And,FWIW,drill bits typically do not have pilots. They will pretty much follow another drilled hole,usually,but you can get some runout. Especially on an older.worn,light duty hobby gunsmith lathe. A #2 Morse tailstock is not quite the same rigidity as a #4 Morse tailstock.
If you were to predrill and introduced runout to the hole the reamer body would try to follow,the wobbly hole would try to introduce side loads to the pilot.
So now you have one of three situations. A floating reamer holder (good tool!) will allow the reamer body to wobble with your drilled hole,perhaps breaking off the pilot.
Or,if controlled by a perfectly true and rigid tailstock (A great blessing,but not every gunsmith lathe has one) you can just about bet the cutting flute contact will not perfectly agree with what the tailstock wants to do. If one or two straight flutes dig in,the cutter pressure goes way up.Things get ugly.Watch how the chips fill the flute . If one or two or three flutes fill with chips faster,one side of the reamer is cutting heavy...and you are cutting oversize.
There is one decent alternative to using a roughing reamer...but you'd have to melt a few brain cells and have the skills. And it will take some time,and maybe some risk.
Given you indicated both ends of the barrel true to a best fit pin gauge ,
If you have some nice,rigid little solid carbide boring bars,you could rough bore an undersize chamber,leaving steel for the finish reamer.
I've run a Monarch EE that was machine enough,and Ive done some small hole tapered bores to a shoulder that were the equivalent to boring a chamber. (plastic injection mold work) But manually boring a chamber versus using a reamer does not make a lot of sense. Time is money.
A good CNC lathe might make rough boring multiple chambers practical,but unless Ruger or Savage or another mfgr has found it more profitable..
Probably best to stick to tried and true.
A free,unsolicited tip. Chip control is critical. Pack enough chips in a reamer flute,the chips will gall and gouge. Pack few more,your reamer will break.
Thats reasonably obvious. But this next part is critical. Meticulously clean all,I mean ALL traces of chips from the chamber and reamer flutes before you run the reamer back into the chamber,every time. If you get even one sliver of a chip over the cutting edge of a flute,it will gouge a ring in your chamber,and maybe even chip the cutter.
I'm not racing the clock,but each time I clear chips,I wash ,brush,and blow off the reamer in a can of solvent. I blow out and run a patch or two through the bore,and visual the chamber area before the reamer goes back in.