A member has posed the question about how to properly install and index the gas block on the AR. Not hard to do, but there are a couple of things to look for.
The first thing is to make sure the gas block is the proper diameter for your barrel. (There are four different diameters. This is easy to determine with a caliper.)
And make sure the gas tube is the right length for your gas system. (There are four different gas system lengths.)
Install the gas tube roll pin in the gas block. Just start the pin in the hole so it doesn't fall out. Use needle nose pliers or vice grips. Geiselle make a special tool for this for those that do a lot of barrel work.
Next insert the gas tube with the gas hole facing the barrel. If it is installed upside down, the gun won't cycle. (Seems obvious, but I've seen it done.)
You can use the appropriate size drill bit shank as a slave pin to align the holes. Gently tap the roll pin in until flush.
There is a tool called the Gas Block Genie to help mark the barrel to properly align the holes in the gas block and barrel. But you can also do it with a straight edge pocket ruler and a Sharpie.
Low Profile Gas Blocks:
If you don't have the Genie tool, take a straight edge like a 6" pocket ruler and draw a line with the sharpie on the top of the Barrel going forward from the gas port to an inch or so past when the front edge of the gas block will be. Eye ball it and make sure it's straight. Now take the gas block and find an allen key and insert the short leg into the gas block port with the long end to the front. Eyeball it and get it straight. Mark the front face of the gas block with the Sharpie to have an index mark to match up with the mark you made on the barrel.
Some barrels may have a dimple to locate the set screw on the bottom of the gas block. If so, this should align everything as the fixture to drill the dimple indexes off the gas port. if not, use the above procedure. The hole in the gas block is larger than the gas port in the barrel, so a small amount of misalignment can be tolerated. But keeping with FTQ, we try to get it as close to right as possible.
Gas Blocks with a Pic Rail:
If your gas block has a picatinny rail on top, you can use it to establish alignment. These gas blocks typically clamp to the barrel with 2-3 allen screws, so dimples won't help.
Use two 6' pocket rulers and lay one across the top of the upper receiver. Lay the other across the top of the gas block rail. Get behind the rifle and see if the two rulers are parallel.
A2 FSB:
These are typically secured with two taper pins, so wherever things were when the holes were drilled is what you're stuck with. Usually not a problem unless you install a FSB that did not come with the barrel from the factory.
In High Power, the taper pin method is not used as it creates a stress point in the barrel that is not good for accuracy. So, High Power gunsmiths D&T the taper pin holes in the FSB for set screws and mill four little flats on the barrel as a place for them to seat. With this arrangement, the stress issue is eliminated, and you can fine tune the windage zero so the receiver witness marks show zero.
To do this, you need a piece of plate glass. Invert the rifle and set it on the glass. Use only two opposing screws to get all four contact points touching the glass. Easy to tell. If the rifle doesn't rock when you wiggle it, you're GTG. Then tighten all four screws.
Barrel nuts:
Some barrel nuts may have holes or notches that the gas tube goes through before entering the upper receiver. If these holes/notches are not in proper alignment, the gas tube with not be in alignment with the BCG gas key, even if it seems to pass through the upper easily.
Barrel nuts that have wrench flats do not engage the gas tube and make things a little easier.
Regardless of the type of barrel nut, you want to check to make sure the BCG gas key slides over the gas tube as easy as possible. The gas key has a internal taper and will self align if it's off a little. We want to ensure that this alignment is as good as possible.
The check this, strip your BCG and slowly slide it into the upper. You should just barely be able to tell when the key slides over the tube. If the BCG stops, but can be nudged a little to go forward, the tube and key are not aligned. If it's off a lot, the BC won't go all the way forward.
Here's a key and tube that is obviously not aligned.
To determine which way the tube needs to move, deflect it at the mid point. Realize the end that goes in the key will move in the opposite direction that you are deflecting the middle.
If you have a notched or hole barrel nut, give the nut a tweak in the direction the end needs to move. This is trial and error and make take a few nudges to get it right. In the end, you want to barely be able to tell when the key over rides the tube. Yes, it will function if the tube is slightly off and the key hits it a little off center, but what happens is the side of the gas tube will wear until it doesn't seal well and at first you will notice the bolt not locking back on the last round. Eventually, it will short stroke and not pick up the next round in the mag. may take 1000 rounds before it gets there. But the more it's off, the faster it will wear.
Barrel nuts with the wrench flats are much less prone to misalignment. If you have a low profile gas block and no barrel dimples, you can rotate the gas block a tiny bit and see if that helps. If not, you can try and tweak the gas tube a little. This is a slippery slope as it's easy to overdo it and then have to try and bend it back.
Adjustable Gas Blocks:
These install the same as the non-adjustable.
As mentioned, the AR gas system is over gassed for when the rifle gets extremely dirty in combat. Since most of us here do not subject our rifles to battlefield conditions, you can turn down the gas setting and the rifle will be a little softer to shoot.
Start with the gas screw turned in all the way. Back it out 1 turn.
Load 1 round in the mag and fire. The rifle may or may not eject, but not lock back.
Repeat the 1 round and shoot and open the gas setting half a turn each time until the bolt locks to the rear.
Then open the setting another half turn for when the rifle gets dirty. As long as the gun has proper lubrication, this should be all the adjusting needed.
