SIG 516 and Mk318 issues

[ABNAK]

Have a "new" (NOS technically) SIG 516 I finally got around to shooting today. It was a SIG lower with a 14.5" chrome lined 516 upper. I wanted to zero my irons and Aimpoint T-1. Used 40rds of Mk318 and 40rds of M855 "green tip". Once the zeroing was done I just plinked at some clays I had set up on my 100yd berm.

The M855 didn't miss a beat. The Mk318 was constantly short-stroking or at best failing to lock back, even on the "adverse" gas setting.

I thought Mk318 was a little hotter than M855? Got me to thinking about a thread I had posted last year about a Hodge barrel and different types of ammo using an LMT EBCG. I looked back at it and saw where the Mk318 had about a 50% failure to lock back on an empty mag, the highest of the rounds I tested that day; fired and cycled well otherwise (unlike today). Of the three types of rounds I tested then (Mk318, Mk262 IMI "Razor", and M855A1) it seemed to have the least "oomph" for cycling. Did not test regular M855 that time like I did today.

So this time there was a piston gun and no EBCG, Razor ammo, or M855A1, just plain old M855 and Mk318. It would seem that the Mk318 still seemed to lack a little in energy to cycle the system. Yes, it was a piston gun this time, but Mk318 was consistent in being the "weaker" round for cycling purposes.

While I didn't compare lot numbers, the Mk318 I shot today was a different batch than what I shot last year. It is disappointing as I have >1,000 rounds of it socked away and it's a damned good bullet terminally. Thought I'd throw this out there and see if anyone had similar experiences.

FWIW, both batches of Mk318 (last year and today) were Federal "white box" T556TNB1.....there was no "X" prefix anywhere. The M855 I shot today was Federal XM855 from a 1K round bulk pack.





EDIT: gonna have to try some Mk262 "Razor" and M855A1 next time.

[BFS]

Sometimes failure to lock back can look like a short stroke, but is actually the reciprocating mass moving so quickly the magazine spring does not raise quickly enough.

It can throw a guy off, because it behaves the same as an undergassed issue.

[drtywk]

As it has been said a bunch on this site and many other places already, the X designator in Federal ammo has nothing to do with quality, so I wouldn’t let that be a factor with your issues. I would hazard to say that you’ve got a bad lot/batch of ammo, especially since you’ve seen this issue with two different guns.

[ABNAK]

As it has been said a bunch on this site and many other places already, the X designator in Federal ammo has nothing to do with quality [I was mentioning it to head off the inevitable comments to that effect!], so I wouldn’t let that be a factor with your issues. I would hazard to say that you’ve got a bad lot/batch of ammo, especially since you’ve seen this issue with two different guns.

That almost has to be the case. Granted, two different "lots" if you will (not number-wise but bought from two different vendors over a year apart) yet similar evidence of less cycling energy. It works absolutely fine in a couple of other AR's to include my wife's new 25th Anniversary carbine with a BCM 14.5" midlength gas system. Sucks in these limited cases 'cause I like Mk318 for a SHTF stash, but enough of my guns tolerate it fine to make this a relative non-issue.

[ST911]

That almost has to be the case. Granted, two different "lots" if you will (not number-wise but bought from two different vendors over a year apart) yet similar evidence of less cycling energy. It works absolutely fine in a couple of other AR's to include my wife's new 25th Anniversary carbine with a BCM 14.5" midlength gas system. Sucks in these limited cases 'cause I like Mk318 for a SHTF stash, but enough of my guns tolerate it fine to make this a relative non-issue.

Take any apart or chrono them yet?

[ABNAK]

Take any apart or chrono them yet?

No, I haven't. These particular boxes came from Bone Frog FWIW.

[pinzgauer]

As it has been said a bunch on this site and many other places already, the X designator in Federal ammo has nothing to do with quality, so I wouldn’t let that be a factor with your issues. I would hazard to say that you’ve got a bad lot/batch of ammo, especially since you’ve seen this issue with two different guns.

