Solved!

Ha!

You guys may remember that I was having serious issues with the piston Sig 516 upper I got a while ago and some .223 ammo. I kept getting double feeds, failure to feed, failure to eject, and jammed cartridges. I usually didn't take all that long to have one of those things happen. I have had to bang the buttstock on the ground (outdoor range) or have the local guy work on it to get it loose. It has always come out, but it has been a major PITA.

The ammo is reloaded, but appears to be normal otherwise. It functions apparently well in my other gun, although there have been a few issues with it. However, FAR fewer and less severe (like no stuck rounds in the chamber).

Today I went to the range with a variety of ammo, and sure enough, within 2 mags I got a stuck round in the chamber. Took some work to get it out.

Then I tried some 5.56 bulk ammo that I bought.

TADA! Ran through 8 mags with nary a hitch. Tried a different .223 load - failure to eject, and failure to feed, all within 5 rounds. Back to the 5.56 - no hitch! Rapid 3-round bunches - no problem. This thing just ATE the 5.56.

So don't let anyone tell you that there is no difference between 5.56 and .223. I clearly can show a difference. And I have a friend who bought a high-end AR piston gun who is having the same issues. So if you have a piston gun, be careful of .223 loads.

Devereaux wrote:...The ammo is reloaded, but appears to be normal otherwise... It functions apparently well in my other gun, although there have been a few issues with it. However, FAR fewer and less severe (like no stuck rounds in the chamber)...

Who did the reloading?  Commercially remanufactured ammo?  Gun show ammo?  Your own loads?  I think it sounds more like a 'reloaded ammo' problem more than a .223 vs 5.56.  Try some factory .223 ammo and see if it has any issues--my guess would be that it eats the factory ammo just fine.  If I was to take a guess, I would say it might be something in the sizing process.  Some autos like the brass on the small side (like using a small base die).  Sometimes it takes pushing the shoulder back just a tad more. When I first started loading .223 I had some real loading pains. Everything seemed to go wrong. The ammo that I chamber checked in a little Rossi single shot .223 rifle would jam in the AR's. I ended up just having to push the shoulder back a tad more for the autos and it all worked worked fine after that. Anyways, glad it isn't a gun problem, ammo problems are almost always easier to fix.

It was remanufactured ammo from Tactical Ammunition. I also tried a small amount of .223 ammo from Atlanta Arms, which I bought through a site called sendmeammo.com. It was a site recommended by Tim Lau on Modern Service Weapons.

The problem seems to be that the weapon doesn't cycle properly with that ammo. It will shoot some rounds, but it ends up not ejecting a case or double loads a round or, at least in the case of the Sig, getting a case stuck in the chamber and not loading properly. Then you cannot either get the bolt to close OR get the cartridge removed. (At least for a while.) I have had a few FTF issues with the YHM upper, but never a stuck case, whereas I have probably had at least 6 on the Sig. At other times it will hang in trying to load but you can pull the cartridge out. Occasionally you can then load the round without issue.

The gun shoots fine on single shot, where you get the shot but no recoil (twist the gas selector to closed). You obviously have to work the bolt with the charging handle but at least it seems to work fine in that state. But when you try to shoot semi-auto you start getting the issues.

THEN you switch to 5.56 ammo. Shoots fine. No hang-ups, FTF's, FTE's - just shoots. You can shoot 3-sjhot groups without any problems.

Ahhhh, interesting. I had a sig 556 carbine for a few months before the big Obama AR scare. It cycled .223 PMC stuff just fine--but I didn't keep the gun very long. Somebody wanted to pay 50% more than I did, so I let them buy it from me. Does your upper have various gas settings?

Only 2 - on and off. Unfortunately. I would have loved a system like on the FAL.

Been thinking about this some. Perhaps piston guns have somewhat different physics. The gas is caught in a closed chamber and works a piston, so you get no dissapation. That may mean you need a bit stiffer recoil spring. So if you shoot somewhat weaker ammo, your cycling isn't consistent.

Note that I don't get the issue with EVERY round - just periodically. For some time I thought perhaps it was just the ammo. It was, after all, reman'd. But it seems to work fine in my DI gun. And then I had the issue with the other .223 ammo.

Because I was at an indoor range I had to have the gunsmith clear the rifle stuck case. HIS comment when he shot some of the original ammo was that it wasn't cycling correctly. But when I gave him the 5.56 ammo, it worked like a charm. Even sounded a bit different.

I wonder if that upper needs some break in time with some hotter ammo?

That, too, is a possibility. I have lots of 5.56, so I guess shooting that for a while is in the cards. I can always go back and retry the other stuff.

