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Upper/Lower Reciever Play-Some Insight

30413 Views 41 Replies 33 Participants Last post by  522rcali, 
U
...it is perfectly normal for you to have a certain amount of play between your upper and lower recievers in the 55x series of weapons...and that includes the 556...these weapons were designed primarily "for the military" to be used "by the military"...under battlefield conditions...

...in military use it is important for the weapon to be realiable, durable and easy to maintain and repair... using readiably "interchangeable" parts that do not require '"gunsmithing" or "fitting"...the design of the weapon and resulting design prints...although specific... allow for a "certain" amount of print tollerances...which is not only inherant to the production process...but also a factor when replacing worn or broken parts with the new...so that the demensional relationship between these parts are such that proper functioning is achieved...we usually see this as a +/- of some unit of linier measurement...so that a part is defined as being within the acceptable parameters as long as it conforms to a certain demension plus or minus so many hundreds or thousands of an inch or whatever units of measurements the print uses...
...sometimes a situation is created where two or more parts...that have a mechanical relationship... are either on the "long side" or "short side" of the print tollerances...but still within acceptable print parameters...and due to their mechanical relationship...sometimes work together to create a situation where the mechanical relationship remains acceptable or becomes unacceptable...this is sometimes referred to as "tolerance stacking"...the parts may either fit to tightly or not at all...or fit to loosely...and function may or may not be affected in either case...the designers and engineers determine what is acceptable or not acceptable...that is ...how tight is too tight or how loose is to loose...

...in order to have readily interchangability of parts in a "military type" weapon...and in order for that weapon to remain largely unaffected by the sometimes "adverse" enviromental conditions found during it's deployment...including operator neglect for whatever reason...the design tollerances are generally greater...and when they work against each other...and become cumlative...problems may or may not arise due to this "tolerance stacking"...

...if the vertical play between the upper and lower reciever is very excessive....and other "tollarence stacking" issues come into play...such as the position of the bolt catch pin, location and demensions of the mag catch cutout in lower, demensional differences in the mag catch, location of the cutout in the mag itself (re 556), the height at which the reciever rails allow the carrier to cycle (raising the bolt head) relative to the lower reciever, and the demensional varriation in the carriers themselver...and so on...you may have issues with the weapon...this is "rarely" encountered though...

...unless you are experiencing feeding issues...especially those that involve the bolt "riding over" and failing to pick up a cartrage and chamber it consistently...i would not be concerned... this issue is usually mag related anyway...so be sure you are using reliable.mags and good ammunition...before you jump to any conclusions...


...something i have done in the past is to use a ~ 3/4 inch...or slightly smaller...O ring...made from an oil resistent material...with a 1/16 inch wall...roll it onto the rear lug...until it sits above the hole...prior to install you might put a little silicone grease on the O ring...and relieve the 4 sharp edges on the lug in this area...so as not to cut the rubber wall...

..O rings are cheap...so buy a dozen or so...they will need to be replaced from time to time...

...if a 1/16 inch wall O ring does not reduce the amount of play substantually...i would return your rifle to sig...on their dime...to correct the issue by either changing out the lower or replacing the rifle...

...i've posted this thread to help alleviate some of the concerns that a few members "may" have if they notice "some play" between their upper / and lower recievers...comments are certainly welcomed...however...stay on topic...i will not allow this tread to become a 55x versus 556 issue...and any replies leaning in that direction will be deleated...
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U
...as an aside...a small amount of play is desirable in a weapon with a steel upper and an aluminum lower that is pinned together in two places...such as the 556...especially when the weapon is subjected to temperature extreames...the thermal coefficient of expansion of aluminum is about twice that of steel...sometimes more depending of the alloy...

...just my 2 cents again...
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Easy Inexpensive Fix

ullie said:
...as an aside...a small amount of play is desirable in a weapon with a steel upper and an aluminum lower that is pinned together in two places...such as the 556...especially when the weapon is subjected to temperature extreames...the thermal coefficient of expansion of aluminum is about twice that of steel...sometimes more depending of the alloy...

...just my 2 cents again...
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:arrow:

There is a rubber AR 15 thingy that can be inserted under the rear AR Pin.
It is made only for the AR.

