View Full Version : Is AR frame over engineered and over built??
From the posts in the GUN PICTURES..
I was always concerned with the AR upper & lower fame's ability to withstand abuse. I remember the M16's we had in Nam and how the frame would dissolve around the front pin from the sweat in our hands.
I figured that they were engineered for a mouse(5.56) round that did not need substantial "meat" in the material build..
Looks like I was off by a large margin.
I would personally rank it with a stout .45-70 load.
TR
Thanks TR for the 1st hand info.. I was thinking that if everyone compares the 50 wulf to a 45.70, then the recoil should be similar. But on the flip side, everything I read on the AA web site and forums says you can use a standard AR lower with stock USGI buffers & springs and not brake the frame??
The specs for an AR upper & lower frame must be way over engineered???
From the AR15 sub-fora: AlexanderArms, Industry Partner
Posted: 12/20/2012 4:14:08 PM EST
gonna order a 50 upper. and im just wondering what buffer and spring i need for it in a carbine stock?
AlexanderArms, Company Rep
The Beowulf will run happily on a regular carbine spring and buffer and does not demand any changes. Using an H2 will slightly smooth the action if you want to upgrade.
Posted: 12/23/2012 2:47:45 AM EST
What about mating it to a Bushmaster lower with an A2-style fixed stock? Can I still get away with not changing the spring or buffer?
AlexanderArms, Company Rep
Yes, as long as the spring and buffer are in spec they will work well. The Beowulf was designed around the rifle stock and buffer/spring set up.
Has anyone ever experienced AR frame failure, due to excessive recoil or other ammo related issues??
:munchin
PS: Ordered a case of ammo,, now the wait.. :mad:
SomethingWitty
02-11-2013, 14:14
No.
The AR uses a steel locking collar which the bolt locks into. The locking collar/locking lugs on the bolt are what contains the pressure of cartridge; the aluminum upper has nothing to do with containing the pressure. The take-down pins also have absolutely nothing to do with the with system itself; they basically serve the same function as the pins that hold the trigger on a bolt-action rifle. If you built an AR-15 upper without a gas-port, it would function just like a straight-pull bolt action rifle; Hell, you could even use it without a lower by using a ball peen hammer to strike the firing pin.
The force at which the bolt carrier is propelled backwards is a function of the gas-port dimension/location; and is basically a fixed value. There is no logical reason to have the bolt to cycle faster or sooner; doing so would be very dangerous.
The only reason that an AR-15/10 would shear at the front or rear take-down pins is because someone fell on it at just the right angle, or tried to use it as a pry bar/lever. Maybe there is some stress on the take-down pins holes because of some slop between the lower and the upper, but it is very miniscule.
No.
The only reason that an AR-15/10 would shear at the front or rear take-down pins is because someone fell on it at just the right angle, or tried to use it as a pry bar/lever. Maybe there is some stress on the take-down pins holes because of some slop between the lower and the upper, but it is very miniscule.
I know the mechanics of the AR action, thank you.
My question dealt with the kinetic energy after the bolt was unlocked and started the ejection. After the bolt is unlocked there is no metering of the rearward movement other the mass of the piston and resistance of the recoil spring.
In affect the 5.56 bolt must be creating the same kinetic energy as the 50 Beowulf,, or something will break???
Also,, I didn't say shear,, The frame actually dissolved from perspiration. 15 to 20 % of the M16 we carried had moderate to severe rot in the area of the front pin.
Maybe it was from that green bottle (bug juice) aftershave you were fond of using at the time... ;)
Maybe it was from that green bottle (bug juice) aftershave you were fond of using at the time... ;)
:D:D:D
SomethingWitty
02-11-2013, 20:43
I know the mechanics of the AR action, thank you.
My question dealt with the kinetic energy after the bolt was unlocked and started the ejection. After the bolt is unlocked there is no metering of the rearward movement other the mass of the piston and resistance of the recoil spring.
In affect the 5.56 bolt must be creating the same kinetic energy as the 50 Beowulf,, or something will break???
Also,, I didn't say shear,, The frame actually dissolved from perspiration. 15 to 20 % of the M16 we carried had moderate to severe rot in the area of the front pin.
I am not familiar with corrosion on early M16's.
