Ok got this new 551 upper and it has a little bar welded on the side that will not let it mate to the 556 lower I have. Has anyone seen this or has a fix? Thanks.

I will post pics later

This message has been edited. Last edited by: SigCarolina,

Years ago I posted a thread about my attempt to mate a SAN 55X upper to a 556 lower. Lowering the 55X down it hit an interference point with the 556 lower, preventing full closure. I probably could have used enough force to slam past the interference point, but it would likely have marred both the upper and lower, which I was trying to avoid.

One member posting in that thread didn't notice there was an interference point while assembling his parts and seemed to just use sufficient force to push past the interference point. A second forum member reported using a Dremel to eliminate the interference point.

I tend towards being a bit more precise and meticulous and really didn't want to modify either assembly an iota more than was absolutely required, and I really did not want to modify the 55X Swiss upper so I decided to do all my modifications to the 556 lower. I vaguely recall that most (if not all) of the interference point was the slightly wider aluminum shelf of the lower and I only had to remove a very small amount of metal depth, but over an extended area on the right side of the lower, and close to the internal mag release mechanism housing, which was tight to get into. I used very small hand files and worked slowly so it took me several hours over a few days, but it helped me control exactly the amount of material that needed to be removed.

My fix worked well and the assemblies fit together perfectly after the mod. I had planned on finishing the bare exposed metal but I ended up selling the 556 lower and the buyer seemed ok with the mod.

Without seeing pictures of your issue I'm not certain we are considering the same problem. I'd post pictures of my fix, but they are stored on one of my external hard drives that I'm having connection problems with and can't access the pics right now, so it would be helpful if you could post pictures of your assembly issue.

Thank you for the reply. That is the exact issue I am having. I will buy a small file tomorrow drink some coffee and file till it fits like tit to mouth.

Yep. I had to dremel the lower where it hit the auto sear release lever block (SIG calls it the carriage block).

Be aware that you will not be able to use USGI aluminum mags, as the top forward edge will hit the Swiss trunion. US 556 models have a relief cut in the trunion to accommodate AR mags. P-mags will fit tightly and will not drop free. USGI 20-round mags fit perfectly.

There's no single answer as SigSauer in the US made a lot of changes to how they made 556 lowers over time. On the one I fit, I had to open up the rear corners of the lower as Sig left them round profile and of course the Swiss upper is sheet metal and square. The other thing I had to modify was the front interference at the bolt catch. This meant removing the bolt release to be able to clearance it. A small milling bit in a dremel, cleaned up with a file did the trick.

YMMV.

Bottom of page 7 is my rifles. Got the 551 and 2 556s. The file worked great. Now for a scope and range time.
https://www.snipershide.com/sh...autos.6253392/page-7