...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...
...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...
