JeepHammer
SHOOTER
Moderators, if you find ANYTHING factually wrong with this information, correct or remove it immediately PLEASE!
Gunsmithing, building accuracy rifles was part of my job in the Marine Corps,
Marines build, or rebuild their own special weapons, and this is a safety issue that I don't see much of anything posted about.
Upper receiver are forged or cast by manufacturers.
Threading the front of that upper, where the barrel mates and a barrel nut holds the barrel in place on the upper MUST be square with the bore of the upper,
The forging & threading process will distort the mating surface for the barrel.
99 times out of 100 YOU will have to square this surface.
On the left, a new, forged, milspec upper receiver.
On the right is another new, forged, milspec receiver that's in the process of having the upper/front barrel mating surface lapped (ground) square to the bore.
This is so you know which surface needs correction...
The above picture shows the high spots being worn away first (shiny) while the other side isn't touched yet.
With a bar centered in the Bolt Carrier Group (BCG) bore of the upper receiver to guide the material removal exactly square with bore,
This allows the bolt carrier/bolt to line directly up with the chamber nut locking lugs.
If the barrel is cocked sideways, or an air gap under part of the barrel mating surface,
The bolt lugs simply CAN NOT engage the chamber nut squarely, and the firing pressure will be transferred to one or two lugs.
This is a VERY good way to sheer the chamber nut or bolt lugs... VERY DANGEROUS putting that much pressure on just one or two lugs.
This is a picture of the barrel mating surface completely lapped (ground) square (90*) to the center line of the upper receiver bore.
Notice the polished ring isn't the same width all the way around...
I only take enough off to square this surface with the bore.
*IF* you have chambering issues (too much needed to be removed) then shims are commercially available and perfectly acceptable to use. This is actually the way most high accuracy rifles are adjusted for the Nth degree adjustments.
The easiest tool for facing this surface off is a commercially available, and inexpensive gunsmith tool called (ironically) a lapping bar.
Just measure the bore shank and make sure the one you get doesn't exceed the maximum diameter of upper specifications.
You don't want to oversize the BCG bore while facing off the upper...
In this picture, you see lapping compound (grit) for the cutting g surface,
Lubrication for the pilot bar that fits into upper receiver, and the lapping tool itself (stainless steel bar).
Above are untouched & finished upper headed for accuracy builds.
By aligning the BCG with the barrel/Chamber nut, the locking lugs can make contact, keep contact all the way around during firing.
The locking lug issues usually present as failure to eject, failure to lock into battery instead of catastrophic failure, but don't discount catastrophic failure.
As lugs with the most pressure applied to them, they flatten, deform, mushroom, and sometimes sheer off.
The added deformation often shows up as malfunctions, from added friction and poorly fitting parts, it's an indicator the lugs are failing.
-
Since I make money gunsmithing, I don't normally give away tips on how to build a better rifle, but this is a safety issue that's EASY to correct by even a home builder.
*IF* you are going to forego a qualified gunsmith (not recommended) the lapping bar & lapping compound are inexpensive & commonly available in any 'Gunsmith Tools' section of major suppliers.
You can hand turn the lapping bar, or use a LOW SPEED drill motor, and it takes less than 30 minutes even by hand.
Lubrication on the guide, lapping compound under the head, rotate until you get a polished ring all the way around -- done.
Gunsmithing, building accuracy rifles was part of my job in the Marine Corps,
Marines build, or rebuild their own special weapons, and this is a safety issue that I don't see much of anything posted about.
Upper receiver are forged or cast by manufacturers.
Threading the front of that upper, where the barrel mates and a barrel nut holds the barrel in place on the upper MUST be square with the bore of the upper,
The forging & threading process will distort the mating surface for the barrel.
99 times out of 100 YOU will have to square this surface.
On the left, a new, forged, milspec upper receiver.
On the right is another new, forged, milspec receiver that's in the process of having the upper/front barrel mating surface lapped (ground) square to the bore.
This is so you know which surface needs correction...
The above picture shows the high spots being worn away first (shiny) while the other side isn't touched yet.
With a bar centered in the Bolt Carrier Group (BCG) bore of the upper receiver to guide the material removal exactly square with bore,
This allows the bolt carrier/bolt to line directly up with the chamber nut locking lugs.
If the barrel is cocked sideways, or an air gap under part of the barrel mating surface,
The bolt lugs simply CAN NOT engage the chamber nut squarely, and the firing pressure will be transferred to one or two lugs.
This is a VERY good way to sheer the chamber nut or bolt lugs... VERY DANGEROUS putting that much pressure on just one or two lugs.
This is a picture of the barrel mating surface completely lapped (ground) square (90*) to the center line of the upper receiver bore.
Notice the polished ring isn't the same width all the way around...
I only take enough off to square this surface with the bore.
*IF* you have chambering issues (too much needed to be removed) then shims are commercially available and perfectly acceptable to use. This is actually the way most high accuracy rifles are adjusted for the Nth degree adjustments.
The easiest tool for facing this surface off is a commercially available, and inexpensive gunsmith tool called (ironically) a lapping bar.
Just measure the bore shank and make sure the one you get doesn't exceed the maximum diameter of upper specifications.
You don't want to oversize the BCG bore while facing off the upper...
In this picture, you see lapping compound (grit) for the cutting g surface,
Lubrication for the pilot bar that fits into upper receiver, and the lapping tool itself (stainless steel bar).
Above are untouched & finished upper headed for accuracy builds.
By aligning the BCG with the barrel/Chamber nut, the locking lugs can make contact, keep contact all the way around during firing.
The locking lug issues usually present as failure to eject, failure to lock into battery instead of catastrophic failure, but don't discount catastrophic failure.
As lugs with the most pressure applied to them, they flatten, deform, mushroom, and sometimes sheer off.
The added deformation often shows up as malfunctions, from added friction and poorly fitting parts, it's an indicator the lugs are failing.
-
Since I make money gunsmithing, I don't normally give away tips on how to build a better rifle, but this is a safety issue that's EASY to correct by even a home builder.
*IF* you are going to forego a qualified gunsmith (not recommended) the lapping bar & lapping compound are inexpensive & commonly available in any 'Gunsmith Tools' section of major suppliers.
You can hand turn the lapping bar, or use a LOW SPEED drill motor, and it takes less than 30 minutes even by hand.
Lubrication on the guide, lapping compound under the head, rotate until you get a polished ring all the way around -- done.
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