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Thread: Building first Grendel- what are common pitfalls.

  1. #21
    Super Moderator LRRPF52's Avatar
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    Gas tubes and alignment
    Another hiccup point I see with a lot of the new generation of DIY AR15 builders is gas tube quality and alignment. For me, I always just paid close attention to it and figured that it needed to be as centered as I could possibly get it.

    One technique is to set the upper upside down and use your stripped carrier to test-fit articulation between the gas key and the flange on the gas tube that seals up into the carrier, as this duplicates carrier location in the raceway when under cartridge stack spring tension. Check alignment of the gas tube with the carrier by sliding it back and forth in the area where the gas key opening engages the flange on the gas tube. In AR15 diagnosing, we call it "clipping" when the key and tube bind against each other, causing wear.

    This is most difficult on RLGS, and easiest on pistol or CLGS.

    Gas tubes are not created equally, and many out there will wear quickly, especially if they are mis-aligned. As soon as the flange wears down, the gun will short-stroke.

    Bill Alexander used medical grade stainless tubing for his gas tubes. Not sure if AA does it anymore, but it was a feature on the rifles for many years that most never knew about.

    For military armorers, there are procedures to ensure correct gas tube alignment, and it is part of the TDP for inspections on rifles and carbines for the military.

    Carrier Keys
    Carrier keys are not created equally and most on the market are not made to the Mil-Std or Spec. A lot of companies figured they should harden the carrier key, whereas the TDP calls for the key to be softer so that it doesn't wear the gas tube.

    With the growing wave of nitrided parts, AR15 manufacturers just ask their BCG suppliers to nitride everything, figuring it's better that way than the "lowest bidder" Mil-spec, assuming that engineers, designers, and military inspection/certification contract officers haven't been working on these components since the late 1950s.

    LPKs
    Lower parts kit selection is another area that gets overlooked.

    I was helping a good friend of mine with his 300 BLK build the other day. Took out the LPK and looked at the hammer, which was totally different than any other LPK hammer I've seen in decades. I got to looking more closely at it, and saw that the reinforcement rib along the length of the hammer was missing on the right side, which is what the M16A2 hammer needed in order for the Burst disconnector clutch arm to have clearance.

    It looked like someone reverse-engineered the M16A2 hammer, minus the Auto Sear hook on top for some reason. Maybe a foreign MIM shop took samples of Foreign Military Sales M16A2s sold to them, who knows. The MIM'd hammer also had surface cavitation from shrinking after cooling. Maybe it would be a great hammer, and maybe not. Most of us drop in quality after-market triggers anyway, but I've sometimes used standard hammers for JP FCG set-ups, bobbing the hammer if it was a quality one with good metallurgy and inspection and proof codes. That hammer I would not install in a lower after inspecting it.

    If you ever get a chance to compare Colt hammers, triggers, disconnectors, selectors, bolt catches, detents, springs, bolts, carriers, gas rings, etc. next to whatever the mix of the month is of after-market small parts, you might be surprised at just some of the differences you can see with the naked eye.

    Gas Rings and Carrier Bores
    Gas rings and carrier bore machining/chrome lining are another failure point. On a poorly-machined/finished carrier bore, you will have tool marks galore. Chroming over them doesn't help, and they will grind the crap out of your cheap gas rings for breakfast.

    This is why I check my carrier bores for uniform smoothness before installing them in a rifle. Not all gas rings are made from Mil-Std alloys, so the cheap ones will wear quickly, allowing fouling to flow past them.





    Every little part on a true Mil-Spec gun has a history behind it that may or may not have had early issues, which were then fixed with metallurgy, processes, testing, and refinement.

    The after-market doesn't seem to care much about a lot of these areas, and goes for the cheapest imitation parts they can source from wherever they source them from to make good margins.
    Last edited by LRRPF52; 12-02-2017 at 08:19 PM.
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  2. #22
    Quote Originally Posted by zcostilla View Post
    Yes, I understand thatís how headspacing works. Itís fairly common know,edge. And the customer service rep basically guaranteed that it would headspace correctly. Sorry I didnít word that more clearly. I didnít mean to imply that it was an extra service.
    And I didn't mean to imply that you did not know how a ARs work, etc.

    At the same time, there's a large history of people who try to piece together a Grendel from a variety of parts that have challenges that are largely avoidable. And sometimes the manufacturers peddling parts don't help that.

    Talking about headspace is one of those danger signs I have found.

    Due to some of the early manufacturers trying to use 762x39 bolts areas that add complexity with some companies selling Grendel stuff.

    Use of the weaker/incorrect bolt is not really a "set the headspace" issue as much as it would in create an extremely dangerous excessive headspace condition.

    So there's long litanies and tales of Woe with people buying off-spec bargain barrels and or bolts and having issues making it work.

    Thus the recommendation from most that if you're just getting started and not familiar, start with a known good barrel and bolt combination. SAAMI chamber, proper Grendel compound throat, etc.

    I'm wishing I bought one of the barrels that you bought, so no worries there. And hopefully this thing will go right together and play well for you. I have not found Grendel to be picky once you learn about bullet seating depth and things of that nature in reloading that are less forgiving than bolt guns.

  3. #23
    Quote Originally Posted by LRRPF52 View Post
    Carrier Keys
    Carrier keys are not created equally and most on the market are not made to the Mil-Std or Spec. A lot of companies figured they should harden the carrier key, whereas the TDP calls for the key to be softer so that it doesn't wear the gas tube.

