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Posted: 4/12/2013 10:13:06 AM EDT
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Ok, so I have my BCM 16 inch midlength gas system barrel that I finally have pulled out and started working on - I bought it two months ago. I got the BRD Engineering dimpling jig (and pinning jig too) and I put that on the barrel last night. It was great! I saw that the factory dimple was a perfect (and I do mean perfect) 180 degrees from the gas port (as the BRD Engineering jig indexes one of its two mounting screws into the gas port). Ok, so this told me I could safely drill the other dimple. It worked great and these jigs rock!
Now, enter my "issue".... I put my VLTOR low profile gas block onto the barrel (the gas block that the BRD Engineering jigs are made for), lined up the set screw holes with my dimples, installed the gas block with the provided set screws, and then noticed that the gas block does not sit snug onto the barrel shoulder. Now, there is a notch for a military style handguard stopper on the barrel right up on the shoulder of it - this is used in conjunction with like an iron site type gas block... SO, I am thinking I am ok here. But! I did see a vid : http://www.youtube.com/watch?v=RHYSgiYN6Q4 at about the 4:06 mark where he talks about this same exact thing on his barrel and how he moved the gas block to be flush up against the shoulder of the barrel at some point AFTER his build... Yeah, I don't want to do this! So, since I am going to be also pinning this low pro gas block, how do I tell if my gas port is perfectly lined up with the gas opening in VLTOR low pro gas block? I don't want to ream everything down, shoot, and then unream it all to then pin the gas block after I see that it works. I want to pin it up front without having to strip back down and pin... You now all see my dilemma here... This gas block thing IMHO is the main thing that is a pain in the rear with these builds. My going back and forth in my mind on what I want to do with it has held my build up for quite some time... I am thinking about trying to get like a shallow gunsmithing pick or something to try and stick into the gas block's opening and see if the gas port is perfectly aligned... The BRD jig though is telling me I should be right on. I don't give a crap about the gap if the gas block works! Sorry, I am a chatty batty today, but I haven't found anything online that addresses this potential issue yet.. |
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Tighten the set-screws up with some Vibra-Tite on them. Shoot a couple boxes of ammo through it. Remove the gas block. See where the burn mark is over the gas port. Adjust from there.
I drill and tap my gas blocks to take a thicker set screw. I also go a bit longer. YMMV. |
| Yeah, problem is, I am going to pin this gas block and what I don't want to do it button everything up, shoot it, then take it all back down to the stripped barrel to pin tha gas block. There has to be an easier way to check my gas port on the gas block. I am thinking some really small gunsmith pick or something... |
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Hehe - you just opened up a can of worms. Look online. Guys that are even using red Loctite and doing things like staking the set screws are having their gas blocks cant on them when firing.. The only sure fire way, and I now am convinced of this after many hours of research, is that a dimpled, set screw, pinned gas block is the ONLY sure fire way that the low pro gas block won't move on you over firing a lot of rounds...
Look at this page below for the BRD Engineering jigs I am using for this. One jig is for dimpling the barrel for the set screws (I used that one last night) and the other jig is the one I will use next to pin that gas block. This is the only way you are for sure good to go on these. Keep in mind also that the iron site gas blocks are pinned like this at two points and this is why many use these across the board on their builds... http://www.brdengineering.com/ |
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And now for a picture of what I am talking about below. You can see the little gap between the left end of the gas block and the shoulder it should be resting on. My hunch is that this is on purpose as a military hand guard stopper would sit in that gap if I were using an iron site instead of a low profile gas block. I would be just fine with this gap if I knew it was good to go but... you have that vid I mentioned above - go to about the 4:06 point in this vid: http://www.youtube.com/watch?v=RHYSgiYN6Q4
and you will see what I mean. This guy had the same issue and he ended up moving it back to close that small gap. I am wondering if he was having firing problems before he did this - this is what I am trying to figure out here.... I don't want to pin that gas block and realize afterwards that it needs to be moved. I want to figure this out without having to button it all up, fire it, then pin. No sir Any ideas? http://i177.photobucket.com/albums/w207/bison9/IMG_0214_zps3c4a3128.jpg |
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Quoted:
And now for a picture of what I am talking about below. You can see the little gap between the left end of the gas block and the shoulder it should be resting on. My hunch is that this is on purpose as a military hand guard stopper would sit in that gap if I were using an iron site instead of a low profile gas block. I would be just fine with this gap if I knew it was good to go but... you have that vid I mentioned above - go to about the 4:06 point in this vid: http://www.youtube.com/watch?v=RHYSgiYN6Q4 and you will see what I mean. This guy had the same issue and he ended up moving it back to close that small gap. I am wondering if he was having firing problems before he did this - this is what I am trying to figure out here.... I don't want to pin that gas block and realize afterwards that it needs to be moved. I want to figure this out without having to button it all up, fire it, then pin. No sir Any ideas? http://i177.photobucket.com/albums/w207/bison9/IMG_0214_zps3c4a3128.jpg Your hunch is correct. This is by design. You generally should not butt a gas block up against a barrel shoulder, the gas port in the barrel is generally drilled to accommodate the handguard cap. The gas port in the gas block is oversized, to account for small variance, however, so even people who do butt the gas block against the should tend to not have any issues. This becomes very evident when you remove them later, you can visibly see the misalignment from the carbon stains. |
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Ok cool. That does make sense. And the BRD Engineering jig (which I trust the heck out of...) and the factory dimple all lined up to what you are seeing now in the pic above so I think I am good...
