BCG hitting gas tube

[Trumpinator]

My 1st AR10 build
Im building an Aero M5. The LPK & Charging handle are in shipping and should be here by Saturday. The Aero M5 BCG arrived today so I tried to slide it into the Aero M5 complete upper that I bought and its stopping as it hits the gas tube. Does the CH have to be installed for the BCG to go into battery?

 

[roostercogburn98]

No

Gas tube should slide into the port on the gas key. Might have to bend the gas tube slightly, but it should not be a hard stop for the bolt to close. Just push the carrier from the back and it should go into battery. Unless the gas tube is messed up terribly.

Pics help

 

[USK308]

My 1st AR10 build
Im building an Aero M5. The LPK & Charging handle are in shipping and should be here by Saturday. The Aero M5 BCG arrived today so I tried to slide it into the Aero M5 complete upper that I bought and its stopping as it hits the gas tube. Does the CH have to be installed for the BCG to go into battery?

Gas Tube should line up in the middle of the gas key slot (see pic); CH does not have to be installed, the BCG should slide over the gas tube (note: Aero M5 upper, Radian CH, Rifle Length gas tube; Also using an Aero BCG)

 

[Trumpinator]

Thanks. I'll see if I can get it lined up.

 

[USK308]

Thanks. I'll see if I can get it lined up.

Your BCG will slide all the way up to where it contacts the front of the CH; so allow that small amount and it should all be good to go!

Make sure gas block is properly set over gas port and that the gas tube is lined up as I pointed out in my pic; everything should be good to go.

If not, more details on items so the Hide can help diagnose & troubleshoot!

 

[Trumpinator]

Ok, so I was able to get my pinky onto the gas tube and gently pulled up just a smidge and the BCG dropped right in with no friction from the gas tube. (edited to keep on topic)

 

[USK308]

Ok, so I was able to get my pinky onto the gas tube and gently pulled up just a smig and the BCG dropped right in with no friction from the gas tube. Also got the trigger installed. The Trigger pins have a long side and short side, does it matter which way they go in? I installed them long side in first (from the groves). Heres some pics.

View attachment 8338676
View attachment 8338680

View attachment 8338681
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Your gas tube should NOT require your pinky to lift up. If the gas block / gas tube don't slide over the barrel and through the hole into the upper smoothly without interference, your barrel nut might not be properly timed. Every time you squeeze the trigger, your BCG will cycle and if the gas tube isn't allowing full movement, your rifle will give you fits (and possibly a whole lot more).

As for trigger & hammer pins, they can go in either side...shouldn't matter.

 

[mtrmn]

Many large frame ARs use, or attempt to use, AR15 gas tubes designed for AR15s. The gas blocks will usually work on both large and small frame ARs interchangeably. The large frames require a different height measurement above the bore. So when AR15 tubes are used on large frames, the rear end of the tube may not be parallel to the bore or the line of travel of the BCG. Due to the bends in the tube, the small frame tube will usually be pointing slightly up where it enters the gas key because it is bent for an AR15. Hence the reason you had to tweak it with your finger. It now enters the gas key, but it's still at an angle and can provide a small amount of drag resistance to the BCG going into full battery. It can also easily get out of position again.

You can remove the BCG and charging handle and look through the charging handle track from the rear of the upper using a good light. The end of the tube should be well centered like you're looking through a peep sight. If it's slightly high or to one side, you may need to remove the gas block/tube assembly so you can VERY slightly bend the rear end of the tube where it enters the key. OR get an AR10 gas tube of the proper length.

Once you get it where it looks centered properly from the rear, you can more easily test the amount of resistance provided by the gas tube/key interface by stripping the BCG down to the bare carrier. Turn the upper upside down, and slide the carrier into the upper. Point the barrel down slightly and allow the weight of the carrier to move it forward. You should be able to see and /or feel any resistance when the key meets the tube. You may also be able to see the tube move slightly if it's still rubbing when it enters the key.

