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FX Barrel Liner Indexing

No, you do not need to tear down the shroud and liner in order to index. You do need to remove the shroud and the threaded tensioner (10mm open end wrench), mark the end of the muzzle with a sharpie, then shoot once all is back together.
Repeat, moving the sharpie mark position each time. You should be able to move it with your fingers but sometimes it will require a plier to do so. Just use something like a piece of leather to protect the liner sides from galling.
There is no need to remove the liner.
It's somewhat tedious but has a lot of value.
If you try this without reassembly, it won't work as there are different harmonics at play if the liner is not tensioned and shrouded.

mike
 
No, you do not need to tear down the shroud and liner in order to index. You do need to remove the shroud and the threaded tensioner (10mm open end wrench), mark the end of the muzzle with a sharpie, then shoot once all is back together.
Repeat, moving the sharpie mark position each time. You should be able to move it with your fingers but sometimes it will require a plier to do so. Just use something like a piece of leather to protect the liner sides from galling.
There is no need to remove the liner.
It's somewhat tedious but has a lot of value.
If you try this without reassembly, it won't work as there are different harmonics at play if the liner is not tensioned and shrouded.

mike
I always wondered (while indexing my barrel) if it was possible to use a shooting bag and instead of doing all of that, would you be able to accomplish this by rotating your entire gun 90, 180, 270 degrees and using the crosshair axis to shoot at the same zero. I wonder if you could get a good idea of the barrel index by rotating the entire gun instead of removing/twisting the barrel.
 
I always wondered (while indexing my barrel) if it was possible to use a shooting bag and instead of doing all of that, would you be able to accomplish this by rotating your entire gun 90, 180, 270 degrees and using the crosshair axis to shoot at the same zero. I wonder if you could get a good idea of the barrel index by rotating the entire gun instead of removing/twisting the barrel.
Maybe just rotate the target
 
Can somebody here, please explain the process of indexing an fx impact. Can you see the red line on the liner without taking the whole damn thing apart? Just asking for a step by step, thank you to anyone who can answer this, in advance.
It appears there is usually a black mark on the outside of the barrel that should be discernable once the barrel nut is removed. I would mark the position of the barrel nut so it can be retorqued to the same position each time. Mark the position of the black mark as if it were on a clock face for reference, if you can't see one make a black mark with a sharpie, say at 12 oclock high. Removal of the barrel is NOT necessary or recommended as getting the O-rings back in exact position is hit or miss at best. Make a large target with a single aimpoint and shoot 2 or 3 rounds to see where it groups and write the clock position that produced that group. REMOVE barrel nut, rotate the barrel in a one hour increment clockwise, retighten barrel nut then shoot another group. Your group will change in reference to the aimpoint and previous group, write the barrel position down and the group it produced for reference then repeat the process till you do a full rotation of the barrel, then observe the pattern that the barrel has produced, it will be roundish, or an oval in shape. The result you are looking for is to find the barrel position that produced the highest point in the circle or oval and then set that as the finished position you want to reassembly with. This will supposedly give you the best performance between point of aim and scope correction. If you need more detail reference indexing on the forums in a general internet search, there are much better descriptions of the process and it's advantages than mine, good luck and we'll try to answer any questions that we can.
 
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Does anyone have a suggestion as to how to permanently mark the muzzle? After indexing years ago I scratched the index mark on the outside of the liner, but it’s not visible with the muzzle nut installed. I’ve used both paint and marker but they’re only good for one assembly. Since the liner rotates with muzzle nut installation I find it important to see the mark with the nut in place. Scratching the face of the muzzle would have to be a bad idea.
 
Does anyone have a suggestion as to how to permanently mark the muzzle? After indexing years ago I scratched the index mark on the outside of the liner, but it’s not visible with the muzzle nut installed. I’ve used both paint and marker but they’re only good for one assembly. Since the liner rotates with muzzle nut installation I find it important to see the mark with the nut in place. Scratching the face of the muzzle would have to be a bad idea.
I just used a permanent marker to mark the outside of the barrel and carry the mark around the front of the barrel. It is visible from inside the lock nut viewing from the outside.
 
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Yes yes I have read. I'm specifically asking about an fx impact m4. And I repeat from above must you tear down the entire barrel every time you want to rotate the barrel??
No sir, just rotate it a measurable amount and see how it changes point of impact. Find the highest possible point of impact and lock it down there.
 
