AR15 - CHECK YOUR BUILDS!

[Capbyrd]

I've invested heavily in AR specific tools over the past year or so and increased my knowledge a good bit.  I'm planning some armorer's classes in the future to take my understanding of the systems even further.  

I guess a lot of my friends know that so they've been asking me for help in assembling kits or replacing parts or diagnosing issues.  Something that I've come across a lot of recently are headspace issues and receiver extension issues.  I had to send a bolt back that wouldn't close on a 5.56 go gauge in any barrel that I had.  The company was great about it and they replaced it without issue.   I also had a friend bring a rifle by that had the same problem.  It was a home built setup that he bought second hand from a shop.  Luckily the shop owner knew who made the bolt and took care of it.  

 

Another rifle was a complete upper from a reputable company.  The rifle headspaced fine but using one of my tools, I noticed that something was off between the receiver and the barrel extension.  Or so I thought.  I thought maybe the extension was clocked wrong inside the receiver.  Even though it functioned fine, you could see wear on the lugs of the bolt where they were having to push past a bind to lock into place correctly.   On further inspection, it appeared that the barrel extension lugs were machined poorly and had little tabs that were causing the binding.   The company asked for pictures and then asked for it back.  They replaced the barrel and sent everything back quickly and it was fantastic.  I ran it through an inspection and everything checked out great.  

Most of these were pre-panic rifles and each one had some small issue.  With the current state of things, check your parts before you get out and run your rifles.   I'm worried about QC dropping even with the best names out there.  

And if you are in the Memphis area and want or need anything checked out, I'm happy to help.   I'm not a gunsmith and I will happily admit that I don't know everything but I know a good bit and I enjoy doing this stuff.   (Also, since I'm not a gunsmith, I don't charge anything.  It's purely a hobby for me at this point.)

Edited December 9, 2020 by Capbyrd

[Erich]

2 hours ago, Capbyrd said:

 With the current state of things, check your parts before you get out and run your rifles.   I'm worried about QC dropping even with the best names out there.

^^^^  x 10. I'd recommend doing this before you build, especially if you dont plan on getting to it right away. I have gotten into the practice of doing a thorough spec check on part arrival as I have noticed increasing defect rates over the last 3 to 4 years. Irrespective of expense and brand. With todays turnarounds, its especially important.

The barrel check mentioned is a great start. I'll add a few more that I run through on parts as they also effect BCG and stacking tolerance.

1. Check upper extension pin slot for spec. Out of spec allows the extension to overclock also leading to bolt lug contact and peening. Pin gages are best, spec .126 +.002. Interference fit at .125+ is ideal.  I recently had an upper that  was at .137.  Manufacturer is a big name and due to their design it could not be shimmed. They stopped production to run thru as they prod staff had a WTF moment when they inspected it.

2. Ideal extension placement is flush fit of extension to the inside receiver face. If you get any setback your BCG runs forward and the stacking tolerance could have your carrier not push the buffer off the detent pin, resulting in the buffer face having hammering against the pin on each firing cycle.

3. BCG carrier flush to rear of the receiver pretty much guarantees your stacking tolerance would be an issue for the item above.

4. bolt to carrier resistance should support carrier weight when held on end and better if its got a bit more resistance. Carriers coatings allowance sometimes are high so diameter allows excess ring blow by leading to being undergassed.

Gas system stuff is its own novel.  

[Capbyrd]

3 minutes ago, Erich said:

^^^^  x 10. I'd recommend doing this before you build, especially if you dont plan on getting to it right away. I have gotten into the practice of doing a thorough spec check on part arrival as I have noticed increasing defect rates over the last 3 to 4 years. Irrespective of expense and brand. With todays turnarounds, its especially important.

The barrel check mentioned is a great start. I'll add a few more that I run through on parts as they also effect BCG and stacking tolerance.

1. Check upper extension pin slot for spec. Out of spec allows the extension to overclock also leading to bolt lug contact and peening. Pin gages are best, spec .126 +.002. Interference fit at .125+ is ideal.  I recently had an upper that  was at .137.  Manufacturer is a big name and due to their design it could not be shimmed. They stopped production to run thru as they prod staff had a WTF moment when they inspected it.

2. Ideal extension placement is flush fit of extension to the inside receiver face. If you get any setback your BCG runs forward and the stacking tolerance could have your carrier not push the buffer off the detent pin, resulting in the buffer face having hammering against the pin on each firing cycle.

3. BCG carrier flush to rear of the receiver pretty much guarantees your stacking tolerance would be an issue for the item above.

4. bolt to carrier resistance should support carrier weight when held on end and better if its got a bit more resistance. Carriers coatings allowance sometimes are high so diameter allows excess ring blow by leading to being undergassed.

Gas system stuff is its own novel.  

Indeed!  I run through all of those checks.  I also check to make sure that the BCG doesn't bottom out during travel.  

Pin gauge sets are one of the next things up on my list.