It will, however, fit the Sig 556 if you put it in right. All you really need to do is watch, while you close the upper and lower, that the rubber deally stays in place as you slide the pin to close the upper and lower.

I ordered 5 of these to use in my AR's and Sig's and they work 100% in both. You just have to watch what you're doing in the Sig. When it's closed and pined you will know it's working. If it isnt just open it up and try again. The washer suggestion works too but this seems to work the best for me. Different, working, solutions for the same problem.

Ther are, probably, other solutions for the same problem.

If they work for you - GREAT!

If you want to leave it as is - fine!

Either way, that's probably not what's causing any in-accuracy if any!
The sight is attached to the receiver. If it moves, so does your sight, It equals out.


HAVE A GREAT DAY !!! :lol:


Tom


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ullie,

In your post you talk about the 55x series rifle being designed for "military use" and in military use it is important for the weapon to be reliable.

Question:

Just how reliable is my 556?

While doing research on my next rifle purchase, I came across videos on the web of the Magpul Masada being dunked in a river and then immediately fired. Smashed in mud and grime and fired without a flaw. I can't find any videos of the 556 going through these "tests".

Can the 556 do this? Can it take the abuse like the Masada or FN SCAR? I don't want to do this to my rifle because I would probably get sick.
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Aaron said:
ullie,

In your post you talk about the 55x series rifle being designed for "military use" and in military use it is important for the weapon to be reliable.

Question:

Just how reliable is my 556?

While doing research on my next rifle purchase, I came across videos on the web of the Magpul Masada being dunked in a river and then immediately fired. Smashed in mud and grime and fired without a flaw. I can't find any videos of the 556 going through these "tests".

Can the 556 do this? Can it take the abuse like the Masada or FN SCAR? I don't want to do this to my rifle because I would probably get sick.
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:arrow: No sweat ! It's action is an upgrade of the AK 47 system as is the Galile, valmet and others. These are the most reliable systems in the world - BAR NONE !!! Even the FN SCAR. and the Bushmaster Masada have many mechanical aspects of the AK. So, without a doubt, the Sig 556 is as rugged as it gets. Look at it's parents - the Sig 551 series. - one of the most sought after riflesa in the world ! The Masada and the SCAR are both GREAT rifles butt so is the Sig 556, especially with the right furniture.

Tom



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Aaron said:
ullie,

In your post you talk about the 55x series rifle being designed for "military use" and in military use it is important for the weapon to be reliable.

Question:

Just how reliable is my 556?

While doing research on my next rifle purchase, I came across videos on the web of the Magpul Masada being dunked in a river and then immediately fired. Smashed in mud and grime and fired without a flaw. I can't find any videos of the 556 going through these "tests".

Can the 556 do this? Can it take the abuse like the Masada or FN SCAR? I don't want to do this to my rifle because I would probably get sick.
Click to expand...

http://www.biggerhammer.net/sigamt/550/550techinspection/
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again very good information, thanks Ullie! :)
This is true for most all new generation military rifles- think about how many parts the US military has in piled up over the years for support of the M-16 platfrom. I do know a little about government contracting- they don't sole source this stuff- they put it out to competitive bid. The governement provides specifications on parts and quantities, delivery dates and points. They may or may not award higher vaule to exceeding the specifications, on a rifle part I would think within specification and price would be the drivers.
So tons of parts different sourcing, they will have to have play to just drop in and go- in the field I can't imagine having to file dremil or sand to get a decent fit.
And as pointed pointed out- loose probably is better in the real world, termperature variations and or dirt would play havoc on a really tightly built rifle.
One one my primary gripes with the AR is that the AR needs to be kept so darned clean to be happy, and how much the gas recoil system is contadictory to that!
Arron ask a interestng point- and I don't know- what was the torture testing Sig USA did to the 556 platform.
As pointed out though, the 556 is a evolutionary step on the 551 platform, and the 551 (agan as was pointed out- thanks Bob RI) has been tortured beyond anything I will ever do to a gun.
IMHO, the 556 combines the best of the 551, with some of the decent aspects of the AR in it's lower design (except for that darned take down bolt spring! :twisted: ).
I don't have a gun I would be willing to do it to, or the time, or the ammo, or the money, or the health right now, but I would be fun to see what a 556 would hold up to! Purely from the scientifc point of view of course! :wink:
From the esoteric point of view, the Sig 551 was tortued, the AR as been tortured also. The Sig 551 came though aces, the part of the AR that have been seen as deficient are in the upper, hence all the improves piston uppers on the market, but not in the lower.
So from the research point of view (and coming from a research nebbish) while there is no actual testing data readily avaliable, the component would suggest a platform what will prefrom well-
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If you have play in your upper to lower receivers with a quick and easy fix you can have a rock solid fit.

