AIAT started to pierce primers since switched to H4350

[Kevin1]

The rifle has a large firing pin and a 26” barrel chambered in 260 by MHS.
My load is Lapua brass, BR2, 147 ELD.

My initial load was with 42.9 gr of IMR 4451. I was getting 2812 FPS and no issues.
I reduced that load (didn’t need that much speed) and settled on 41.7 gr IMR 4451 with an average of 2786 FPS….Again, no issues.
H4350 became available and I switched powder. I did a load dev from 40.5 to 42.1 with no apparent pressure signs.
I settled on 41gr of H4350 for an average of 2774 FPS. But here’s the thing, I’m getting some pierce primers. Out of 23 rounds I fired today, I got 2 pierced primer. My previous outing I had the same problem with 41.3 gr.

How do you explain this? Can this be because my batch of brass is at the end of its life (8 times reloaded and no annealing)?

 

[Supersubes]

Has nothing to do with brass age. You have any velocities on the pierced loads? I’d just back it off some more. Maybe it’s something with the h4350 pressure curve? Hard to say, but clearly too much pressure.

 

[Ein]

I don't shoot 260 so Im not familiar with loads or velocities. Ask yourself the following questions. Are your seating depths varying enough round to round so as to inadvertently jam a bullet into the lands occasionally? Are the primers that don't pierce flat? Is your velocity consistent with a mid-range load? Are the ones that pierce random heavier charges? Verify that one with a chrono. Is your firing pin radiused nicely and consistently? If your answers were No, No, Yes, No, Yes, then bush your firing pin hole down to ~1/16" diameter. Have the pin turned down for a snug fit in the hole. Should solve your problem.

I've done it when mild/midrange loads would pierce as well as to fix a similar problem while knowingly shooting hot loads. The velocity/accuracy combo was awesome. It worked both times, but the brass was short lived in the second case.

 

[Kevin1]

The bullet has 0.01" jump and I don't believe jamming is an issue.
I didn't chrono the pierced primer loads. Here's a pic of the brass with 41gr of H4350. There might be some pressure signs on most of primers.
I'm surprised that I get pressure signs at a lower velocity with H4350 vs IMR4451. I thought it would the other way around. Next time I'll chrono the remaining rounds and see if I get an unusual high velocity for the pierced primers.

 

[m1match]

Other than the 2 pierced primers, you primers show no signs of being anywhere near max pressure. They aren’t excessively flattened. I’d have to guess that it’s a firing pin issue. The large pin Accuracy International rifles are known for piercing primers sometime with normal pressure loads but your loads should be comfortably below maximum. I shoot a 6.5 CM AIAW with BR2 primers and 42.5 gr of H4350 in a Hornady case, giving around 2750 fps out of a 24” Bartlein bbl and I get minor primer cratering but never pierced primers. If it were me, I’d call Mile High Shootingto ask for their opinion, since they’re an AI service center they’d know what to do and they can bush your bolt if it needs it.

 

[MarinePMI]

Yeah, this is a common issue with the older large firing pin AI's (they have since all been changed). You can play with the firing pin protrusion and hope to find a solution, or you can send the bolt off to be bushed. Many here can do the job. I sent mine to Stuteville, and had zero complaints about the work or the estimated lead time. Both were spot on.

 

[Redeye3]

Yeah, this is a common issue with the older large firing pin AI's (they have since all been changed). You can play with the firing pin protrusion and hope to find a solution, or you can send the bolt off to be bushed. Many here can do the job. I sent mine to Stuteville, and had zero complaints about the work or the estimated lead time. Both were spot on.

Same here with AIAE mk3 bolt, sent to LRI and all problems went away.

 

[Kevin1]

Or I just go back to IMR4451....BTW does MHS do the bushing as well?

 

[MarinePMI]

I'd expect, that as an AI certified repair depot, they'd not do it (since it meets spec as sold), and they'd ask you to procure an new bolt and have them set the head space. I may be wrong though (this is just a guess), so it'd be best to call MHS and see.

 

[Supersubes]

I’ll be the dissenter here. I see nothing but pressure on the backs of those cases. Many of the cases have scrubbing or ejector swipes on the case heads. I bet I’d find more if I could spin some of them to change the light. You should have felt some sticky bolt lift assosicated with those marks. If you’re a neck sizer, and your cases were on the fat side to begin with, that would have masked the bolt opening force a bit. Many of those primers are about as flat as a br2 will get. While the br2 isn’t a magnum primer, it does have a magnum cup, so they are very stout. You said in your initial post that you’ve never seen any pressure signs, even at higher charges than this. Given what I’m seeing above with regard to pressure, I think you’re just missing those signs.

It also looks like your pin profile has sharp edges like my AW pin has, which acts like a sheet metal punch, so that’s certainly a factor for the piercing. I’ll be back with some pics to show you what I’m talking about.