I'm not sure that's as clear-cut as you're saying.

The federal X ammo meets SAMMI specs. Which is case dimensions and maximum pressure only.

In many cases in the past it did not meet the tighter NATO spec, and that's why it's being sold on the civilian market.

Federal is pretty cagey regarding this in their web sites, documentation, and if you call and ask.

All they will say now is: " offers consistent accurate performance at a price that's perfect for high-volume shooting. The loads feature quality bullets reloadable brass cases and dependable primers."

In a page they have since taken down they stated it was first run, first quality product made at the Lake City Army ammunition plant for Federal cartridge and is made to Federal specifications for commercial ammunition.

It's clear why it was taken down... they have answered in support requests and in interviews that XM ammo had "insignificant issues" that disqualified it for M usage.

XM is ammo that failed individually or in lots to meet the NATO spec but still was safe for federal's commercial specs.

There are other manufacturers like IMI which state that their M193 conforms to Mil-C-9963F. Testing from people like Molan bore that out.

Federal XM is not bad, but not great

[Ranger86]

Maybe I misunderstood your post, but I got the impression you changed uppers to the 14.5". Both of my 516 uppers that I bought that are other than 16" came with a buffer and spring with a note stating they must be used for reliable performance. I believe their piston system needs a matching buffer.

Probably not your issue, but I thought I would mention it if you got the 14.5" upper that didn't include the buffer.

Sent from my SM-G960U using Tapatalk

[ABNAK]

Maybe I misunderstood your post, but I got the impression you changed uppers to the 14.5". Both of my 516 uppers that I bought that are other than 16" came with a buffer and spring with a note stating they must be used for reliable performance. I believe their piston system needs a matching buffer.

Probably not your issue, but I thought I would mention it if you got the 14.5" upper that didn't include the buffer.

I originally bought the gun with the 16" nitrided upper. I replaced that with the 14.5" chrome lined upper and sold the 16" one to a buddy. I used a HeavyBuffers.com anti-tilt buffer (which is supposed to be an H buffer). The one that came with the SIG originally was a plain-old, flat-faced H buffer without the anti-tilt nub on it. Now, assuming the anti-tilt buffer was indeed an H weight like it's advertised, it *should* have worked fine. After cleaning the 516 I popped the original flat H buffer back in just to rule that out next time I shoot it.

EDIT: I looked up a HeavyBuffers.com H buffer and it is 4.0 ounces. A standard H buffer is 3.8 ounces. Not sure whether that 0.2 ounces would make a functional difference though.

[kyjd75]

I originally bought the gun with the 16" nitrided upper. I replaced that with the 14.5" chrome lined upper and sold the 16" one to a buddy. I used a HeavyBuffers.com anti-tilt buffer (which is supposed to be an H buffer). The one that came with the SIG originally was a plain-old, flat-faced H buffer without the anti-tilt nub on it. Now, assuming the anti-tilt buffer was indeed an H weight like it's advertised, it *should* have worked fine. After cleaning the 516 I popped the original flat H buffer back in just to rule that out next time I shoot it.

EDIT: I looked up a HeavyBuffers.com H buffer and it is 4.0 ounces. A standard H buffer is 3.8 ounces. Not sure whether that 0.2 ounces would make a functional difference though.

It shouldn't make a difference. I have several 516s, including 14.5 uppers that did not come with any buffer or buffer spring, as well as a 16" barrel that I had John Thomas cut to 14.5. All of these are running regular H buffers, with the exception of one 14.5 that is running a Geissele Super 42 with a Geissele H buffer. All of these have been 100% reliable for me. Granted, I normally only run 5.56 (IMI or Federal M193), but I have run Federal .223 through a couple of them, with the same results. Complete, absolute reliable functioning. I personally am not familiar with the ammo you are having trouble with, but I would suspect that if your weapon functions reliably with good 5.56, then it has to be a strong suspect.