Re: .300 Win. Mag, Loads 210gr,SMK
Pray tell just what kind of "custom" did you "put together"???
Will your barrel twist rate handle the 210gr bullets? Probably but if you have an 11 or 12 twist rate it may not do so well.
Was your barrel throated long enough to handle that particular bullet? Most .300win mags, unless your gunsmith knew in advance, are throated for 180 or 200gr bullets. Do you know how long the ogive measures on that matchking bullet?
"Putting together" a rifle out of some generic parts, or stuff your gunsmith gave you a "good deal" on; and then expecting it to handle a specific and specialized longrange load is only rarely going to work out.
Most guys who "put together" a custom know the reamer specs, twist rate, and know enough of their starting component interplay, or have seen that Sierra's accuracy loads are often right-on.
Wish you the best of luck.