt tail and firing pin assembly just like AI originally intended.

AFAIK if you buy a new AXMC in 308, it will come with the LFP 308 bolt by default (so it's 100% interchangeable with the 300WM and 338 bolts) unless you ask the dealer to swap out the default AXMC LFP 308 bolt for the SFP AXMC 308 bolt assembly.

You can't use the factory SFP in a bushed bolt, as the profile and amount of the material behind the bolt face of a factory SFP bolt head is substantially different than a bushed LFP bolt because of the process used to install the bushing in the bushed LFP bolt face. You also can't use the factory SFP bolt tail assembly in a factory LFP bolt body/head, as AI made the firing pin profiles between the SFP and LFP different enough it won't work.
 
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#34
This is great information @Kiba

What I didn't mention previously is that my AXMC is also a LH model...not sure that would make a difference in any of this.

You say that
"AI still makes a LFP 308 bolt for the AXMC, part number AI-20012 (which is the one they've offered ever since the 6-lug AXMC was released) and the standard large firing pin bolt tail assembly will swap between the LFP 308, 300WM, and 338 bolt bodies."

I wonder if the AXMC LH 308 bolt body is also LFP. It is PN AI:27604BL. Would you happen to know. I can always call MHS
 
#35
Nik H said:
I wonder if the AXMC LH 308 bolt body is also LFP. It is PN AI:27604BL. Would you happen to know. I can always call MHS
Click to expand...

If the LH 308 AXMC bolt body is bring sold as a bolt body only, I'm 95% sure it's LFP.

As far as I know AI only sells the AXMC SFP 308 bolts as a complete assembly which includes the bolt head, bolt body, bolt tail, and firing pin assembly.
 
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#37
lowlight said:
the new ASR is the switch for the AXMC to SFP...

The AXMC is large, the new ones that replaced them are small
Click to expand...

That is good news if the ASR and AXSR will be all SFP from the factory including the magnum bolts. No questions then on which bolt tail and firing pin works with which bolt body, no having to have bolts bushed and giving up the AI warranty, and best of all no concerns about cratering or blanking primers (unless you get really, really, REALLY stupid with your loads.)
 
#38
Kiba said:
If the LH 308 AXMC bolt body is bring sold as a bolt body only, I'm 95% sure it's LFP.

As far as I know AI only sells the AXMC SFP 308 bolts as a complete assembly which includes the bolt head, bolt body, bolt tail, and firing pin assembly.
Click to expand...

I actually looked in the manual and it says that a single shroud/FP assy is supplied with the MC. Move it from bolt body to bolt body. I would assume that I have a LRP 308 bolt.

Still waiting for a magazine solution for short action cartridges.
 
#39
Was going to take pictures of the AXMC SFP assembly next to the LFP and saw this:
20200419_102835.jpg


Haven't used the SFP at all yet but it's nice and bent. Popped it into the bolt body and this happened:
20200419_103700.jpg


Sent an email to Tim at AI NA (when I received full SFP bolt assembly from EO there was no extractor on the bolt). Should be resolved quickly but still annoying...
 
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#40
to switch from AI for a second

other custom, high end factory rifles (not a rack grade r700)
DTA, Caydex, GAP etc

are the pins closer to the AI large or AI small

how about the pin to hole clearance, i think the PVA website targets .003"

are other tighter (possible issue in foul weather) or looser

thanks for the info great stuff so far
 
#41
Most custom actions are around .060ish pin diameter. Guess I never measured fp aperture to see what tolerance was. I know I've had some rds during loadwork that had heavy lift and shiny ejector marks, but primers looked perfect.
 
#42
6brshooter said:
Most custom actions are around .060ish pin diameter. Guess I never measured fp aperture to see what tolerance was. I know I've had some rds during loadwork that had heavy lift and shiny ejector marks, but primers looked perfect.
Click to expand...

With all this good info coming in Im wondering if there is a ratio or sorts:

Pin diameter to protrusion

Pin diameter / protrusion to spring strength/lbs

Pin diameter to hole/clearance

After that, what is min clearance pin to hole to work in adverse conditions.

Design on “perfect” the lean left or right depending on rifle/weapons system ....bench vs hunting etc
 
#46
carlinetor said:
Thanks, I know this, but then to remove the firing pin I need to unscrew the small screw and then unscrew the firing pin?
Click to expand...
There are two set screws. The one you can see which is acting as a lock for the one you cant see below it. It should unscrew after that they are both removed. I dont know the uncompressed length of the spring, vs pin thread length. There may be a special tool for this. Be careful. Somebody who's attended the armorers course would probably know this.
 
#47
Supersubes said:
There are two set screws. The one you can see which is acting as a lock for the one you cant see below it. It should unscrew after that they are both removed. I dont know the uncompressed length of the spring, vs pin thread length. There may be a special tool for this. Be careful. Somebody who's attended the armorers course would probably know this.
Click to expand...
Thank you so much for the valuable information.
I will be very careful...
 
#48
So a quick questio. I am planing to shoot 6br in my AT. Am I going to need to change to the small firing pin to do this or is the large pin going to work?
 
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