AR 15 Bolt Tail

[prepare]

For the purpose of gauging what are the GO, NO-GO OD dimension specs for the bolt tail?

[lysander]

For the purpose of gauging what are the GO, NO-GO OD dimension specs for the bolt tail?

Diameter 0.2501 to 0.2505 for a length of 0.560, this must be concentric to the 0.5280 and 0.488 diameters to within 0.002" total.

[prepare]

Diameter 0.2501 to 0.2505 for a length of 0.560, this must be concentric to the 0.5280 and 0.488 diameters to within 0.002" total.

So less than 0.250 would be No-Go?

[Evil Black Rifle]

Does not comply to print? True. Non-functional or malfunction inducing? Probably not.

[Clint]

The bolt tail to carrier interface is one of the most critical fits the whole gas system.

If out of spec, it can definitely cause function issues on barrels with non-over sized gas ports.

It is a major source of gas leakage in many lower quality or nitrided bolt carrier groups.

Does not comply to print? True. Non-functional or malfunction inducing? Probably not.

[titsonritz]

The bolt tail to carrier interface is one of the most critical fits the whole gas system.

If out of spec, it can definitely cause function issues on barrels with non-over sized gas ports.

It is a major source of gas leakage in many lower quality or nitrided bolt carrier groups.

Major critical.

[Pappabear]

The key to winning this shit show is to buy from LMT etc...we have been through this but shit still needs to be fixed.

PB

[prepare]

What would be an appropriate No-Go gauge for the bolt tail?

[lysander]

The bolt tail to carrier interface is one of the most critical fits the whole gas system.

If out of spec, it can definitely cause function issues on barrels with non-over sized gas ports.

It is a major source of gas leakage in many lower quality or nitrided bolt carrier groups.

Actually, the drawing lists this dimension as a "Major" characteristic, not a "Critical" one.

Testing in 1971 showed that bolt tail with deviations as large as 0.0005" below specification (0.2496") will give acceptable performance.

"INVESTIGATION OF BOLT/BOLT CARRIER CLEARANCES IN THE M16A1 RIFLE," Grandy, Duffy, Horchler and Ehle, Frankford Arsenal, 1971
A brief series of tests and evaluations was made to provide experimental and analytical data to the U.S. Army Weapons Command In support of the M16A1 Rifle Bolt Dimension Investigation. Both firing tests and a mathematical model confirmed that bolts with diameters 0.0075 and 0.010 inch below specification do not give reliable performance because of excessive gas leakage between the outside bolt shaft diameter and its mating diameter in the bolt carrier. Normal rifle function occurred with bolts 0.0004 and 0.0005 inch below specifications.

What would be an appropriate No-Go gauge for the bolt tail?

For a new bolt - 0.2501"

For a used bolt discard if the diameter measures 0.2497"

EDIT: Critical characteristics are characteristics that, if not within specified tolerances, will result in dangerous or unsafe conditions. Major characteristics are characteristics, that if not within specified tolerances will result reduced reliability or durability. Minor characteristics are characteristics that, if not within specified tolerances, will result in reduced reliability or serviceability, but not to the degree of a Major characteristic. Mandatory characteristics are characteristics that must be as specified, but do not necessarily fall under the any of the above three characteristics.

Examples:
Believe it or not, the bolt does not have any Critical characteristics.
The width of the ejector groove is a Minor characteristic.
The material is a Mandatory characteristic.

[Evil Black Rifle]

Since most civvy uppers are over-gassed, it probably will be fine.

If the rifle is for self-defense, or job related, obviously a bolt that complies with the GI print dimensions should be obtained.