It will be interesting to see how it runs after 500 rounds of 5.56 through it

The other little tidbit is that I have a friend who bought a high-end piston AR, and he is having the same issues with .223. I know everyone notes the difference in pressure but no one usually counts it for being very important. Perhaps in reference to the strength of a chamber it isn't, but perhaps for cartridge function it is. Remember that Garands will beat themselves to pieces if you use hunting ammo and not the stuff specially designed for them (M2 Ball) - unless you install one of those adjustable gas plugs.

Well, I've been talking to all kinds of people about this. It seems all agree that this is a pressure thing. One of the supporting reasons is that on single shot (when you turn the piston off) it works fine. The other is that in a well lubed weapon, FTF is a cycling thing, and that's either not enough pressure or too strong a spring. The latter is unlikely as a long term issue as it cycles 5.56 very well.

I am having some difficulty finding accurate pressure data. It seems the 5.56 has a higher operating pressure, but info is a bit sketchy as the military has no requirements to adhere to any SAMMI specs. They make their own ammo, to their own specs.

Apparently one of the differences in chambers is that the 5.56 has a more gradual transition or throat to the lands & grooves. This was done to accomodate the somewhat longer bullets found in the SS-109 and M856 ammo. This difference allegedly can cause dangerously increased pressures if fired in a standard .223 chamber, far exceeding even 5.56 specs. 

There also appears to be a real pressure difference. Best I can figure it, it is about 5-8,000 CUP, but maybe some more. This kind of difference is somewhere in the 10% +/- range. As I'm sure all would agree, that's a lot of pressure difference. Auto's are sensitive to operating pressure. Recollect that shooting regular hunting ammo in a Garand will destroy the gas system and pound the weapon apart. It requires installing the adjustable gas plug so one can moderate the amount of pressure in the system.

Finally there are differences in the physics of a piston gun. Most particularly, a piston gun has more mass to move, it having to move a piston, rod, and BCG. The DI gun only has to move the BCG. So the springs in a piston gun will probably be somewhat lighter. BUT with the increased mass, the piston gun may simply not respond to a lessened pressure spike as well, as it takes more force to move the mass.

All this implies that there are differences in piston vs DI guns, with the piston ones being more persnickity. Or so I conclude.

You guys thoughts.

I found this while surfing the webs:

http://sigtalk.com/sig-sauer-rifles/15497-sig-sauer-516-review-problems-issues-fixed-solved-ftf-cycling.html

I would think that a 5.56 rifle still should be able to cycle .223 ammo, otherwise, what's the point? All of the AR's I have been around (specifically DI guns) have been able to cycle both types of ammo reliably as long as they are chambered for "5.56". I would think that Sig would want to fix this issue or potential issue by having 3 gas settings, (off, on, and on with more gas). Isn't that the big marketing "pro" for piston guns, more reliable?

Perfect, Corn! But I didn't realize the gas piston had 3 positions. I will have to find a manual somewhere and look that up (time for google I guess).

But that article describes my problems to a tee.

Well, I looked a little harder at my Sig. There are a number of dots there. The one where the regulator is vertical is how the thing came. But there is one further over that I can't seem to get to, and there is at least one between closed and normal. The on-line manual says there is the normal position, one closed, one for shooting suppressed, and one "other" or "distressed". I expect that one opens the port wider as they don't recommend using it very long.

So I guess it's call Sig and find out what all goes.

Cool, keep us updated.

Talked to Sig this morning. Apparently I have an older model as they have changed how the piston switch works.

BUT there are several "dots" on the outside of the selector switch. I was unable to turn it to one of them, but this morning, with the addition of a small screwdriver (one of a set of Forster ones that has the tip broken off - I use it for "stuff") I got enough leverage to turn the switch in any direction. SO now I have to go back to the range and try out the various settings to see what they do.

And they are sending me a manual since I got none with the upper. It's the rifle manual, but it obviously should apply.

Sounds good, I'm betting that with it turned to the right gas settings you will be able to cycle .223 ammo.

Range again yesterday with the 516.

Went through 100 rounds of 5.56 without a hiccup. Shoots fine, and good groups when one works at it. At 30 yds with a silohoute you can consistently put body shots in with double taps and fast shooting.

But the .223 ... Still won't shoot right. I tried regular, then something towards the cut-off. Not sure if it's the suppressed slot or what, but it doesn't work there. Then I shot the dirty slot, or whatever Sig calls it when you are not functioning correctly and want to blow out the system. STILL won't shoot well. Matter of fact, you can FEEL the difference. The .223 just feels less powerful. The case when it comes out - and it does reasonably regularly - doesn't fly out with the same velocity, nor does it go to the same spot on the ground but further forward.

Still haven't put a 1,ooo rounds through it, so it may take some more shooting to make any final decisions, but so far, no joy.