I took a friend of mine shooting yesterday.  He had a new 458 socom build for his son for Deer Hunting that he wanted to get sighted in.  It was brand new, never been fired.  It's a KAK upper (I've never heard of that) with a Spikes Lower.  Upper had a free float rail and a stainless barrel.  Nice set up.  Light too, which I think is nuts with that caliber.  LOL.  My buddy also owns an RRA 458 socom that works just fine.  

OK, so he goes to load his first round and it FTF's.  And then the round was stuck in the chamber.  So I did the ole charging handle against the bench and cleared it.  Basically it did that a few times.  I looked at his mags and I don't know what kind they are.  They are black with a gray follower that specifically said 5.56 on them.  He said he has to relieve them in the front to allow the round to pass through.  I don't know if they make 458 specific mags or not, but his RRA 458 socom runs fine with how he has it set up, so...

But it seemed to me that the mags were rubbing on the bolt, slowing it down, but I couldn't really see any thing that weird looking.  There was a little bit of a scrape on the BCG right where the firing pin is but it looked normal.  We did figure out that if he loaded one round, the bolt would close all the way and fire the round and eject it.  But obviously that's no good.  

He said the upper was real tight getting on the lower on the front pin.  Like he had to hammer it a bit.  And then we tried to swap it to his RRA lower and he said the back pin was really tight.  I put it on an RRA lower that I was shooting that day and it went on fine.

So....  it's all tight for some reason, I didn't think to try to have him use my lower though.  Duh. I wasn't sure if they used the same buffer set up but he told me later they do just have a standard buffer.  

If I had to guess it seems like a mag thing. Or the lower is just a bit off that when the mag seats, it's just a bit too close and tight to allow it to function.  He tried loading by charging handle AND bolt catch.  Either way it would get stuck.  But it loaded ALMOST all the way.  The FA wouldn't seat it though.  

Has anybody else have this happen?

What magazines is he using?

This...

Also, who made the barrel? I know some of the offerings are reverse engineered, rather than reamed off the correct dimentions.

Between just those two variables, that could cause all sorts of hiccups.

Kak I don't think is an authorized mfg.  That means they used a ptg reamer which has been reverse engineered from a 458 case.  Thus they have problems loading some ammo, 405gn jfp is one that I have heard has problems.  If available try a couple different bullet types.  Also have him get an SBR case gauge.  This way he can check his ammo.  Though if he has a barrel that used a ptg reamer.......

Go to www.458socomforums.com. tons of info.

Eta,  if the round fits in a standard ar15 mag it should work.  The polymer mags have a tendency to swell too much loaded.  Also the pmags have a center ridge that gets in the way of some loads.  Good old gi mags and lancers seem to work the best.  But the socom was designed to use standard mags.

I told him all what you guys said.  I guess he is actually a member over at that 458 forum.  So he knows a thing or two.  I forgot to mention he was using handloads.  He called the guy at KAK and he told him to make sure he's sizing his cases all the way down.  Because he uses a progressive, the guy says there is a little bit of play on the shell carrier and that makes a difference.  My friend also did say he was aware of the reverse engineering thing but he didn't seem to be too concerned about it.  I think he's going to try to run some factory ammo through it and see.  

What's weird about his mags are they are marked D&H.  Which AFAIK are USGI mags.  At least they have or had a contract with the .mil.  But I've never seen military mags be black.  But I also am not an expert on things and there was a few years I was not posting on here too much because of bad health and all that.  But I thought all USGI mags were the light grey.  Which I have.  The followers look newer too.  Not the old grey but the newer grey with a big ole 5.56 in the front of the follower.  I'm used to the old grey follower or green.  Which the green were all the rage at one point and then you got the orange that magpul was putting out or something.  I have no clue.  

Anyways.....  if the chamber is tight and his ammo is a bit out of spec, it would make sense.  What I forgot to suggest yesterday was to drop the first round in the chamber manually and then seat the mag with some ammo in it.  And maybe the recoil impulse would help move things along a bit better to fully seat the next round.  

I kept thinking it was mags too.  He says he files down the front of the mag at the top so the case mouth doesn't get hung up there.  And I'm all like, "Doesn't anyone make a 458 specific mag?"  And he says you're supposed to be able to just use 5.56 mags but he relieves them a little anyways because it seems like they can get slowed down on that front lip of the mag.

Bet you a buck he is seating and crimping in one step or using a roll crimp.  He is over crimping and that causes the base of the shoulder to swell and that jams up the works.  First, don't seat and crimp in the same step.  If you crimp at all, and crimping really is not needed, use the Lee Factory Crimp die.  Pretty sure either not crimping or using the LFCD will fix his problems even with the bastardized chamber.

If your rifle is built correctly standard 5.56 mags will work perfectly, just like Marty and Tony designed it to.

