Re: 700 action / 300 RUM ?
Your factory Rem 700 LA magazine box will likely allow a 3.660" OAL, +- .010 or so.
Also, Wyatt's Outdoor offers an extended mag box, which will let you load out to about 3.8" OAL. Gunsmith will have to fit this.
Your gunsmith will only be able to tell you what the throat length is for that specific 300 Ultra reamer he is using.
If it's a SAAMI spec reamer, you've already got the wrong reamer... you'll want a tighter than SAAMI spec neck diameter and possibly a shorter throat in order for these bullets to contact the lands while fitting down in the mag.