View Full Version : Primer Strike Offset
How much offset is acceptable? I was inspecting brass from a new upper and there is noticeable offset. The upper has functioned for the 300 rounds I have through it. I swapped bolts and the results are the same so it is likely related to the chamber. Not a function issue so far, just curious to see what the general consensus is.
60699
How much offset is acceptable? I was inspecting brass from a new upper and there is noticeable offset. The upper has functioned for the 300 rounds I have through it. I swapped bolts and the results are the same so it is likely related to the chamber. Not a function issue so far, just curious to see what the general consensus is.
60699
Who makes the weapon?
It's a KAC
Thanks, I would contact them and see what they have to say. Personally, its getting to be a bit on the edge of what I would like to see, but not every weapon is perfectly aligned, and as long as it groups well I wouldn't personally be worried. Again, I would still contact KAC (Jack L on this board) and see what their thoughts are.
Will do, appreciate it. My other ar's all have some small degree of offset but you'd really have to be looking for it to notice.
Looking forward to their reply. Can’t really say why, but that would concern me.
geezer john
I'm interested to see what they have to say. I'll admit, I have never really looked at my firing pin strikes if the round went off without issue.
The Army did a test of eccentric firing pin impacts back in 1943.
This test was with Calibers .30, .45 and .50. It was found that with normally radiused firing pins as long as the ecentricity of the impact was within 0.020 inch of center, normal ignition should occur.
If the firing pin tip is blunter than the typical 0.078 to 0.090 inch spherical tip greater eccentricity can be tolerated, and conversely, narrower tips can tolerate less.
Rough estimate from the picture is you're about 0.010 to 0.012 off center.
Thanks lysander..that's interesting. Per KAC it's common and not an issue. I'll probably run a few hundred more rounds through it this weekend to see if it continues to run well.
Interesting. I'll have to look at the KAC gun we shoot to see if it's doing similar. Reliability wouldn't be a concern at all if it were my gun.
People have been reporting so many KAC issues lately that it's quality (or quality control) now seems far below BCM and Colt.
And this is only reported issues, I'm sure there are at least 10x unreported issues - people simply not willing to admit them publicly or decide to ignore minor things. And 25x unnoticed issues which require specialized tools or extra attention to identify.
Not impressed by it at all. I also don't buy into what excusers are going to say "everyone will have issues". Yes, to some degree, but too many issues pop up lately with a lot of Tier 1 manufacturers which really look like systemic quality control issues.
People have been reporting so many KAC issues lately that it's quality (or quality control) now seems far below BCM and Colt.
And this is only reported issues, I'm sure there are at least 10x unreported issues - people simply not willing to admit them publicly or decide to ignore minor things. And 25x unnoticed issues which require specialized tools or extra attention to identify.
Not impressed by it at all. I also don't buy into what excusers are going to say "everyone will have issues". Yes, to some degree, but too many issues pop up lately with a lot of Tier 1 manufacturers which really look like systemic quality control issues.
Except this apparently isn’t an issue... and what other issues are you referring to?
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Except this apparently isn’t an issue... and what other issues are you referring to?
I'm not particularly keeping track with regards to KAC or any other brand, but just from the top of my head some issues I've read about KAC - issues with stripped lowers (something about trigger pins or something), few people reporting sandcutter carriers gas keys coming loose, feeding/cycling issues (might be older 2+ years and/or feed ramp related), ammo sensitivity issues (some people saying it will cycle everything, others saying it will only work with M855), MAMS 5.56 having issues with welds. And granted that most people babying their KAC rifles.
I have a very high respect for KAC as they are one of the few companies that actively innovate in the field and have a very good customer service. From reading timelines some KAC problems reported initially are quickly fixed as later iterations almost never report a similar or repetitive issues (which is a good indication of a positive feedback loop in production).
It might totally be a false perception on my part, as I think people tend to pay more attention to issues with a higher end products (KAC, LMT, HK, etc.) rather than say Palmetto or S&W.
Firing pin offset might or might not be an issue. As OP rightfully stated it might be indicative of another problem like a chamber alignment. I wish a qualified gunsmith would chime in with the comment on this just for a general knowledge. I'm interested to find out if this is something one needs to pay attention to or not.
aix01, I think you are blowing things out of proportion. Back off of what appears to be a falsehood in regards to the "rampant" issue with KAC. Have enough guns out there and any one can have a defect.
People have been reporting so many KAC issues lately that it's quality (or quality control) now seems far below BCM and Colt.
And this is only reported issues, I'm sure there are at least 10x unreported issues - people simply not willing to admit them publicly or decide to ignore minor things. And 25x unnoticed issues which require specialized tools or extra attention to identify.
Not impressed by it at all. I also don't buy into what excusers are going to say "everyone will have issues". Yes, to some degree, but too many issues pop up lately with a lot of Tier 1 manufacturers which really look like systemic quality control issues.
When people spend $2500 minimum(and upwards of 5k+) for an AR they tend to latch on to any and all issues, no matter how small... "my new KAC has a spec of dust on it, their QC has gone to shit"... And some of it ends up being a bragging post...with a non-existent problem... "hey look at my new 5k dollar SR25 ACC, ABC, XYZ in the ultra rare pink cerakote...that I think doesnt have the barrel timed properly as the bullet doesnt engage the rifling quite right"...
I dont mean that as a dig on the OP. Ive honestly never looked at firing pin strikes except to read them for pressure. Ive never thought being offset was an issue. If the gun goes bang, im pretty happy. If its an accuracy based gun and its accurate, im pretty happy also. But im much more function over form.
I didn't intend to start any back and forth on KAC here. They were very responsive via email and while not ideal the offset has not impacted function. I am going to keep an eye on the brass. So far I've run federal tbbc and xm193...5.56. I have some black hills and mk318 that Ill try as well as the offset could also be further exaggerated by the brass itself. And I do hold "tier 1" manufacturers to a higher standard. If this was a PSA I wouldn't care and probably would never have created a thread or checked brass in the first place. KAC makes a great rifle and my only intent was to make sure this particular example is gtg with the primer condition.
Why this is not an issue:
1) The diameter of the recess for the case head in the bolt can be 0.385" in dia. The diameter of the military case head can be .371". Commercial brass may show a greater variation. If the extractor pushed the case head all the way the the opposite side of the recess the firing pin can be displaced up to 0.014" or more.
2) The military chamber allows up to 0.008" radial clearance between the case and the chamber walls. Commercial chambers and brass may show greater variation.
3) There is a 0.002 allowable off-center tolerance on the firing pin hole and the center of the recess in the face of the bolt.
4) The bolt carrier has some clearance in the upper 1.000" bore and the bolt had some clearance in the carrier bore (I'm not going to look them up right now), so there will some radial movement allowable.
All of this will add up. As long as the firing pin strike is within 0.015" and there is reliable ignition, it is not an issue.
@lysander, thank you for the explanation. I ran another 200 trouble free rounds this past weekend. Interestingly, Black Hills 77gr TMK had less deviation than the federal loads. I suspect this is due to the quality of the brass and primer pocket consistency. In any case, the rifle is shooting very well.
60822
Interestingly, Black Hills 77gr TMK had less deviation than the federal loads. I suspect this is due to the quality of the brass and primer pocket consistency.
Black hills usually runs L.C. brass which isn't magical. I'd guess that there's something else causing the variance... hotness of load, etc.
I've loaded a ton of almost every brass out there. The only glaring issue that comes to mind is Fiochi's (GF) brass with the flash holes that are way off center. Primer pockets are always concentric or seating primers would be a nightmare.
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