I thought it would be interesting to post some observations from things I have seen over the years, and specifically in regards to things I have seen over the last few months teaching my AR15 Armorer Course.
Problem- Bolt Carrier Group wouldn’t move freely inside the upper receiver. The person in question had a 20" upper assembled by someone “knowledgeable” in AR’s. turns out the gas tube was tweaked and the carrier key was hitting the gas tube. Because this person was inexperienced he just kept forcing the carrier into the upper.
Solution- Using a flat tip screwdriver we placed it into the upper between the receiver wall and the tube and then did a slight tweaking. The tube straightened and the carrier moved freely afterwards.
If you are going to build or have someone build you an AR make sure that you understand the parts and tools required to complete the task. If someone else is building it make every effort to determine what their competency and skill level is beforehand and thoroughly check the weapon. If necessary have them walk you through what they did or provide a spec sheet of the details.
And if you are working on someone elses rifle, don’t let them see you pick up the big screwdriver and start prying.
I’ve adjusted gas tubes that way, and some people were not at all happy about it. I guess I should have used the old “Hey look, there’s Elvis” routine, and tweaked it while their back was turned.
Great idea for a thread. In your experience, if one takes care to ensure the gas block is perfectly centered, how often will a gas tube need tweeking to allow the BCG to run freely? Do you consider bending the gas tube kind of a last resort when someone has already pinned the GB or dimpled the barrel or is it more routine? Are the gas tubes themselves ever out of whack?
The gas tube is a bendy bit of stainless steel. Just the effort of wrestling it into the front sight upright, and threading it back through the upper receiver can put a tweak in it.
For me, checking the fit of the gas tube, and the carrier key over it is a normal part of installing a gas tube, and I pretty much tweak every one. Some a little, some a lot, but all get checked.
It’s been my experience that if you align the block and the barrel nut properly, it is almost always going to be straight. What usually happens is the the barrel nut is the main culprit because it isn’t centered with the gas tube hole in the upper. With a low pro build it should be a no-brainer. I install the barrel nut first and tighten it. Then I install the tube into the gas block and slide it into place to check for any issues and will then tighten the set screw just enough to hold it so I can look into the upper and see if it’s straight.
I had the same problem with my carbine, gas tube was misaligned just enough to rotate the BCG slightly and cause the bolt lugs to hit the barrel extension. In my case, it was because the notch in the barrel nut was not aligned correctly. The fix was to remove the gas tube, loosen & re-torque the barrel nut to properly align the notch and re-install the gas tube.
Hi Iraqgunz- An excellent idea and your first tip is an excellent one. I recently finished my build and spent quite a few minutes being sure to line up my gas tube.
How do you suggest checking alignment? I removed my bolt from the carrier, put my finger in the back of the carrier and moved the bolt back and forth in the upper to be sure I didn’t feel any off-center resistance. Is there any other means of doing so? I also check for uneven wear on the gas tube once I have fired the rifle for a while and as you’ve noted I “tweak” it a bit if needed.
That is one of the best ways that I have tried bc it makes sure that the tube and key are aligned with each other(in theory should be dead center of the channel, but not always…by very small margin of course)
If you tighten or loosen the barrel but while the tube is installed you can watch how very little movement of the nut causes the tube to cock to one side or the other…it really exaggerates the small amount of movement applied to the nut
Thanks for the tip IG. Small things like this are one reason i will not buy an upper or lower that was built in a garage. Unless it was yours, grants, or one of the highly respected experts that are a part of this forum.
For me it would be scary to see one of my local smiths tweek my gas tube with a screwdriver. If it was you i would not even second guess your method.
If someone was building an upper and the barrel nut was torqued to 30-35lbs but the tooth for the barrel nut divided the gas tube tunnel perfectly, how would you go forward in the build? I always have extra barrel nuts that I try if this happens but what if a spare nut isn’t available?
This is the method i use as well. I have also taken a sharpie and colored the inside edge of the gas key to see if the gas tube was rubbing slightly. Although the sharpie method may not hold true, I still do it.
Loosen the barrel nut and repeat the process. I usually snug the bbl nut, loosen and repeat a minimum of 5. This allows the threads to “seat”. Then after tightening to 30-35 ft lbs, if the gas tube hole is still blocked by a bbl nut tooth, I tighten the nut to next available groove.
If I’m wrong in my process, someone please correct me.
Brownells has two tools that are very helpful in this regard.
The first and one that I use on every barrel install is the barrel nut alignment gauge (080-216-014 $4.73). By slipping the gauge into the upper at the gas tube port you can tell instantly if your nut is aligned with the gas tube entry hole in the upper. Saves a lot of time.
The second is the upper receiver lapping tool (080-000-182 $26.95). If you are going for a specific torque number, you can change the barrel nut alignment by lapping the receiver face. It will also true the face for a more solid interface if the upper is not completely true.
I am usually not overly concerned about hitting a number unless building a precision rifle. Generally I aim for a torque range of 40-60 ft. lbs. on most builds and have had excellent results.
I use the same technique. Generally I don’t worry about alignment after the fact if everything is smooth after assembly. I will do a check later, but that’s just as insurance.
Adco sells a great barrel nut alignment tool. Rather long and fits snug that’ll let you know if your gas tube is going to bind or not. Hell, they make several tools specifically for AR building that are great and priced pretty decent.
Having an extra barrel nut is not a bad idea. Over the years I have had one or two barrel nuts that came close to the 80 ft-lb torque limit in order for the gas tube to pass through without binding. I would prefer the torque was in the 35-45 ft-lb range.
Brownells has the alignment tool and everything else you need to build and repair AR-15s. That’s where I picked up all of my tools.
Ok the gas tube is made of stainless steel the barrel and barrel nut is steel and the upper is aluminum. Now when these different metals heat up and expand at different rates would that not cause the gas tube tube to be tweak even more when the rifle gets hot if the gas tube was not install correctly?
That hasn’t been my experience. Consider how many variables determine the location in space of the open end of the gas tube;
Axial alignment of front sight rings to front sight gas tube tunnel.
Perpendicularity of the gas tube pin hole, and the gas tube locaiton hole for that pin
Bends made in the gas tube by the manufacturer to meet specs
Bends inadvertently added by warehouse, packer, other gear in shipment, storage on shelf and schlepping to your AO.
Inadvertent bending induced by assembly, as you wrestle it into place.
Alignment of barrel nut clearance cut.
Location of clearance hole drilled in upper receiver
Maybe I’ve worked on too many frankenguns, but I go into it assuming Ill have to at the very least check alignment, and probably adjust it to make it a perfect fit.
But a good tip, and one worth having in your bag of tricks.