I finally got out today to test fire 2 SBR's that I built several months ago. (Been laid up with a broken heel & complications related to) The 8"  300 Blk ran like a dream.  Loud but in a good way. (Mailed the Form4 for the Saker762 today) The 12.5" 308 did not cycle. It extracted the rounds but I had to cycle the charging handle each time to chamber a new round. I have a fixed rifle stock on it with the standard spring & buffer for that stock. I didn't have a table and was shooting out of the back of a truck so I could do much trouble shooting. What would be some thing for me to look for the ned time out. Ammo was the only thing available, Russian steel case. I also had a 7.5"  223 Wylde shooting steel case also that did not cycle on my Sendra RR. Thanks in advance for any help offered.

Try changing out to a lighter spring and buffer. Trial and error my friend. I typically bring a ton of spare parts when I got out the range to what runs or not.