He said he does do them in two different stages.  Lee factory crimp.  After I showed him what you said, he said he rolled one without a crimp and chambered it.  (No primer or powder) and it chambered fine but wouldn't extract easily.  Had to ram the charging handle again.  I wonder if that chamber isn't messed up somehow.

If he isn't using a case gauge, that's where I'd start.  I had all kinds of glitches before I got one.  No trouble since.

Hmmmm...  I know he trims them and as was mentioned he was sizing them.  I'll ask about a case guage.  

But here's the kicker.  He's been chambering his rounds and then some Corbon and SBR ammo and they all will chamber, but they won't extract without needing to do the ole charging handle rammed against something hard.  Now I'm thinking there is something up with the chamber that's not right.  He's sending the upper back to them.

Lee dies on the reloads?  I am just getting into the 458 game, but in my research some of the Lee dies have issues.

One problem down, he was over crimping them severely.  If a Lee FCD causes the cases to swell then he must have really been crimping the crap out of them.  I never put in more than one-half turn of crimp if the case mouth is lined up with a crimp groove and when it is not lined up, absolutely no more than one-quarter turn of crimp.  That means, once the crimp collect contacts the shell holder, turn the die down one-quarter more turn.  The only time I have seen the LFCD cause shoulder and body damage is when it is turned down way too far and adding way too much crimp.

On the sticking even with the uncrimped dummy, I'd be looking at seating the bullet deeper.  More than likely it is being jammed into the lands and sticking.  I am pretty sure the PTG chambers are not throated as long as the original speced chambers because it seems the PTG chambers mostly have trouble with the heavier bullets.
Mark the bullet with a black Sharpie and chamber it and I'll bet when he extracts it he will see rifling marks on the bullet.  Seat so the bullet is at least .020 to .050 off the lands, make sure he sizes completely, and adds no more than one-quarter turn of crimp and I bet his problems will cease.  If not, time for some chamber inspection.

This second part had dawned on me too.  But the bolt seemed like it would stop a little further back then what I would've thought getting hung up on rifling would set it back.  I don't know exactly what grain bullets he was using. They were some ballistic tips with a red tip.  I can ask him.  I'll paste your comments about the crimping.  

And thank you for the clarification RRAMODERATOR.  I was going to let him use one of my twenties to see if it was the mag, but it did seem like the case of the round would get hung up on the front lip.  

I have one of Kurt's (KAK) barrels, 16" 458, I also have some time on one of his 8" barrels). It eats every thing I put in it. Kurt does not use a ptg reamer. He had his own made, Lots of trial and error on his part.  Keep in touch with him if you feel it is a barrel issue. Kurt will make it right, he is one of the best people to deal with I have ever been around. By what your describing though, sounds like a issue in loading.

He sent it back to KAK.  We'll see if he gets the same problems or not.  Maybe it's his handloads, but like I said he tried some factory ammo too.

Well, we were at the range again with it today, sighting it in for deer season.  

I'll be honest, I'm like WTF about the whole thing.  He sent it back, the guy tells him it's his ammo.(handloads)  And there's probably some truth to it.  But he ran some factory ammo through it today too and it functioned better than the first trip, but not good.  It barely ever fed a second round.  It's basically a shoot a round and rack the charging handle to seat anther round gun right now for gun season.  But at least it's dialed in accuracy wise.

 I don't know what's going on with it.  He just put a new stock on there that has a big ole rubber buttpad for the recoil, but AFAIK, he's always using standard carbine buffers.  

Furthermore, he got some 10 round pmags and apparently he never has a problem with his RRA socom upper, but I noticed that his RRA didn't lock back on a pmag after the last round.  So I'm all like WTF about this 458 socom stuff.  

There's problem some secret or simple explanation but I have no clue what it is.  If it's not running right with factory ammo, then you can't blame his handloads.  Although it does seem like his handloads made the problem more drastic.  I don't know.  

Are 458 Socoms typically finnicky in comparison to 5.56?

Normally they are not picky at all.  All of my 4 AR .458 SOCOMS have run like a top from the get go.  It sounds like he has a gas problem now.  Check (measure, not eyeball) that the gas block is properly installed and that the gas key on the bolt is not leaking.  If the bolt will not lock back on factory loads something is jacked up with the gas system.  Also check your gas rings to make sure they are not broken.
 While he has the gas block off checking alignment, measure the port in the barrel.  Maybe the port is too small.  When one goes with a barrel not made by someone who is licensed to be making them and using reverse engineered reamers, just about anything can happen.

I mentioned that it might be a gas problem too, because that thought dawned on me.  But it also seems like the chamber is just really tight.  Maybe it's both.  

That's good to know that this isn't common.  He said his RRA has never had any problems.  So.....  But he also said he didn't use lube at all.  He said he never used any in them and I was kind of scratching my head.  I soaked down his bolts with CLP.    That might be the old school way of doing it, I don't know.  But I figured it can't hurt, especially in a new gun.