FG
Well, I think the guy has somewhat of a point, in fact it has been discussed on other forums.... the all copper bullets do not obturate very well at all they are really pretty solid... now on the other hand copper clad lead bullets do obturate to a certain point and force the sabot up even tighter on the lands and grooves.
I shoot Nosler's (til I run out due to price) but they do have lead exposed on the bottom of the bullet... the force of the gas pressure on the the bottom of the sabot also forces the bullet to obturate within the sabot increasing the gas seal as the bullet runs up the barrel.... can it obturate enough to fill the whole barrel? not a chance...
One guy did some experimenting with a 50 cal Speer (no gas check)inserted into a 50 cal ML. The bullet was tight enough with a fouled barrel to offer some resistance going down but on a clean barrel it would slide down really easy, anyway he experimented with this for awhile. While the bullet shot OK from his ML, because it did obturate it was not reliable... some expanded more than other creating different pressures and of course different POI's...
Outdoorsaman - if you are willing to shoot those expensive bullets - you might try some Nosler's Partitions- they shoot very well in all of my ML's, of course they are not as areodynamic as the others. The .451/260's are an easy 175 yard bullet and the .451/300 will get you to 200 with plenty energy to spare.
I shot a toehead whitetail last year @ 170+ yards with a 260 it was a complete pass through - destroyed all the vitals inside the chest and came out behind the right front shoulder.
good luck in your search....