i am thinking of doing an AR10 SBR build. i am looking at bolts and they all seem to be the “lesser” 9310 material unlike the desired c-158 for 5.56. is this because the AR10 isn’t standardized milspec and therefore the manufactures do not feel the need to go with c-158?
9310 isn’t necessarily lower grade than C-158, its just available in smaller quantities than c-158 (but cost per pound is more). Also the larger dimension of the ar10 bolt means that there is more material for added strength so c-158 may be unnecessary.
Also… the bolt on an AR-10 is a ham hock compared to the finer AR-15… and subjected to no more PSI.
I don’t know what LMT’s AR10 bolt is made from, but that thing takes a filthy beating from Pappabear, and doesn’t skip a beat.
The pressure is the same, but the area exposed to the pressure is greater.
This means more force on the lugs. Assuming the lugs area is scaled up the same % as as the surface area expised to the pressure is, the pressure on the lugs should be the same as a 5.56 version.
At least thats me thinking through it mentally, can someone confirm or correct this?
Spreading the pressure out over a larger bolt face will reduce stress on the lugs. Using lugs with greater area will further reduce stress.
Place a nail against a 2x4 and strike it with a hammer. All the force is concentrated into a very small area and the nail is driven into the wood.
Now, strike the 2x4 using the same hammer using the same force. The blow will leave a dent in the wood, but penetration is shallow. The force of the blow is spread out over a larger area.
Yes, the pressure is similar, bu the larger area of the .308 case head results in a larger bolt thrust.
It’s about 40% more. However, the bolt is beefier, proportionally speaking…
Not quite true, the load on the bolt face has to be transmitted through the bolt to the lugs, so the surface area of the face has little or no relation to the stress in the lugs. Take for example the Krag bolt with its one lug. The stress in the one lug is higher that that of a Mauser with about twice the locking area, even though the Krag case head is much larger than that of a 8mm Mauser. Primarily, we are interested in the total shear area of the lugs, and a complex calculation to figure out the bending stress of the lugs.
The area of consideration on the AR design the area is at the base of the lug, seen in orange or red:
(Incidentally, red indicates that the stress has exceeded the yield limit of the material. This is why AR bolts are subject to low cycle fatigue.)
****ing love it. Proving once again why engineering isn’t something you learn from hitting a nail with a hammer and then comparing what happens when you hit a 2x4 with the same hammer.
Same pressure with more area (bolt face) results in higher force. Assuming the lugs are the same size, this means higher force and stress on the lugs.
As lysander said, the greater lug area brings stress back down.
Your nail analogy is same force, increased psi. The 308 produces more force, its the same principal that allows a 308 to throw a 147gr projectile the same velocity as a 223 does a 55gr.
Edit: maybe im using the wrong term. Im saying bolt face meaning where the case and bolt meet. I think you are refering to lug/barrel extension interface.
Quijote, since you’re so clever, perhaps you can tell me what the flaw is in my thinking.
Lysander, Megademic or anyone else, don’t help help. I want Mr. “Quickset” to explain it to me. This isn’t a trick question, I did make a mistake.
While we are on the topic of lug loads and stress(same thing?) is the chamfered area on the lugs to assist in relieving that as it unlocks or to aid in locking?
Load and stress are closely related but not the same. Load is measured in pounds (at least in the imperial system), and stress is load divided by the area supporting that load or lbs/square inch.
As the to the second part - Are you referring to the small angled cuts on the counter-clockwise, rear corner of the bolts locking lugs? Those are there just to ensure when the bolt rotates is does not hang up on the barrel extension lugs and allows the bolt to cam itself into locked position.
If you are referring to another feature of the bolt, you’re going to have to be more precise in its location, a picture perhaps?
The AR design really does not need “help” during unlocking. In the AR specifically, because the bolt is the gas piston and the bolt carrier the gas cylinder, the load on the locking lugs is very close to zero during the actual act of unlocking, provided the gas system is properly timed. Because there is a time delay between the maximum chamber pressure, the gas port being uncovered, and the time it takes the gas to flow down the gas tube a fill the carrier cavity, the chamber pressure has dropped considerably. The force pushing the carrier backwards (gas pressure in the carrier cavity times the area of the bolt tail flange) also pushes the bolt forward, and this force is very close to the remaining chamber pressure times the area of the case head.
The difference between my LMT MWS BCG and my BA 308 BCG is very noticeable , in hand and in the rifle. VERYnoticeable. According to LMT the 308 BCG is made from the same material as the LMT Enhanced 5.56 BCG with some secret squirrel coating, and even more importantly the same principles of design as the E-BCG. That’s just good news for everyone. (Who has one) I think the bolt is chromed or something. I recall reading it somewhere. (Probably an internet lie.)What I can tell you is that I’ve run my LMT MWS upper hard. Good ammo - cheap crap ammo and it just cycles and cycles with a can without a can. Like an energizer bunny. It cleans up better than any NiB BCG I’ve cleaned by a million miles.
If I build another AR-10. I would without question invest in the LMT BCG. As it stands one of the reasons I bought an MWS Upper was so when I could swap barrels and give me another purpose/better capability.
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Lysander, great, thank you. Yes the locking phase, explains it well.
C158 and 9310 are close in strength but 9310 has a better charpy value(stronger at the notch). More important than material is proper heat treat. Years ago a company made bolts using 9310 but made the mistake of heat treating them so hard they were brittle. Same thing has been going on for over 15 years with one company using C158.
Compressive strength of the bolt is figured by area of the bottom of the lug x7(number of lugs)x the yield strength of the material. Figuring the strength of the lugs really takes a program. It isn’t as easy as a simple compression or tensile formula.
The original AR10 bolts were 8620, later changed to c158. I made ours from 9310 after the results we had from the AR15 bolts. AR10 bolts made from any of the 3 materials above when properly heat treated are plenty strong enough to handle the thrust from a 308.