Just use a longer grip screw. Flip the bolt and nut. Nut should in inside the receiver, so the extra bolt length intrudes into the receiver.

The idea is the bolt end contacts the upper receiver lug that the rear takedown pin goes through.

Install the longer grip screw, then try to close the receivers. If it doesn’t close you have a long enough grip screw.

If it does not even come close to closing, remove the screw and cut the grip screw down with a dremel or a saw. Don’t cut too much you still want it a bit long.

Install the grip screw and try to close again. Repeat cutting and checking until you are about 1mm from being able to push (hard) the rear takedown pin in.

Then take a punch and hammer and peen the end of the screw (where you cut). Then check fit.

When you get the screw the right length you will just be able to get the rear takedown pin in with a reasonable amount a force.

Now you have a rock solid fit between your upper and lower receivers.

I did this with all my SIG 556’s and it worked perfectly.
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What was the measurment of the screw? :D
I used an o-ring like Ullie suggested, but that needs to be replaced every so often. :cry:
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I just knew somebody was going to ask that.

I never checked. I did a rough cut with the screw out of the gun then did the fine fitting with the screw in the gun. Once it was just the right length, I never removed the screw to measure it.
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I opened up my receivers to look at how much longer the screw would have to be.
My grip screw is already turned the right way but, it looks as if the screw was longer it would be too far forward of the lug and wouldn't come close to it. :?
Can you post some pictures?
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M
not to kill the subject but how much gap between the upper and lower is normal?

i can easily see light through the gap and there some play.
maybe ill break out a ruler to be exact.
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I took an AR15 accuwedge and flipped it over with the tab that sticks up in the back ( in the AR ) to the front and pointing downward into the grip screw hole. That way the flat base sits on the shelf in the lower. Press down firmly on the upper, engage the pin and bam, you have a rock solid, no wiggle fit.
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dano36 said:
I took an AR15 accuwedge and flipped it over with the tab that sticks up in the back ( in the AR ) to the front and pointing downward into the grip screw hole. That way the flat base sits on the shelf in the lower. Press down firmly on the upper, engage the pin and bam, you have a rock solid, no wiggle fit.
Click to expand...
My Accuwedges just came in the mail today. I did the same thing you detailed in your post and it works great. I had more play then I was happy with and now it's rock solid. 8)
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You would think a tight fit would be better to keep out dirt like with the M16's. Having a big gap would let water or mud get inside.
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Has anyone looked at making a Tension Pin similar to the one JP Enterprises Inc makes for the AR series rifles? I think that may be my next project in my shop. All of the other cures I have seen have been band aid fixes. Something on this order would be a solid metal to metal lock up.
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I cut my accuwedge in half and just use that. Tight as a rock now.
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coffey said:
You would think a tight fit would be better to keep out dirt like with the M16's. Having a big gap would let water or mud get inside.
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Umm, these are not "air tight" anyway. Loose specs mean better reliability, though an accu-wedge cures those wobbly blues.
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ak4me said:
coffey said:
You would think a tight fit would be better to keep out dirt like with the M16's. Having a big gap would let water or mud get inside.
Click to expand...
Umm, these are not "air tight" anyway. Loose specs mean better reliability, though an accu-wedge cures those wobbly blues.
Click to expand...
When trying to shoot good groups its very annoying to have the lower wobble back and forth. The newest AR15 I built has a super tight fit with the upper and lower. I like it.
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