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I've been lucky, my Saiga 12 only had 2 FTF's in the first 300 rounds, perfect since.

What ammo are you using? The usual culprit for the problems you describe is using too-light ammo, the S-12's prefer heavier loads to cycle properly. Look at Tromix's website, they have a page outlining a lot of details relating to ammo choice and gas settings:

http://www.tromix.com/saiga-servicetips.htm

From Tromix:

Ammunition: Most Saiga-12’s will cycle all 1-1/8oz ammunition. I do not recommend firing any of the super light 1oz shells. The very best cheap, light, birdshot ammo is Winchester AA, Super Sport, or Super Handicap. It runs about $4.50 per box and cycles Saiga’s like a champ. DO NOT ATTEMPT TO FIRE WINCHESTER “UNIVERSAL” AMMO SOLD AT WALMART. YOU WILL HAVE NOTHING BUT PROBLEMS. Don’t call me if your gun does not work with this ammo. Any high base heavy loads should work fine in all guns, including buckshot, slugs, and steel shot waterfowl ammunition.