Randall,

Thanks for the extra information. I find it very helpful to better understand the parts at play in this system. I have always had some concern over the ‘backtrust’ issue in this weapon system using the 7.62x39.

I am looking to have a good parts supply for this weapon. However, finding quality parts is another mater when it comes to this combination as I have found.

With your experience what do you think the rounds/bolt rate would be? Do you have any suggestions as a good place to start looking for a good quality replacement bolt for my AR?

I for one am very glad that you made your post as this has been the best source of information beyond load, point, shoot that I have come across.

Thanks Again,

ArticWolf

Originally Posted By engineer2001: Should I even worry about getting a mid-length gas system if all I intend to do is shoot in a range-based semi-auto fashion for thousands upon thousands of rounds over years of use and properly maintain the weapon?

Fear not, the US military has 1000's of carbine length M4's in use.
If you have the choice, go mid-length, but don't choose mid-length just because everyone says you have to.
Get what you want, take care of it properly and if it's quality kit, it will serve you just fine.

Now my personal 3 gun match rifle is an 18" barrel with rifle-length gas system and a big brake on the end.
You want to talk about smooth shooting...
It will almost keep a penny sitting on the top of the barrel without falling off between shots.

Originally Posted By AR15barrels: If you have the choice, go mid-length, but don't choose mid-length just because everyone says you have to. Get what you want, take care of it properly and if it's quality kit, it will serve you just fine.

That's what I wanted to hear.

Thank you!

A big thank you to Randall Rausch for posting all of the info on the AR's gas operation! It has given me some insight on a problematic AR-15. I now know it was having a short-stroking problem, but I didn't know it was called that at the time. Some modifications were made to a "plain jane" A3: YHM handguards, flip front and rear BUISs and a 6 position collapsible stock. I had told my friend that perhaps something's wrong with the buffer spring in the stock, but now I'm not so sure. Could it simply be that my friend didn't tighten the the gas block of the flip front sight? I don't recall the front sight being loose, however...

Should the gas block be checked out, or should the fixed stock (and everything else connected to it) be reinstalled?

Originally Posted By AR15barrels: Yes, it could be as simple as the gas block not fitting right, or not being aligned to the gas port.

After passing on the suggestion to check the gas block, my friend informed me that he was able to tighten the screws a little more. He also said that he tightened them to the point where he thought he was going to break his allen wrench!

Now he just has to find the time to go to the range and hope he doesn't have to pull the charging handle for each round he fires.

Those YHM flip-up front sights need to be forward about 1/32" from the shoulder on the barrel.

This has me wondering: if the gas block can be put "too far" back, wouldn't the gas tube (since it must be pinned to the gas block) collide with the bolt carrier, or cause some sort of fitting problem during (re)assembly? While I don't claim to be an expert at building an AR, I think it's safe to say my curiosity is due to Black Rifle Disease.

I would like to say thanks to all the people on this thread who have helped me fix my problem. This is my first post so I didn't even have to ask, there is enough info right here for me to figure out my problem and fix it in house. That means I didn't have to send my upper back and wait how-ever-many weeks to get it back so I could shoot the flippin' thing. I just bought my first AR about a month ago, a Bushie 11.5" bbl. I read the probs with shorty's and was afraid I got one of those. There are 3 of us here who bought shorty's (another 11.5" and a 10.5") and they have had no probs. I, on the other hand, thought I bought the shortest single shot in the state. My problem was a short stroke and through this thread I learned how the gas system works and have a better understanding of the gun. I thought I would have it fixed when I learned I needed an "H" buffer. I got one and still had the same trouble. When I started really looking at my gun, after reading this thread, I noticed a "powder shadow" on the gas tube in the gas block. This was my problem. The gas tube and the gas block were not lining up correctly. Why? No roll pin. The roll pin that holds the gas tube in place had not been installed and the gas tube travelled forward and misalligned on the block. Bushmaster is a top notch company and I'm sure this one just fell through the cracks, I just got lucky and bought the one the quality assurance (QA) inspector pencilwhipped, lucky me. Anyway..... Thanks guys, I learned a lot from this site and will keep coming back.
When I get my Gemtech Halo I'll post a pic.
Thanks.


Goodnight Baby! I'll miss you.
VF-21 Freelancers 1989-1992

I've been trying to figure out what all that gas was doing in my rifle's bolt and this is the best information that I've been able to find anywhere.
During my search, I've also been tearing apart my AR and trying to see how the parts interacted. I have not seen anything in this thread about the firing pin. It has a flange that protrudes substantially above the bolt and, it seems to me must catch a substantial part of the gas pressure pushing backward. This would extract the firing pin from the bolt face and I was wondering what part this plays in function?

Gas tube mushroom & alignment questions:

What exactly is the mushroom?

On my new sabre AR, I hear a scraping as the bolt carrier slides the last bit forward.
I am thinking it's the gas tube touching one side of the key interior as the key slides over it rather than going dead center into the key.

Is this a misalignment problem, or of little concern. The rifle runs fine, the scraping noise just got me wondering.

Thanks!

Originally Posted By 2ndChildhood: What exactly is the mushroom?

It's the "uncurcumsized" end of the gas tube.
Look at the last 1/8" of the tube in this picture:

The end of the tube should enter the key cleanly. If needed, pull the hand guards and tweak the tube over the top of the barrel to get the end of the tube aligned with the carrier key.

Now if the tip of the gas tube enters the key fine, but as the carrier starts to touch the barrel extension, the key rubs the side of the tube after the mushroom end, then you will need to tweak the end of the gas tube protruding inside the upper receiver to get it straight, then go back and tweak the tube about the barrel to realign.

As long as the barrel nut was correctly installed (read the tube can move around in the upper a bit, and not bound up on one side of the barrel nut spines), the key should have some movement to self correct after the initial mate with the key.

Great Job Randall!

Originally Posted By JFA: The gas tube I got from Bravo doesn't have a mushroom, it's straight. JFA

Something's wrong...
Post a close-up picture here.

Randall,

Here's a couple pics. Let me say first that I tried gas tube into the key to see if it fit okay and it did not, not even with some force. So I chucked it into the lathe and stoned with a very fine hone stone (about like crocus cloth) and that did it.





JFA