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I put together a build for a friend of mine. Group buy barrel and bolt. I used a Rise Armament 140 Super Sporting Trigger. Single stage 3.5 lb.
Getting some light primer strikes. This has become worse with the cold weather. I swapped out the trigger with another of the same that has worked flawlessly in one of my builds.
Still getting light strikes.
I'm going to pull and swap out the firing pin next. And maybe the bolt.
Anything else I should think about? Anything in particular I should look at?
Initially, I was thinking about maybe a bad trigger. Now I'm wonder if there is a small burr on the pin or in the bolt.
Toolcraft bolt carrier, I believe. I'll have to check.
Thanks
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11-26-2018, 11:50 AM
(This post was last modified: 11-26-2018, 11:55 AM by grayfox.)
New and stiff bc/bolt/gas rings, or maybe if you used any lube, slowing it down?
how many rounds downrange? I would try to get 50 factory rounds downrange then look at it.
O yeah, measure that FP before simply swapping. Are you using the toolcraft bolt?
Toolcraft was (and maybe still is) using a shorter fp by ~0.11 in its bcg's, so putting in a milspec fp could lead to pierced primers w/ their bolt. Has to do with how they made the bolt to accommodate the deeper face depth, they didn't add the +0.11 onto the rear of the bolt like it should be (the real Grendel bolt has the 0.136 face depth and the longer tailpiece so you can still use a milspec firing pin).
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11-26-2018, 08:46 PM
(This post was last modified: 11-26-2018, 08:48 PM by VASCAR2.)
Toolcraft initial 6.5 Grendel bolts were 2.80 long from the rear of the bolt tail to the face of the bolt. The bolt face depth of the Toolcraft 6.5 Grendel bolt should be .136. Alexander Arms chose to use a bolt length of 2.810 to account for the deeper bolt face and the ability to use milspec firing pins. Ive measured all of my firing pins and bolts and was amazed at the various lengths of bolts and firing pins whether 6.5 Grendel or 5.56/223. With a 2.800 length bolt you need a .01 shorter firing pin.
The AR-15 has a specification for firing pin protrusion, deviate from this spec and you can have failure to fire or pierced primers. The minimum firing pin protrusion is .028 and the maximum is .036. Brownells sells a firing pin protrusion gauge. In essence there is a tolerance of .008 between the length of the bolt and firing pin length that need to be in spec. It is possible to get a bolt that is to long or short and I found significant variation in firing pin overall lengths.
I have not found an official document for the exact milspec for firing pin overall length but I believe it is 3.279 plus or minus tolerance. If you measure bolt and it is 2.810 + FP length 3.279 = 6.089. Your two measurements together need to be within .008 of 6.089. As example if you have a shorter 2.80 Toolcraft bolt your firing pin needs to be .01 shorter or approximately 3.269. Adding 2.800+3.269=6.069. Your bolt length plus firing pin length needs to be within .008 of 6.069.
To avoid confusion it appears recent Toolcraft bolts are now shipping with 2.810 length bolts and milspec length firing pins. In measuring my 6.5 Grendel bolts I found where my bolts were within .004 of the stated 2.810 length. Interestingly the longer bolts or shorter bolts were paired with FPs to acheive the proper FP protrusion. It appears that during assembly the the assembler has a batch of bolts and firing pins and checks to see if the selected bolt/firing pin are within spec for firing pin protrusion. If the bolt/FP is out of spec another firing pin is used to get the FP protrusion within specification.
For these reasons I dont recommend swapping firing pins without first measuring the bolt and firing pin length. I found these same variations in manufacturing tolerance when comparing my 5.56/223 bolts and firing pins. In every case the bolt and firing pin were within the .008 spec for firing pin protrusion. It is not uncommon to find .004 variation in bolt or firing pin lengths when comparing numerous AR-15s.
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When I get it from him, I'll check those dimensions. I used a group buy bolt. The bolt carrier, I believe, was toolcraft.
