Armalite's Deprecated Lug- Thoughts?

[lamarbrog]

The AR-15 bolt can be thought of as starting with eight lugs around its circumference. One is cut away to provide room for the extractor. During firing, this asymmetry causes the bolt to warp slightly due to uneven support and put approximately 80% of the force load on the two lugs adjacent to the extractor. It's no surprise that the majority of bolt lug failures occur at one of these two lugs. ArmaLite's solution is to balance the force by only using
six of the seven lugs for load-bearing, restoring symmetry under load. This change is accomplished by simply shortening the back of the lug opposite the extractor so it does not engage the barrel extension. ArmaLite's engineering study determined that this change alone reduces the stress on the two problem lugs by about 40%.
Link to an article referencing what I am talking about. (http://demigodllc.com/articles/the-armalite-m15a2-infantry-rifle/)

What is the general consensus on this alteration? I just heard about it a few minutes ago- not sure how it slipped past me.

I am usually one who is all about stock components. Not a fan of the fancy coatings, piston kits, and so on. What I like about this, though, is that doesn't affect parts interchangeability. Stock bolts and altered bolts can replace each other without issue it seems.

Of course, on a fighting rifle preventative maintenance usually dictates that bolts are replaced before they get to that 10.000 rounds mark where they are exceptionally prone to failure... but it is a simple enough modification that if it works I don't see what the harm is in potentially increasing bolt life.

Does anyone have more information on this? Numerical data? Quite honestly, this is the first AR alteration that has gotten me excited in quite a while. Does Armalite have a patent on it? Are any other companies doing this yet? Am I an idiot if I briefly contemplated modifying my own bolt?

[lamarbrog]

I guess not quite as interesting as I thought it was...

[Paulo_Santos]

It does sound interesting. As long as it doesn't change the way the AR operates, I don't see anything wrong with these bolts.

[lamarbrog]

I really just need to start sending you PMs instead of posting threads... I think you're the only one who actually pays any attention to them anyway. [BD]

[Paulo_Santos]

I really just need to start sending you PMs instead of posting threads... I think you're the only one who actually pays any attention to them anyway. [BD]

LOL. How much are those bolts anyway? I couldn't find a price on them.

[lamarbrog]

My understanding is that all Armalite M15 bolts are configured in this manner. If that's true, it looks like they're $73.

http://www.armalite.com/ItemForm.aspx?item=EB0040&ReturnUrl=Categories2.aspx?Category=159d4b3c-5072-4c09-9823-0396f4668a10

It says they're 158 Carpenter alloy... but makes no mention of HPT, MPI, shot peening, etc.

Edit: called Armalite, spoke with "Tim". All of their AR10 and M15 bolts have their patented "Relieved Lug".

[AR-10]

Looks like a good concept.

I'm ordering one of Harrison Beene's Superbolts for my Recce, he designed his bolt with rounded lugs rather than square which supposedly makes for a stronger bolt.

I will post some side-by-side pics here for comparison when I receive it.

[M. Gale]

The failure of the lugs adjacent to the extractor grove are not just a bolt design issue. The quality of both the barrel extension and bolt are critical to bolt life. If the working surfaces of the bolt and barrel extension are not cut properly, bolt thrust will not be equalized across the available lugs. The relieved lug design is suspicious due to the fact that it removes 28% of the load bearing surface even if they are structurally weaker.

[lamarbrog]

...... I have read this post at least five times, and still can't figure out how you're reaching your conclusion with the information given.


The failure of the lugs adjacent to the extractor grove are not just a bolt design issue.
No, they're not just a bolt design issue- but if the arrangement of the lugs is an issue, what is to be gained by not resolving it?



The quality of both the barrel extension and bolt are critical to bolt life. If the working surfaces of the bolt and barrel extension are not cut properly, bolt thrust will not be equalized across the available lugs.
No argument there. Inferior products fail more quickly. This is completely irrelevant to whether or not a relieved lug is a good idea. This entire section of your post deals with quality control issues, not design issues.


The relieved lug design is suspicious due to the fact that it removes 28% of the load bearing surface...
And here is where I can't figure out how you came to the conclusion that the relieved lug design is "suspicious". It removes a load bearing lug, yes. The average load per lug is increased, yes. However, the average is immaterial- what matters is the maximum load on any given lug, because once one lug fails the bolt is trash. The deprecated lug increased average load, but decreases max load on any one lug. I don't see how this is "suspicious".


... even if they are structurally weaker.
I give up- what does this even mean in the context?

[Volta]

"The average load per lug is increased, yes. However, the average is immaterial- what matters is the maximum load on any given lug, because once one lug fails the bolt is trash." Like breaking threads one versus twisting them together and combining their strength. I forget where I heard of this before but it peaked my interest. i would really like to see some test data. My next thought was that of an ar15 from a parallel universe. Instead of replacing one of eight lugs with an extractor, imagine a bolt with a smaller number of stronger, evenly spaced lugs(5 or 6) and the extractor goes in between them.

[lamarbrog]

"The average load per lug is increased, yes. However, the average is immaterial- what matters is the maximum load on any given lug, because once one lug fails the bolt is trash." Like breaking threads one versus twisting them together and combining their strength. I forget where I heard of this before but it peaked my interest. i would really like to see some test data. My next thought was that of an ar15 from a parallel universe. Instead of replacing one of eight lugs with an extractor, imagine a bolt with a smaller number of stronger, evenly spaced lugs(5 or 6) and the extractor goes in between them.

I, too, have thought about the possible benefits of, say, three lugs. However, the problem with this is it creates parts which are not interchangeable.

[M. Gale]

"The average load per lug is increased, yes. However, the average is immaterial- what matters is the maximum load on any given lug, because once one lug fails the bolt is trash." Like breaking threads one versus twisting them together and combining their strength. I forget where I heard of this before but it peaked my interest. i would really like to see some test data. My next thought was that of an ar15 from a parallel universe. Instead of replacing one of eight lugs with an extractor, imagine a bolt with a smaller number of stronger, evenly spaced lugs(5 or 6) and the extractor goes in between them.

The AR-15 family of weapons will never have any bolt design other than an 8 lug pattern. In the 50+ years the weapon system has been around, every modification you can think of has been tried, at least twice. The 8 lug design unlocks in only 22.5 degrees of bolt rotation. A 6 lug design would increase the the rotation to 30 degrees and a 5 lug design would take 36 degrees to unlock. The length of the cam pin track in the bolt carrier and the clearance required in the upper receiver for the cam pin would jump dramatically. Building a bolt from Aermet 100 would solve the only "problem" remaining with the weapon.