Hornady doesn’t make factory 260/147 eld ammo I don’t think, but they do have load data published. In that data, they show bigger charges with powders similar to h4350(they didn’t test h4350), but much less velocity(2650 max). There are people here claiming pretty crazy velocities with the 147, but to my eye, you’re pushing the 147 at 140 velocities. This is based upon owning a bunch of .260’s and creedmoors over the last 10 years. Every one of them made 2750-2800 with a 140. My 147 loads are high 26xx.

 

[LRI]

I can help:

https://www.longriflesinc.com/colle...ing-m700-striker-pin-hole?variant=49376970189

 

[hollowoutadime]

I'm impressed. Approx turn around time for an AI bolt?

 

[Kevin1]

Here’s another interesting fact.
Below pic is the brass shot from factory 260 130 ELDM. I shot the same box (16 rounds when barrel was new, the 4 remaining rounds a few days ago after 1000 rounds)
The 6 cases to the left: Shot when the barrel was new a few month ago
The 4 cases to the right: Shot a few days ago. Barrel has around 1000 rounds, Average MV of 2926 FPS
I didn’t chrono with the factory load when the barrel was new, but I know the barrel has speed up a little since.

I can see the 4 primers shot recently are showing some rough edges.

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[Jethro3898]

Piercing a primer sucks when it keeps the firing pin from falling on the next round....it really sucks when you happen in a match and you only have 90 seconds to solve it. I tried the "firing pin turn" thing and it didn't work for me. Save yourself a lot of heartache and send the bolt to Chad at LRI. Chad took care of mine and the problem went away. Price was very fair and his turn around was quick as well.

Oh yeah, my rifle has only shot Prime 6.5 ammo and maybe a box of Hornady. Never shot a single handload. Before I had the firing pin worked on I would pierce a primer every 15 rounds or so.

 

[LRI]

I'm impressed. Approx turn around time for an AI bolt?

Week to 10 days. Just depends on where you catch us in the cycle.

 

[redneckbmxer24]

Several pieces of your brass in the first pic clearly have ejector marks, are they from this firing or a previous? If they're from this firing then your load is high pressure. No surprise your piercing if this is the case.

I've shot thousands of rounds through LFP AI's with H4350 between 243, 260, and 6.5 creeds and I've never ever pierced a primer. I keep my pressures reasonable but by no means are they low. Currently running a 243 barrel on my AXMC which are all large pin, I've got over 1500 rounds through this barrel now with H4350 and no pierced primers. Charges for 105 hybrids and 115 DTACs have both been just over 42r.

 

[Supersubes]

Here’s another interesting fact.
Below pic is the brass shot from factory 260 130 ELDM. I shot the same box (16 rounds when barrel was new, the 4 remaining rounds a few days ago after 1000 rounds)
The 6 cases to the left: Shot when the barrel was new a few month ago
The 4 cases to the right: Shot a few days ago. Barrel has around 1000 rounds, Average MV of 2926 FPS
I didn’t chrono with the factory load when the barrel was new, but I know the barrel has speed up a little since.

I can see the 4 primers shot recently are showing some rough edges.

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Looks like 3 of the 4 have heavy ejector swipes as well.

 

[MarinePMI]

I get ejector swipes on my brass in my AI as well, and with relatively mild loads.

An ejector swipe is not always an indicator of over pressure loads.

 

[redneckbmxer24]

I get ejector swipes on my brass in my AI as well, and with relatively mild loads.

An ejector swipe is not always an indicator of over pressure loads.

A little swipe is one thing but there is clearly round spots on a few of those pieces where it tried to flow back into the ejector pocket. That's indicative of over pressure.

 

[Kevin1]

I have a local match this week. I’m going to switch back to 41.7 gr of IMR4451 that gives me 2786 FPS. I’ve been using that load for the past several hundred rounds and never got a pierced primer. I’ll be using new Lapua brass. I’ll take pictures of the fired brass and report back.

One thing that makes me nervous with bushing the firing pin, is when I read on LRI site “This will alter the headspace dimension of the rifle slightly. With the recent implementation of our new milling center and probing, we can now control this tolerance quite well. Expect your headspace to grow by .0005"-.0015".” Not to mention that it might void the AI warranty.

Honestly, I don’t think I need any bushing work. I’ll just do another load dev with milder charges. I don’t mind losing 100 fps with the 147 ELDM as the ballistics will practically be the same at up to 1100Y which is the max distance I shoot.

I think my barrel has speedup too much. I’m getting 2926 FPS in a 26” barrel with factory 130 ELDM vs the advertised speed of 2840 FPS for a 24” barrel. That’s 86 FPS more for an additional 2” of barrel.
I also think the pressure curve of the H4350 might be more conducive to higher pics vs the IMR4450.