However in regards to the second aspect, the force used, and the velocity that the bolt unlocks is going to be affected by gas port pressure, the size and location of the gas port, and the powder/bullet combination being used. Chamber pressure is not going to affect how the bolt unlocks. In other words, the gas-port on a .223 Remington is going to be a different size than the size of a 50 Beowolf (if it produces more gas port pressure).
Peregrino
02-11-2013, 20:52
JJ - Saw the same thing in El Sal during my first tour. Those were VN era rifles too. Once the anodizing had worn off, the acid in the Soldier's sweat was sufficient to eat the aluminum. Because of the way they carried the rifle it was common to see rot around the front of the mag well. One of the things I will say about the "Soldier of Fiction" crew from that time was that they provided a decent amount of weapons maintenance support, including repairing many those receivers by milling out the rotten part and filling the resulting hole with epoxy (I think it was actually JB Weld but that was a few years ago and my memory isn't what it was). During my second tour it was nice to see the US Govt finally replacing the decrepit crap the El Sal troops started the war with.
As for the strength of the receiver - it's more than sufficient. All the real stress is in the bolt/barrel extension lockup. A good upper to lower fit without any slack and it's a unitized system where recoil forces are transmitted directly back to the shooter with no slop to start knocking things loose. So long as chamber pressure, timing, and the pressure curve (mostly gas port pressure) fall within a fairly narrow range, the weapon design can handle just about any cartridge without self-destructing. When the pressures/timing doesn't fall within the acceptable range (as I'm working through issues with a 300BLK that is just barely outside the ideal parameters) then you've got a lot of compromises to make to restore reliability.
An internet search should give you a decent idea about potential problems. If you are going to shoot it a lot, I would recommend acquiring a spare bolt and inspecting the active one at regular intervals for cracking in/around the locking lugs. Personally, I doubt you really need to be worried about basic action strength. If I understand you correctly, you're expressing concern about the bolt carrier velocity during the extract/eject portion of the cycle. That's a function of gas port pressure. Ideal is 19,000 psi; IIRC the M4 carbine is around 24,000 psi hence the extraction problems that plague some weapons. The .50 Beowulf is somewhere in the middle, depending on the load you're using. FWIW, the system is somewhat self-regulating. Once the action has unlocked and the bolt carrier key has cleared the gas tube, excess gas is dumped into the receiver - no more acceleration w/potential for excess velocity and associated battering.
Justinmd
03-11-2013, 20:05
There's a lot to consider when looking at the strength of the AR platform. I personally do not consider them overbuilt by any stretch. However the fact the large bores have been in use for awhile tends to show it will be ok.
Breech strength or "integrity" of a rifle depends on a few different factors. One is the overall peak chamber pressure of the round and directly related to that is the amount of that pressure that gets translated into force on the bolt head. The larger cases will transmit more force to the bolt because of the increased surface area however the large bore round may operate at lower chamber pressures.
How this force is handled depends largely on two elements, the amount of area of the lugs that is in contact with the lug abutments in the barrel extension, and the amount of area that attaches each lug to the bolt body (called shear area) along with the shape of the lug (length vs width vs height)
AR's are notoriously weak in the bolt area, in almost every .gov/.mil endurance test I've read on the M16/M4 at least one lug shears off if not two (those being closes to the extractor. This is with 5.56 ammo. Over the years various blends of steel have been tried to combat this problem (one mitigating factor is the bolt getting heated up so much)
Additionally AR uppers are not super strong for hanging off a long heavy barrel and this is why you see the beefed up aftermarket uppers (and things like Mark Larue's OBR barrel attachement).
If you look at the path of the recoil forces in an AR, it seems almost surprising it holds up. The force is transferred through the bolt to the barrel extension to the upper receiver in an area that is not particularly thick, is threaded, and has a notch cut in it creating a stress riser. From there is is transferred to the lower through either one or both takedown pins, or by direct contact with the buffer tube tower which also doesn't seem very strong to me, then through a threaded joint to the buffer tube, then to (in some cases) a single steel pin in a hole in the aluminum tube.
The kinetic energy of the bolt carrier assembly can be controlled to a large degree to be similar whether in an 5.56 or 458 SOCOM.
Justin