After doing more Googling about the MGI setup I decided against it.
Took a chance on the SIG 556R and have had it out three times, so I thought I post here instead of starting a new thread.
First time out I used the lame red dot that it came with, SIG lists it at $199 on their website, what a joke! I'd much rather it'd had come with the $142 "diopter" sights also listed on their website.
My initial result using the Bulgarian "slabside" metal lined plastic mag it came with was not good, having several bolt over base feed failures and many failure to fire with Golden Tiger ammo, although all the primers appeared well dented. Next I tried a steel comblock mag that has been fine in my Saiga loaded with Wolf FMJ. Perfect function and I was beating up the steel plates as by then I'd got the red dot reasonably well zeroed despite the frustration with the Golden Tiger ammo.
Feeling pretty good I gave a Tapco mag a try, also loaded with Golden Tiger. Again about 10% failure to fire and two bolt over base feed failures.
Next another random comblock steel mag loaded with Wolf JHP, again 30 rounds perfect function.
Finally a steel comblock mag loaded with Golden Tiger. Perfect feed and ejection on the rounds that fired. I picked up the 10 or so "duds" and stuffed them into a steel mag, all fired the second time with no feed or ejection issues.
I cleaned the rifle, especially the firing pin channel, and mounted Midwest Industries SIG 556 flip up front and Magpul flip up rear for BUIS, lost the red dot after using it to get the BUIS in the ballpark, and then mounted a Millet DMS 1-4X scope in a QD mount.
Tried the Bulgarian plastic mag and Tapco mags again loaded with Tula FMJ and took them out after the first bolt over base feed failure, stripped out the rounds and loaded them into a steel comblock mag wich then functioned without issue. I then ran three other steel mags loaded with Brown Bear, Wolf JHP and Tula with 100% function. No failures to fire with these three ammos.
Third time out I just brought only steel mags including a Korean than most folks bad mouth, although they've always worked for me. I had a mag each of Brown Bear, Tula, Wolf JHP, and Golden Tiger. Had 100% function with the Korean mag loaded with Tula, 100% function with the Brown Bear, Three failures to fire with the the Golden Tiger, and this time one failure to fire with the Wolf JHP. Again all went the second time.
Overall I'm very happy with this rifle and have had none of the feed or extraction/ejection issues Sturmgerwher showed in his youTube videos. I'll try the Bulgarian plastic and Tapco mags in my Saiga and if they fail I'll toss them down range and use them as reactive targets
Condition were not suitable for any accuracy testing (10-30 MPH wind gusts) but off sand bags at 50 yards, waiting for reasonable calm I got groups with the Tula about half the size I generally get with my Saiga. Next time I can get out on a calm day I'll try a head-to-head with my Saiga, AR 7.62x39 upper, and the SIG 556R.
I'd be hot to try a stronger hammer spring, or do a minor mod to increase the firing pin protrusion a tad like I had to do for my AR 7.62x39.