I didn't understand how the seating depth would affect stability either - I would've thought that the projectile would have aligned correctly as it went into the rifling, even if it were slightly crooked initially as the result of a large throat. I wasn't expecting this many problems either and was open to all advice (opinions?). And, after all, there was evidence that those projectiles seated out went through the target straight and there was no evidence of this for the deep seated projectiles - all evidence for these were completely sideways through the target.

The guy that gave me this advice was shooting 45/70 cast loads that were going really well and consistent (1010fps) - he was using Trailboss. He had also set his chrony up what I thought was an unusaul configuration. He didn't have the screens on and it was mounted well below the boreline, probably in line where the screens would have gone - but it worked evrytime.

The projectiles are 0.458/0.4585 as best as I can measure. These are not a gas check style so I'll try Trailboss (should get around 1000fps) and experiment with seating depth. The get caught up in the action if seated right down to the crimping grovve (ala typical CZ fashion for the second last round in the magazine with round nose) but even with only the one round in the magazine. When they were seated out this didn't happen.

I may try the Westcastings (higher velocity rating) and AR2205 again at some stage in the future. I won't be casting my own at this stage (battling for shooting time and reloading time as it is) and will have to rely on "ready rolled" - these are relatively cheap anyway.

Also, I tried a round with the projectile seated out (as per my first attempts), no crimp, no powder and a standard CCI primer (CCI200) andf it drove the projectile easily an inch into the rifling proper! It took a decent effort to knock it back out.

Oh yeah, the guy at the range also had a bullet mold special made in the states for his rifle. He was going on about the seating depth, meplat, bullet diameter etc and said there wasn't a standard mould available that was suitable (he also said the microgroove barrel on his Marlin was 0.460 and, even though molds are listed at this size, they are generally not imported in Australia). There is a website where you can design the mold and they will manufacture for you accordingly, and the cost is reasonable.