    With the growing wave of nitrided parts, AR15 manufacturers just ask their BCG suppliers to nitride everything, figuring it's better that way than the "lowest bidder" Mil-spec, assuming that engineers, designers, and military inspection/certification contract officers haven't been working on these components since the late 1950s.
    What's your read on the nitrided vs hard chrome keys?

    I was pondering that the nitride might be a little bit easier on the gas tube bell then the hard Chrome as it's a bit softer. I did used a nitrided alconyl/stainless tube on one recent mid-length build that I liked.

    Like you say... Parts is not parts. Huge differences between LPKs and BCGs.

    I used to use Daniel Defense trigger groups until the ALG ACTs came out which are my current favourite if I don't go with a premium trigger.

    DD, LMT, Colt, or toolcraft/AOP contract bolt carriers all have noticable finish differences compared to generic delton/dpms/whatever. To me it's not worth cutting corners there.

    They are not cheap, but I'm very pleased with the later production AA bolts with the ionbond whatever coating. Much nicer bolt than my early AA non hard use bolt or generics.

  4. #24
    Super Moderator LRRPF52's Avatar
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    The underlying base alloy and hardness of the steel gas key is what is called out in the TDP, followed by hard chrome lining of the bore inside of it.

    The internal diameter has to be within a very tight spec, just like the gas tube flange has to be within a very tight spec for OD.

    Same with the bolt tail and the bolt tail bore in the carrier that it protrudes through. If that is off, the gas system is too open and will leak there.

    With modern manufacturing methods, we don't normally see a lot of problem areas here, but if you ever have short-stroking, they are some additional points to look at on top of loose keys, mis-aligned blocks, loose blocks not press-fit to the journal, loose tube-to-gas block fit, etc.

    Gas blocks press-fit to the journal is another one. The TDP calls for the sight tower/gas block assembly to be fit tightly to the barrel without leakage, then pinned with the taper pins of course.

    I don't recall seeing much gas leakage on any of my issued M16s and M4s, but I see it all the time on low profile gas block AR15 DIY builds and after-market Vismod AR15s.

    The LaRue Stealth 2.0 gas system is really nice, functions the smoothest I've felt so far of all my Grendels. He uses a little ferrule to secure the gas tube into the gas block so you don't have leakage there.

    The ArmaLite AR10 SASS and the KAC Mod 2 gas systems are interesting improvements over the basic tube insertion with tiny roll pin to secure.
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  5. #25
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    Quote Originally Posted by LRRPF52 View Post
    I don't recall seeing much gas leakage on any of my issued M16s and M4s, but I see it all the time on low profile gas block AR15 DIY builds and after-market Vismod AR15s.
    Yep, It's amazing that such a basic piece as a gas block can vary so much. My buddy decided to go into the gas block business and I was shocked at how much thought he put into it. It paid off though. They aren't 100% leak proof, but are better than most that I've seen. Basically it boils down to quality materials, good tolerance control and tapering the roll pin hole. Some of the cheap imports either leak a bunch or can be a real bugger to install.

    AXIS MFG Gas Block.png
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  6. #26
    Bloodstained zcostilla's Avatar
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    Based upon the info given in this thread, I looked up more information specific to the 18” BCA barrel I ordered. BCA website it uses 300BLK feed ramps and 0.070” gas ports. Will I need an adjustable gas block? I wonder if anyone here has an 18” BCA barrel with up with a standard gas block.
    -Zac

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  7. #27
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    Quote Originally Posted by zcostilla View Post
    Based upon the info given in this thread, I looked up more information specific to the 18” BCA barrel I ordered. BCA website it uses 300BLK feed ramps and 0.070” gas ports. Will I need an adjustable gas block? I wonder if anyone here has an 18” BCA barrel with up with a standard gas block.
    I'm running an 18" BCA barrel and standard gas block. No issues. I wouldn't expect any either as long as you don't put a suppressor on it.
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  8. #28
    Bloodstained zcostilla's Avatar
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    Quote Originally Posted by Frontier Gear View Post
    I'm running an 18" BCA barrel and standard gas block. No issues. I wouldn't expect any either as long as you don't put a suppressor on it.
    Thanks for weighing in! It sure simplifies things if I can use a standard gas block. I’m not planning on suppressing the rifle. And thank goodness for YouTube, as I can see how the alignment pins really make this easier to install everything.

    https://m.youtube.com/watch?v=cwmUewVwHdM
    -Zac

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  9. #29
    Bloodstained zcostilla's Avatar
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    So I couldn’t pass up another sale, and ordered a complete 6.5 Grendel BCG from Bear Creek Arsenal. My BCA barrel arrives tomorrow (ordered on Cyber Monday) but I had to divert it to a UPS store because I’ll be at my son’s basketball tournament. I also ordered a nitrided gas block and SS gas tube from JoeBob Outfitters, and arrival time is TBD on the BCG, gas block, and tube. Since they’re Christmas gifts from my wife I have to wait until we get back from FL to install them, so it won’t matter. Only things left to get are magazines and a scope. Looking at a used Redfield Widefield 6X on eBay if it sells under my max bid. Not a fan of Kentucky windage, but it was made before Redfield’s quality took a dump, and will work for close in stuff. If I don’t win the auction it might be another month to get the SWFA SS in 6X, unless the demo in 6x9 HD is still a available. As much as I prefer a fixed power scope, the HD glass at a modest price increase might be too much to pass up.

    So any magazine brands/models to avoid? I’m looking at 10-rounders.
    Last edited by zcostilla; 12-08-2017 at 05:23 AM.
    -Zac

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  10. #30
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    They have six of these in stock. I'm 95% sure they are E-Landers.

    https://www.andersonmanufacturing.co...ound-magazine/

    I have hit-and-miss luck with the C-Products mags.
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