.458 SOCOM AR feeding issue

My friend asked me to look at his .458 sOCOM upper since he knows that I built one that runs well and his does not. If the magazine has two rounds in it, all is good - it will load the first round manually and the second round will load semi-automatically like it should. It will not work if there are three rounds in the magazine and the first round will get jammed partially into the chamber and will have a dent in the case. The issue repeats with various magazines as well so it seems something in the way the bolt strips the rounds is being affected by magazine spring pressure. Anybody have any experience with this or have any ideas of what to look at? Thanks for the useful replies!

Does either rifle have the late M855 feed ramp?

I am not sure what a M885 feed ramp is. The .458 I built was made with a .458 SOCOM barrel which included the threaded on barrel extension which also has the feed ramps. I sourced an upper receiver suitable for the larger cartridge and also the appropriate bolt carrier and bolt. The only problem I had with mine was gas and I had to open up the gas port in the barrel several times to get it to cycle. My friends is a factory built .458 SOCOM upper which came assembled with a upper receiver bolt and bolt carrier. I would think that the factory that built it (Bear Creek) would have used the correct feed ramp configuration.

Most aftermarket pieces and parts are not truly compatible with the 458 SOCOM and exhibit an array of mostly feeding a reliability issues. If you go to Tomix site you may can get an idea of what you may need to make it reliable.
I have a Tromix complete upper, and it functions flawlessly with any loads including cast Bullets. I had read about issues others had experienced and decided on a complete upper assembled by them with their parts.

"Dieseldoc" gives the best advice--- Tromix / Tony Rumore did the original development of the .458 Socom. Tromix will provide the answers you need.

Allow me to reiterate. The upper that does not function is a factory built upper specifically for .458 SOCOM by the manufacturer. I do not have a Tromix and my friend is probably not going to buy one and would like this one fixed. I ask to further my knowledge. Perhaps I have come to the wrong place to ask about this problem.

Ok Mr Gazz I’m still going to try to help. You assume that the “manufacturer” will make all of the necessary changes, but for some reason or another they don’t always. We work on these types of problems regularly and see this a lot. One thing that is a needed is, the corners of the three lower bolt locking lugs in the upper receiver need the corners cut away for more clearance. In the photo of the receiver lugs, you can see possibly, where they have been relieved. Compare that to the one you are working with. In the photos of the ejection ports, the 458 requires that the opening be relieved also to make room. One is a 458, and the other is a regular upper. Another thing you may try, to pin point the issue is to take a slow mo video of the ejection cycle with your camera so as to see exactly what is happening.
We have had more than one customer bring in a “factory built” rifle that will not operate. A 338 Spectre and a 450 Bushmaster come to mind, that would not cycle properly, issue was traced to the loads that were supplied, of which only the “manufacturer” could supply. It was obvious as they were also experiencing pierced primers,blown out primers and case separations. So just because a “manufacturer” sells it that doesn’t guarantee that if is worth the ore it took to make the metal. dae

I understand that the manufacturers do not always make something that works otherwise I wouldn't be here asking questions. The upper receiver does have the larger ejection port to accommodate the larger round. I will check on the locking lugs of the barrel extension to see how they compare with your pictures and with my functioning upper. Unfortunately, I can't swap bolts from one to the other as the "complete" upper uses a bolt carrier with a side handle that is held captive in the upper with a screw at the rear of the upper. My .458 has conventional AR type charging handle.

If I understand your original post, when you have 3+ rnds in the mag you get a fail to feed of the first round? If so, I would compare the buffer spring between your rifle and his. My suspicion is that his spring is too weak, so the bolt isn't striping the round with enough force/speed for it to make the jump to the chamber. This can also be caused by a spring that is too strong, but more likely too weak. As the buffer was not likely included with the upper, there is every possibility that your friend didn't install one with the correct spring tension. Have you verified that the timing is in spec?

I only have his upper so I am attempting to run it on my lower which works fine with my upper. As it is, I'm going to have to load some more ammo to continue.