 

[Kevin1]

Other than the 2 pierced primers, you primers show no signs of being anywhere near max pressure. They aren’t excessively flattened. I’d have to guess that it’s a firing pin issue. The large pin Accuracy International rifles are known for piercing primers sometime with normal pressure loads but your loads should be comfortably below maximum. I shoot a 6.5 CM AIAW with BR2 primers and 42.5 gr of H4350 in a Hornady case, giving around 2750 fps out of a 24” Bartlein bbl and I get minor primer cratering but never pierced primers. If it were me, I’d call Mile High Shootingto ask for their opinion, since they’re an AI service center they’d know what to do and they can bush your bolt if it needs it.

Are you shooting 147 gr ELDM as well?

 

[trophyhunter]

One thing you need to check that will get worse with each piercing is eroding of the firing pin. I used to have this issue with my AXMC before having my bolt bushed. It’s very evident in your brass on top pic, it’ll erode your pin and almost create a needle that’ll cause even more piercings.

Getting my bolt bushed by LRI was the best money I could’ve spent. Or you can buy a new small firing pin bolt assembly from Mile High. They do not offer the bushing services.

 

[Ein]

You'll be the first shooter I've ever heard say a barrel has "sped up too much".

That's a pretty bright ejector mark for Lapúa brass. One other thing to check, headspace between a fired and sized case should be pretty close. If a sized case has the shoulder pushed back too far it gives the case a running start at the bolt face when it's fired. I've seen it cause flat primers and ejector marks in low pressure loads.

 

[trophyhunter]

Also to add, you will have MAYBE .0005” headspace growth with the service. After they install the bushing they mill the bolt face and it’ll alter your headspace less then .001”. This will cause nothing in your ammo except now you’ll have an extra .001” to add in full expanded cases. I don’t know how tight your measurements are but my are within .001” of each other so the added was not noted. And I can’t really see what other issues you would need a warranty for that Mile High can’t fix for you. It’s a damn AI....what can go wrong ?? Only other item is the trigger?? but guessing yours is older then a year since it’s a large firing pin so don’t know how long the warranty is on a trigger.

 

[Kevin1]

You'll be the first shooter I've ever heard say a barrel has "sped up too much".

That's a pretty bright ejector mark for Lapúa brass. One other thing to check, headspace between a fired and sized case should be pretty close. If a sized case has the shoulder pushed back too far it gives the case a running start at the bolt face when it's fired. I've seen it cause flat primers and ejector marks in low pressure loads.

Yup!! I failed to mention but I definitely pushed the shoulder back too much during full length resizing with the h4350.

 

[Kevin1]

So with the few primer piercing I had (might be caused by bumping back the shoulder a little too much), might have caused a firing pin erosion, which might also explain the rough edges on the primers of factory ammo (post #13).

I took out the firing pin and it does show some erosion….So I might even get pierced primers in this week’s match with my usual 41.7 IMR4451 load with new Lapua brass…..Well that sucks…

I might have the bushing done, if anything just to smoothen the firing pin. BTW now that I see that MHS is selling SFP bolt assembly (any idea of the price?), I’m less concerned about the warranty and head spacing. Worse case (not that I expect anything to go wrong), I’ll buy a new bolt assembly from them.
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[trophyhunter]

They cost roughly $550 for a complete assemble

Sorry I was wrong. $700
https://www.eurooptic.com/26547BL-AI-Accuracy-International-Bolt-A.aspx

And yes that firing pin is definitely causing some of your issues on piercings. And your headspace is also an issue on the excessive ejector marks.

Also to add, I have 3 bolts currently at my house, 1 is a large pin, 1 is factory small and 1 is LRI bushed. I’ve measured all 3 and they are within .001” of each other when I’m my AX measuring headspace, so I don’t think you’ll have an issue.

I think your sizing is more of the issue here.

 

[Kevin1]

I switched back to 41.7 gr of IMR4451 with new Lapua brass for my local match. As expected I got an average speed of 2777 FPS with a 3 SD.
I had exactly ZERO pierced primers. I agree that the firing pin marks have some rough contour, and that might have been because of the previous pierced primers that damaged the FP, but nevertheless, today everything went fine.
For me, the baseline is my 41.7 GR IMR 4451 load and it’s still performing well. Therefore, I don’t see the need to do a bushing on my bolt. The pierced primers with the H4350 can only be explained by either a too aggressive full-length sizing or a different pressure curve. BTW I was talking to a very accomplished shooter today at the match, who said that this could also be caused by a carbon ring (I’ll also give it a good cleaning).
Having said that, I now realize that launching the 147 ELD at speeds close to 2800 FPS, even in a 26” barrel is a little too much. Going forward I plan to do a load dev to get speeds slightly below 2700FPS. I’m hoping that I can do that with the H4350 and without piercing primers. If not, I’ll just switch back to the IMR44551 and I won’t look back…. Truth be told the IMR4451 is really good!
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