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1075 tech Wrote:When I get it from him, I'll check those dimensions. I used a group buy bolt. The bolt carrier, I believe, was toolcraft.
The carrier shouldn't matter. I also used a group buy bolt and a milspec firing pin without issues. The above post by Vascar is great info!
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I missed that the Op mentioned using a Monster Group Buy bolt which have been very consistent with good QC. It is still possible either the firing pin or bolt/FP are out of spec. When a failure to fire occurs with two different FCG makes you think there is a possible problem with the BCG that might have something out of spec. At least worth checking to eliminate one possible culprit.
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Check primer seating depth and headspace.
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Ok. I checked bolt (from first 2018 group buy). Overall length is 2.814. Bolt recess is .134 instead of .136. Within tolerances?
Firing pin length is 3.278 and I have pin protrusion of .0305.
Upon disassembly, I found an excessive amount of oil and carbon. I would like to think this, combined with the below freezing temperatures, slowed it down enough to not ignite the primer.
But that doesn't explain the 2 misfires during the initial function testing in late summer. However, I believe they were early on with a very low round count and these group buy bolts are tight in the carrier.
Any other thoughts?
BTW, all factory ammo. Inthinknthe initial two were with Wolf steel case. The others more recently were with Federal Fusion.
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I had an issue with a Rise 140 I sent it back and they sent me a new haven't had anymore light strikes. Just my experience I swapped it out with Larue MBT and put in my Sport .223
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1075 tech Wrote:Upon disassembly, I found an excessive amount of oil and carbon. I would like to think this, combined with the below freezing temperatures, slowed it down enough to not ignite the primer.
Any other thoughts?
Only other thing that comes to my mind is -- and this is from a different rifle, bolt-action not grendel -- this other 7-08 had a titanium firing pin, lawyer-like to lessen slam/srop fires, instead of steel. That, combined with low temps and some oil (Rem oil in fact) in/around the bolt/FP, led to 4 light-strike nonfires in a row while I was on stand with a deer in my sights. I think all AR FP's are stainless so probably not an issue here, but it does illustrate how a firing pin can be slowed down to give light-strikes.
btw, I ditched (traded in) that rifle and went with another mfr, with steel FPs I could trust.
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1075 tech Wrote:Ok. I checked bolt (from first 2018 group buy). Overall length is 2.814. Bolt recess is .134 instead of .136. Within tolerances?
Firing pin length is 3.278 and I have pin protrusion of .0305.
Upon disassembly, I found an excessive amount of oil and carbon. I would like to think this, combined with the below freezing temperatures, slowed it down enough to not ignite the primer.
But that doesn't explain the 2 misfires during the initial function testing in late summer. However, I believe they were early on with a very low round count and these group buy bolts are tight in the carrier.
Any other thoughts?
BTW, all factory ammo. Inthinknthe initial two were with Wolf steel case. The others more recently were with Federal Fusion.
I think your bolt face depth, bolt overall length and firing pin are gtg. Only other thing is if you had excessive head space but I would be surprised if that was the issue. You could measure your spent brass to see if there is excessive growth after being fired this barrel. I suspect you have an issue with the Rise Arm. trigger. Have you got access to a different lower with different type of trigger to test the upper.
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VASCAR2 Wrote:I think your bolt face depth, bolt overall length and firing pin are gtg. Only other thing is if you had excessive head space but I would be surprised if that was the issue. You could measure your spent brass to see if there is excessive growth after being fired this barrel. I suspect you have an issue with the Rise Arm. trigger. Have you got access to a different lower with different type of trigger to test the upper.
Didn't measure, but fired brass looked fine. I did swap out the trigger with one that was proven to work in another lower. The same brand.
Now that I've cleaned the bcg thoroughly, we'll give it another go. I'll let you know.
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Assuming pin protrusion is within spec I'd suspect either lubricant thickening or carbon build up within with the bolt causing excess drag.
If the ammo was Hornady American Gunner, I've also experienced missing anvil in primer like another poster has.
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