Once I get my groove to groove measurements from the slug I also measure the inside case neck dia. of once fired unsized brass I plan on using, if you use several different brands especially in rifles with tight chambers it's a good idea to measure those as well as not all brass is the same thickness,their isn't much point in having a bullet sized properly for the bore that want fit in the chamber. I haven't run across that yet but it does happen sometime especially in some of the old war horses.
Next I will look for a mold that cast a bullet around .002" over that groove dia. that will also chamber in the gun once the bullet is sized, I can always size it down smaller if needed but I like to start out larger than I need to begin with. As to gas check I use copper Gator checks I get from Gas Checks - Sage's Outdoors there much cheaper and work just as well as Hornady or Lyman which are actually both the same gas check just sold under different names at different prices. I apply my checks with either my RCBS Lube A Matic or use the Lee or NOE push through sizer. The Lee sizers are fine for most applications but you have to check the final sized bullet to make sure it's sizing to whats stamped on the die. You can always hone them out if necessary if they are off a bit and it very easy to do. The NOE push through body and sizing die bushings sold individually have many more options as to sizes and the tolerances are spot on.
As to the mold I used in the above thread it's a custom mold by Tom over at Accurate molds that I modified the dia. slightly from his original design,basically I enlarged it to .315" on the driving bands so I could size it to .314" for my .312" groove 91/30 Accurate Molds: Custom Bullet Molds The bullet will only fit my 91/30 as it is to long for my M44 which has a much shallower throat as most all the M44's will. However if the bullet will cast large enough to fill the groove of your rifle the Lee 160 gr. TL RN or Lee 185 gr. RN both .312" gas check bullets will shoot very well in both rifles, I used them both will excellent results not only in the Mosin rifles but in my SKS rifles as well.