We’ve been mtbing for 25 years and have tried loads of junk pumps from almost every brand. This one from Rose bikes (was also previously sold as an Axiom pump) is the best ever.
http://www.rosebikes.co.uk/article/xtreme-airik-mini-pump/aid:21224
Cheap, reliable (our original one from 2007 still works like new), does road pressures easily (e.g. 100 psi yet still only holding it with fingertips) but still fast for mtb tyres.
We’ve stockpiled a load and some spare seals – at less than £11 including a secure bottle cage clip it is fantastic (and functionally better than pumps costing 2 or 3 times the price).
There is a 1/4 turn cam that holds it together when not pumping – mrs does a rough canal towpath commute with no ejection or rattles – not sure what it is like on rough gravel.