All the above is my outlook on this common problem and is WHAT I DO to fix it. I would bet that the majority of ARs out there (large and small frame} make excessive contact in that area and nobody knows or cares as long as they don't have problems. I tend to obsess over these little details in an effort to make mine as reliable as possible without spending a fortune on them. YMMV

 

[Trumpinator]

Nicely detailed and articulated Mtrmn, Thank you. I'll take a look.

 

[mtrmn]

Nicely detailed and articulated Mtrmn, Thank you. I'll take a look.

Read through this forum sometime when you get bored. It's somewhat entertaining and informative at the same time. The third post in the linked thread has a picture of an AR15 tube vs a AR10 tube side by side.

Gas tubes, barrel gas ports, and buffer weight

This might end up as a long thread, but it shouldn't be too long. It's a lecture, or a sermon, depending on how you look at it, and I'm not looking for feedback on what I'm about to say. I'm publishing information. In reality, it's pretty damned simple, but manufacturers are making it difficult f...

forum.308ar.com

 

[mettee]

One thing I do before I install a barrel is check that the gas tube is bent correctly and it cleanly engages the gas key. I have had to bend a few gas tubes ever so slightly to get them to line up.

I just think it's part of building your own rifle and something everyone can do and should do. If you don't check it and it's off the tube wears prematurely.

 

[Trumpinator]

Read through this forum sometime when you get bored. It's somewhat entertaining and informative at the same time. The third post in the linked thread has a picture of an AR15 tube vs a AR10 tube side by side.

Gas tubes, barrel gas ports, and buffer weight

This might end up as a long thread, but it shouldn't be too long. It's a lecture, or a sermon, depending on how you look at it, and I'm not looking for feedback on what I'm about to say. I'm publishing information. In reality, it's pretty damned simple, but manufacturers are making it difficult f...

forum.308ar.com

Thanks, reading now.

 

[Trumpinator]

So after reading the 308ar forum thread on the gas system I can see why theres so many gas issues with the lr308. Its the wild west. Theres no standard.
When I started thinking about building my own ar10 thats all I kept reading about was people having gas issues. So I was aware and was the reason why I stuck with the same manufacture for all the parts. I wanted a Cryptic coated Mystic bcg but noticed it was an Ounce lighter than the Aero bcg and didnt feel like chasing a gas isssue due to the lighter carrier. And then I read on here about headspace and how you should match the bcg and upper/barrel when you can for reliability with the lr308 platform. I didnt know there were so many different gas tube options. Im glad I bought a factory assembled complete upper for my 1st ar10 build. That said I may pick up a 18" Criterion barrel and adj gas block down the road so knowing about this gas tube fiasco will certainly help when I do.

 

[B y r o n]

I sort of feel like the odd man out here. Correct me if I'm wrong. Normally the charging handle is placed in the upper receiver before the bolt/carrier is installed. The gas key rides in the groove of the charging handle and basically keeps it centered. If the charging handle is not installed the gas key can lean left or right which might not allow the gas tube to enter it.

As long as the gas block is properly aligned/not tilted the gas tube should be aimed straight down the upper receiver. If the gas block is tilted the gas tube will be out of alignment and again the gas key will hit the end of the tube.

Before bending the gas tube I would attempt to verify if it is hitting the gas key high or low, left or right. Put a charging handle in place and see if there is still a problem.

 

[supercorndogs]

I don't think the gas key to charging handle clearance is providing much as far as properly locating the BCG. The bearing surfaces are on the carrier and run in the bore of the upper receiver.

 

[hlee]

For those bending gas tubes, are you using a purpose built tube bending tool, or just winging it? I haven’t needed to bend one (yet), but I’m a bit concerned about kinking one…

 

[mettee]

Depends how much it's off, typically it's very little, and doing it by hand using a wood block or other object is successful.

You could use thr brownells tool to hold the gas block.