Clean and lube the rifle every few hundred rounds and you should be fine.
The first thing is to make sure the gas block is the proper diameter for your barrel. (There are four different diameters. This is easy to determine with a caliper.)
And make sure the gas tube is the right length for your gas system. (There are four different gas system lengths.)
Install the gas tube roll pin in the gas block. Just start the pin in the hole so it doesn't fall out. Use needle nose pliers or vice grips. Geiselle make a special tool for this for those that do a lot of barrel work.
Next insert the gas tube with the gas hole facing the barrel. If it is installed upside down, the gun won't cycle. (Seems obvious, but I've seen it done.)
You can use the appropriate size drill bit shank as a slave pin to align the holes. Gently tap the roll pin in until flush.
There is a tool called the Gas Block Genie to help mark the barrel to properly align the holes in the gas block and barrel. But you can also do it with a straight edge pocket ruler and a Sharpie.
Low Profile Gas Blocks:
If you don't have the Genie tool, take a straight edge like a 6" pocket ruler and draw a line with the sharpie on the top of the Barrel going forward from the gas port to an inch or so past when the front edge of the gas block will be. Eye ball it and make sure it's straight. Now take the gas block and find an allen key and insert the short leg into the gas block port with the long end to the front. Eyeball it and get it straight. Mark the front face of the gas block with the Sharpie to have an index mark to match up with the mark you made on the barrel.
Some barrels may have a dimple to locate the set screw on the bottom of the gas block. If so, this should align everything as the fixture to drill the dimple indexes off the gas port. if not, use the above procedure. The hole in the gas block is larger than the gas port in the barrel, so a small amount of misalignment can be tolerated. But keeping with FTQ, we try to get it as close to right as possible.
Gas Blocks with a Pic Rail:
If your gas block has a picatinny rail on top, you can use it to establish alignment. These gas blocks typically clamp to the barrel with 2-3 allen screws, so dimples won't help.
Use two 6' pocket rulers and lay one across the top of the upper receiver. Lay the other across the top of the gas block rail. Get behind the rifle and see if the two rulers are parallel.
A2 FSB:
These are typically secured with two taper pins, so wherever things were when the holes were drilled is what you're stuck with. Usually not a problem unless you install a FSB that did not come with the barrel from the factory.
In High Power, the taper pin method is not used as it creates a stress point in the barrel that is not good for accuracy. So, High Power gunsmiths D&T the taper pin holes in the FSB for set screws and mill four little flats on the barrel as a place for them to seat. With this arrangement, the stress issue is eliminated, and you can fine tune the windage zero so the receiver witness marks show zero.
To do this, you need a piece of plate glass. Invert the rifle and set it on the glass. Use only two opposing screws to get all four contact points touching the glass. Easy to tell. If the rifle doesn't rock when you wiggle it, you're GTG. Then tighten all four screws.
Barrel nuts:
Some barrel nuts may have holes or notches that the gas tube goes through before entering the upper receiver. If these holes/notches are not in proper alignment, the gas tube with not be in alignment with the BCG gas key, even if it seems to pass through the upper easily.
Barrel nuts that have wrench flats do not engage the gas tube and make things a little easier.
Regardless of the type of barrel nut, you want to check to make sure the BCG gas key slides over the gas tube as easy as possible. The gas key has a internal taper and will self align if it's off a little. We want to ensure that this alignment is as good as possible.
The check this, strip your BCG and slowly slide it into the upper. You should just barely be able to tell when the key slides over the tube. If the BCG stops, but can be nudged a little to go forward, the tube and key are not aligned. If it's off a lot, the BC won't go all the way forward.
Here's a key and tube that is obviously not aligned.
To determine which way the tube needs to move, deflect it at the mid point. Realize the end that goes in the key will move in the opposite direction that you are deflecting the middle.
If you have a notched or hole barrel nut, give the nut a tweak in the direction the end needs to move. This is trial and error and make take a few nudges to get it right. In the end, you want to barely be able to tell when the key over rides the tube. Yes, it will function if the tube is slightly off and the key hits it a little off center, but what happens is the side of the gas tube will wear until it doesn't seal well and at first you will notice the bolt not locking back on the last round. Eventually, it will short stroke and not pick up the next round in the mag. may take 1000 rounds before it gets there. But the more it's off, the faster it will wear.
Barrel nuts with the wrench flats are much less prone to misalignment. If you have a low profile gas block and no barrel dimples, you can rotate the gas block a tiny bit and see if that helps. If not, you can try and tweak the gas tube a little. This is a slippery slope as it's easy to overdo it and then have to try and bend it back.
Adjustable Gas Blocks:
These install the same as the non-adjustable.
As mentioned, the AR gas system is over gassed for when the rifle gets extremely dirty in combat. Since most of us here do not subject our rifles to battlefield conditions, you can turn down the gas setting and the rifle will be a little softer to shoot.
Start with the gas screw turned in all the way. Back it out 1 turn.
Load 1 round in the mag and fire. The rifle may or may not eject, but not lock back.
Repeat the 1 round and shoot and open the gas setting half a turn each time until the bolt locks to the rear.
Then open the setting another half turn for when the rifle gets dirty. As long as the gun has proper lubrication, this should be all the adjusting needed.
Clean and lube the rifle every few hundred rounds and you should be fine.