Problem is, the only thing that still gets me is that 4:06 point in this vid: http://www.youtube.com/watch?v=RHYSgiYN6Q4 Why would this guy take the pain to talk about this? I am hoping he moved it for his own cosmetic reasons and not because it was a firing issue. |
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Just tighten it up and shoot.
THe port in the block is huge compared to the port in the barrel. This is done to address any misallignment in the block. You will get carbon leakage around the block anyway until it self seals with buildup. Some people put a little red loctite on the bearing surfaces as well. That works, have done it...just smells funny until it cooks off. |
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Quoted:
Ok cool. That does make sense. And the BRD Engineering jig (which I trust the heck out of...) and the factory dimple all lined up to what you are seeing now in the pic above so I think I am good... Problem is, the only thing that still gets me is that 4:06 point in this vid: http://www.youtube.com/watch?v=RHYSgiYN6Q4 Why would this guy take the pain to talk about this? I am hoping he moved it for his own cosmetic reasons and not because it was a firing issue. Ignore the video, you're good to go. |
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Quoted:
Ok cool. That does make sense. And the BRD Engineering jig (which I trust the heck out of...) and the factory dimple all lined up to what you are seeing now in the pic above so I think I am good... Problem is, the only thing that still gets me is that 4:06 point in this vid: http://www.youtube.com/watch?v=RHYSgiYN6Q4 Why would this guy take the pain to talk about this? I am hoping he moved it for his own cosmetic reasons and not because it was a firing issue. Because that guy in the video doesn't know what he is talking about. 
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I didn't mess with the video. Having been a Tool & Die Maker for years. I set mine up mathematically. Once I know where the center line of each hole is. I then use shims on both sided of the gas block when I slide it back, and vertical. Then I either pin it, or drill into the barrel for setscrews.
I've never had a gas block sit up tight to the shoulder. Usually somewhere around 35-45 thou off the shoulder |
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Quoted:
Quoted:
And now for a picture of what I am talking about below. You can see the little gap between the left end of the gas block and the shoulder it should be resting on. My hunch is that this is on purpose as a military hand guard stopper would sit in that gap if I were using an iron site instead of a low profile gas block. I would be just fine with this gap if I knew it was good to go but... you have that vid I mentioned above - go to about the 4:06 point in this vid: http://www.youtube.com/watch?v=RHYSgiYN6Q4 and you will see what I mean. This guy had the same issue and he ended up moving it back to close that small gap. I am wondering if he was having firing problems before he did this - this is what I am trying to figure out here.... I don't want to pin that gas block and realize afterwards that it needs to be moved. I want to figure this out without having to button it all up, fire it, then pin. No sir Any ideas? http://i177.photobucket.com/albums/w207/bison9/IMG_0214_zps3c4a3128.jpg Your hunch is correct. This is by design. You generally should not butt a gas block up against a barrel shoulder, the gas port in the barrel is generally drilled to accommodate the handguard cap. The gas port in the gas block is oversized, to account for small variance, however, so even people who do butt the gas block against the should tend to not have any issues. This becomes very evident when you remove them later, you can visibly see the misalignment from the carbon stains. Not always the case, the YHM low profile gas block #9383 needs to but up against the shoulder. "DO NOT PUSH THE GAS BLOCK AGAINST THE SHOULDER OF THE BARREL! You must leave a 1/32” gap between the shoulder of the barrel and the gas block. *::Exception::* The YHM-9383 gas block should not have a gap between the shoulder on the barrel and the gas block." Directly from YHM site. Not sure about the gas block you are using but check with the manufacturer. |
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Install the gas block with set screws, that way it's in the right spot.
Then drill and pin. Now your pin is in the right spot. If the gap between the gas block and barrel shoulder bothers you, Armalite sells little spacer washers to put in that space. |
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