AR-15/M16/ 308 AR GAS TUBE WRENCH | Brownells

Clamps around those fragile, easy-to-bend gas tubes to prevent damage during both removal and installation and adds a convenient “handle.” SPECS: Steel, blued. Approximately 6" (15.2cm) long.

www.brownells.com

 

[mtrmn]

I sort of feel like the odd man out here. Correct me if I'm wrong. Normally the charging handle is placed in the upper receiver before the bolt/carrier is installed. The gas key rides in the groove of the charging handle and basically keeps it centered. If the charging handle is not installed the gas key can lean left or right which might not allow the gas tube to enter it.

As long as the gas block is properly aligned/not tilted the gas tube should be aimed straight down the upper receiver. If the gas block is tilted the gas tube will be out of alignment and again the gas key will hit the end of the tube.

Before bending the gas tube I would attempt to verify if it is hitting the gas key high or low, left or right. Put a charging handle in place and see if there is still a problem.

If you have a large frame AR that has a small frame gas tube installed, (like MANY seem to have) this adaptation normally causes the tube to rise slightly in relation to the bore centerline. Therefore the centerline of the gas tube is not parallel to the centerline of the bore like it should be, entering the gas key at a slight angle. I stole the picture I mentioned previously from the 308AR forum. Small frame gas tube on top and large frame on bottom, showing the difference in the bends of each. If it doesn't need bending by all means leave it alone, but if all it takes is a little tweak at the rear end of the tube it ain't gonna be nearbout enough to cause a kink.

 

[B y r o n]

I don't think the gas key to charging handle clearance is providing much as far as properly locating the BCG. The bearing surfaces are on the carrier and run in the bore of the upper receiver.

You are correct the charging handle has nothing to do with the locating of the bolt carrier group. However, without the charging handle in place the gas key can lean to the left or right instead of being held upright by the charging handle.

 

[mtrmn]

For those bending gas tubes, are you using a purpose built tube bending tool, or just winging it? I haven’t needed to bend one (yet), but I’m a bit concerned about kinking one…

I've had to bend a few AR15 tubes over the years to eliminate unnecessary friction, but on the large frames I wound up just replacing the tube with a large frame tube because the tubes I had were also too short.
The tubes I did bend only needed to move a few thousandths, so the bending was usually done while it was still installed in the gun. I would insert a pencil or some small block of wood for the tube to bear against at the point where I wanted the bend. (usually about 3/4 to 1 inch from the rear end of the tube) Then use some kind of non-marring lever to pry against the rear end of the tube to move ever so slightly for proper alignment so it wouldn't rub in the gas key. You can apply some prussian blue to the inside of the key to assist in determining where it hits. Usually you can see the tube move slightly and determine what side it's hitting this way.

 

[supercorndogs]

You are correct the charging handle has nothing to do with the locating of the bolt carrier group. However, without the charging handle in place the gas key can lean to the left or right instead of being held upright by the charging handle.

The gas key is bolted to the BCG. It should not touch the charging handle.

 

[msgriff]

The only gas tubes that I know of which are correctly bent for large frames are the ArmaLite AR-10 tubes. Those are longer and will not work with DPMS LR-308 pattern barrels. With a couple of exceptions, DPMS pattern barrels use AR-15 gas tubes, which are supposed to have an offset of 0.156" (+/- 0.005"). A large frame with a 0.750" gas journal using a Mil-Spec gas block needs an offset of ~0.220". Some gas tubes are easier to bend than others, and minor adjustments can usually be done by hand without tools. They do make tube holding pliers if you don't have enough grip strength. I always try to adjust the existing bends, go slow, and grip/clamp as close to the bend as possible.

ETA: Adjustable gas blocks typically require different offsets, and you may find that an AR-15 gas tube will not need adjustments for large frames.

 

[B y r o n]

The gas key is bolted to the BCG. It should not touch the charging handle.

I stand corrected. I just pulled out one of my AR's and removed the charging handle. I thought that the gas key kept the bolt upright but even without the charging handle in place I was not able to rotate the bolt. That means that I was wrong, the gas tube should be able to enter the gas key even without the charging handle in place. Since I've never tried this before, always inserting the charging handle into the upper receiver first, I believed that it kept the bolt from rotating, I was wrong. Sorry for any confusion I may have caused.