Avoid the left-or-right aluminium breech like the plague! On mine the bolt is a loose fit and snags against the end of the opposite groove making cocking a real effort. Also the left hand groove is wrongly positioned so the o-ring doesn't seal and the hammer peg hits the back of the bolt on firing.
Their extended probe bolt is fine though.
Various aftermarket trigger setups are available; I just got a kit from Gmac and I'd recommend it.
Of course you could just polish up all the bearing surfaces and compress the spring a little. Yhat's surprisingly effective.
