BluntForceTrauma Wrote:Reference the photo of the cutaway 5.56 chamber in Page 3, Post #51. A 5.56 chamber first has a 120° cone, or ramp, cut into the breech leading into the chamber, and then the edges are radiused .0625. So, when visually examining the breech, you'll see the flat surface of the breech and then a relatively wide "circle" that represents the angled cone and the radius around the chamber mouth. The smaller that "circle" around the chamber mouth representing a smaller angle and radius the more supported the case web area.
Attached is a completely random and unrelated photo I don't know what cartridges these barrels are chambering; I got the photo by Googling "AR15 feed ramps" but the photo gives a rough side-by-side comparison of the visual differences of different "cone" sizes. The cone on the right, for example, is smaller than the left.
So I forgot that you had some data here regarding the feed cone specs. And it's very helpful in this analysis. But it's not perfect. So I have to make some assumptions which I will clarify along the way. Here goes...
To start, I've been wanting an approximate size of the feed cone depth because we are all mostly in agreement that this is the unsupported area of the case and the failure occurred at an unsupported area. Also remember that Bill Alexander himself stated that he had to reduce it for the grendel round. And per the note above..."the edges are radiused .0625"...I think this is a good starting approximation of the standard M16 feed cone depth which supposedly is the same for an Odinworks barrel. I'll note that it is not perfect, but I'm pretty sure the feed cone depth will be approximately close to this dimension.
Next, I want to compare this to the feed cone depth of an Alexander Arms barrel. But unfortunately, it was stated that this is proprietary information, so we will have to make an assumption here. And remember that Bill stated that this change was critical to the design because of the high pressure in the grendel round. So if it was this important, I think we could assume that he made at least a 30% improvement to reducing the size of the feed cone depth. So taking 70% of 0.0625", I am getting 0.7 x 0.0625" = 0.04375".
So now, let's take the difference between a standard M16 style feed cone depth and this assumed Alexander Arms feed cone. 0.0625" - 0.04375" = 0.01875". This is approximately how much more unsupported the OP's brass was when he switched to using his 12" Odinworks barrel which was reported to have the standard M16 feed cone design. Note also I'm assuming that his other barrels, which had no issues with this load, had feed cones more aligned to Alexander Arms style. That's why the load never failed on those barrels and it did on his Odinworks barrel.
Now let's compare this number to the bolt face depth being out of spec, which is one of LRRPF52's failure models. In past posts, we determined that an out of spec bolt face may expose the brass to become unsupported by an additional 0.005". This comes from the difference from an in spec bolt face to an out of spec bolt face. I'm not going to share the math again since it is in my last post since this one.
Comparing the two numbers above I get 0.01875" vs 0.005". Clearly, the feed cone depth issue is more significant than the out of spec bolt face issue. It exposes almost 4x more brass to being unsupported as compared to a bolt face being too deep.
So to me, based on this, we cannot just assume this issue was just the bolt face issue. The feed cone depth for an M16 barrel design is still a significant issue. It simply exposes more brass to being unsupported per my comparison above.
At this point I want to mention that the excessive head space issue is in the wrong direction with respect to exposing the brass to unsupported wall area. Excessive head space is at the shoulder end of the case and allows the round to go more into the chamber rather than out of it. As such, I'm not accounting for this in my math and moreover I don't think it is a viable failure model...but I'm open to being convinced otherwise.
Now as to the brass itself. It definitely plays a role here and cannot be dismissed as well. With more unsupported brass exposed, weaker brass will tend to fail more often, and stronger brass will tend to fail less. To this point, the first question I have is...would the OP have had the same failure with Hornady or another brass besides Lapua? To this issue, it is still undetermined. But it is entirely possible. I know some folks are saying no way it was due to Lapua brass, but I'm sorry that's not being very objective without numerical evidence. We have 3 threads on this forum alone that are of Lapua grendel brass failing. We have Bill Alexander saying that he designed the grendel working with Lapua brass and he outright states that he had to change the feed cone design to limit the amount of unsupported brass with it. We also have visual proof that Lapua has less overall brass around the case head. (And the only counterarguments to this are non-numerical claims..."Lapua is the gold standard of brass"..."I've used Lapua brass for years without problems"...etc.)
And finally, we have the OP using Lapua brass with an Odin barrel which has the M16 feed cone design that Bill Alexander intentionally redesigned for the grendel round.
So with all that said, all my earlier failure models are on the table and I'm in alignment with lazyengineer that it's a tolerance stacking issue...the barrel feed cone was too long, the brass was too weak*, there was too much powder, and (possibly) the bolt face was too deep. All these combined led to the failure.
*And note that by brass being too weak, I'm open to the fact that there may have been a defect in it and/or other factors besides that it was simply Lapua brass.
Lastly, I'm fine if anyone reading wants to refute this. That's part of having an open forum discussion! And I know we are all working on limited information and have to make assumptions to make progress. Also note that I'm simply trying to understand this failure for safety reasons. Mine and others. Also note that I'm trying to be as objective as possible using limited available numerical data. If you want to criticize me, please provide alternative failure models and provide some data and theory based on the laws of science that can actually be evaluated.