hi have a hardtail with Sram XO rear mech and X9 shifters. its only a couple of rides old but have been experiencing ghost shifting on bumpy downhill sections. Putting it into big ring generally solves the issue, but not really and overall solution.

Anything worth checking or is it the norm when things are bouncing around going downhill(s)?

Probably just cable stretch (perfectly normal after a couple of rides on new cables). Just a gear set up should solve.

I foind that SRAM mechs are far more sensitive to 'B screw' adjustment than SRAM, I had sporadic trouble with an X9 until I set the gap as described in the 'instructions' (6mm clearance jockey wheel to pulley when in small ring front, large sprocket rear)

In typical SRAM style they released a bunch of rear mechs that not only had too tight bushings that hindered shifting but also had adjuster screws that slowly work their way loose over the course of every ride. In some cases the screws fall out and are lost. I would check this first and apply some thredloc to the screw and super glue to the useless plastic insert the screw sits in.

If that fails check cassette, chain and chainring wear.

cheers will check, keep the suggestions coming 🙂

I foind that SRAM mechs are far more sensitive to 'B screw' adjustment than SRAM,

Than Shimano?

If that fails check cassette, chain and chainring wear.

It's a few rides old!

Chain too long would be my shout if putting it on the big ring cures it, too much slack and it'll bounce between gears.

I thought ghost shifting was prevalent on full suss bikes where the compression of the suspension causes the cables to pull and shift gear. Yours sounds more like just the chain bumping off, maybe get a chain device? 🙂

I foind that SRAM mechs are far more sensitive to 'B screw' adjustment than SRAM,

Than Shimano?

Oh, I meant shimano 😳

I have had the problem with all and every SRAM rear mechs and the front mech is not much better. My Deore bits on a 400£ GT were near darn faultless in comparison. Next upgrade to the Anthem, Shimano XT rear mech and XTR shifters… Have to fiddle the X9 stuff every other ride to get them not rub, ghost shift, skip and so on …

It seems that it is me or me and SRAM don't go together as three pairs of Avid Juicy's were just as unreliable and fiddly to sort out.

Sorry not very helpful but had to get it off my chest.