I have a 257 wtby. 257 lazzeroni scramjet and a 25/06 but have never tried the gs custom bullets. I have read on their website about them. Just never have got around to ordering anything from them. These guns are primarily my varmint/coyote guns. The bullets I use the most is the 75grn v-max. or the 115 grn berger vld. I would be interested if anyone has tried the gs custom bullets.
With 75 grn vmax I get 4250 fps. It grouped at under 1/2" at a hundred yds. It is also fairly flat out to about 400 yds. I have trouble keeping barrels with that velocity. I got about 750 rounds out of the last barrel. I don't mind though it is so fun to shoot and kills like lightning. Very messy if you are trying to keep pelts, but what ever you hit with it is grave yard dead. I have had it in a 1-12 twist and i'm in the process of installing a 1-10 twist shilen right now. I never could get it to stabilize the 115 bergers well enough to group well.
I live in Canada and was ordering them from a distributor in Alaska. For me, I was looking at a good 7-8 weeks, depending on when the distributor would put a new order in. Keep in mind though that 2 of those weeks was getting the bullets from the US into Canada.
Things have changed though. As I understand right now, GSC does not have any distributors in the US or in Canada. Because of that, I was informed that I could order directly and I would guess that in that case, the wait could be much less.
If you're not in a hurry, willing to spend a bit more on bullets, and are into experimenting, definitely give them a try. Because of their drive-band design, they do not increase chamber pressure when seated into the lands - but that may or may not be an issue anyway. So far, my .257WBY likes them best 0.080 off the lands. The science behind reloading them is a bit diferent than with lead jacketed bullets.
I'm having great accuracy with the 100gr HV's @ 3940fps. They also make a .257 92gr SP bullet that has an amazing BC of .567, however, because it's a solid SP, it would probably not be much good for anything but target and paper puching because it will not expand. Also, you would probably need a faster twist for some of these. Because copper is lighter than lead, the monometal copper bullets are a bit longer than lead bullets of equal weight and sometimes therefore require a different twist than you would expect.