It appears there is usually a black mark on the outside of the barrel that should be discernable once the barrel nut is removed. I would mark the position of the barrel nut so it can be retorqued to the same position each time. Mark the position of the black mark as if it were on a clock face for reference, if you can't see one make a black mark with a sharpie, say at 12 oclock high. Removal of the barrel is NOT necessary or recommended as getting the O-rings back in exact position is hit or miss at best. Make a large target with a single aimpoint and shoot 2 or 3 rounds to see where it groups and write the clock position that produced that group. REMOVE barrel nut, rotate the barrel in a one hour increment clockwise, retighten barrel nut then shoot another group. Your group will change in reference to the aimpoint and previous group, write the barrel position down and the group it produced for reference then repeat the process till you do a full rotation of the barrel, then observe the pattern that the barrel has produced, it will be roundish, or an oval in shape. The result you are looking for is to find the barrel position that produced the highest point in the circle or oval and then set that as the finished position you want to reassembly with. This will supposedly give you the best performance between point of aim and scope correction. If you need more detail reference indexing on the forums in a general internet search, there are much better descriptions of the process and it's advantages than mine, good luck and we'll try to answer any questions that we can.
I have an m4 .35 800mm barrel and I don't think I can pull this off.... Maybe I'm wrong though. Thank you very much for the awesome response!
 
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No, you do not need to tear down the shroud and liner in order to index. You do need to remove the shroud and the threaded tensioner (10mm open end wrench), mark the end of the muzzle with a sharpie, then shoot once all is back together.
Repeat, moving the sharpie mark position each time. You should be able to move it with your fingers but sometimes it will require a plier to do so. Just use something like a piece of leather to protect the liner sides from galling.
There is no need to remove the liner.
It's somewhat tedious but has a lot of value.
If you try this without reassembly, it won't work as there are different harmonics at play if the liner is not tensioned and shrouded.

mike
I'm having an issue I believe because I have an 800mm m4 barrel.... The .35 is different I think. You couldn't supply me with some photos could you? Thank you for this wonderful response though!
 
Okay.
This works for me, perhaps it will for you too....
I use a slug of the correct caliber of my rifle. It should have a flat base, a slight dish base will also work though.
Simply scribe with an awl to make a visible mark, then push the nose of the slug into the muzzle so that it's snug and won't move.
Do your indexing leaving the slug in place until you have decided that you know where within the 'clocking' you want the top dead center.
Still leaving the slug in place, then tighten the barrel nut into place noting where that slug mark is to assure proper alignment.
Done!
This is what it looks like from the muzzle end. You can see the etched mark that aligns with the red sharpie mark used for that alignment.

mike

Indexing Mark.jpg
 
Okay.
This works for me, perhaps it will for you too....
I use a slug of the correct caliber of my rifle. It should have a flat base, a slight dish base will also work though.
Simply scribe with an awl to make a visible mark, then push the nose of the slug into the muzzle so that it's snug and won't move.
Do your indexing leaving the slug in place until you have decided that you know where within the 'clocking' you want the top dead center.
Still leaving the slug in place, then tighten the barrel nut into place noting where that slug mark is to assure proper alignment.
Done!
This is what it looks like from the muzzle end. You can see the etched mark that aligns with the red sharpie mark used for that alignment.

mike

View attachment 535143
I'm sorry for my ignorance here. I can't quite grasp the purpose of the slug. And how does it not get stuck in there!?
 

dnye1986


The slug isn't pushed completely into the liner, only enough so that it sticks without movement. A slight push with the thumb is all it takes to do that. Since it's conical, it self-centers.
Once it's 'stuck' in the muzzle, then it becomes your marker for clocking, instead of trying to mark your liner end and erasing it with removing/reassembling the liner tensioner.
1.) Remove the tensioner from the liner/shroud.
2.) Mark the 'clock' positions on the side of the liner that is sticking out from the shroud (most go at least 4 clock turns: 12-3-6-9 o'clock for instance).
3.) Insert slug with the etched mark into the liner, NOT all the way in, just enough so that it doesn't move, and is in alignment with say 12 o'clock. It should be sticking out past the liner muzzle a tad.
4.) Reinsert the tensioner watching that the mark on the slug moves to the correct location (Top Dead Center).
5.) NOW, remove slug with tweezer (BE CERTAIN TO REMOVE SLUG BEFORE FIRING!!).
6.) CHECK TO ASSURE THAT THE SLUG HAS BEEN REMOVED!! Test fire to verify POI.
7.) Repeat until all four 'clock' positions have been test fired by moving the liner to Top Dead Center (12 o'clock to 3 o'clock, then to 6 o'clock, then to 9 o'clock) and aligning the slug mark to each new clock setting after moving to Top Dead Center.
8.) After getting the 'clocking' complete, decide which one is correct (normally the one that prints closest to Top Dead Center), then remove the marks on the side of the liner that don't work and leave the one mark that does (say it's 3 o'clock, so remove 12-6-9 o'clock marks).
From then on, after the need to remove your liner for any reason, always use that mark (the one 3 o'clock mark) to align with the slug mark and then tighten the tensioner to align to the Top Dead Center position (AGAIN, REMOVING SLUG BEFORE FIRING!!).
It's a little tough to explain, so if you still have questions, just ask or PM to clarify.

mike
 
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