.003" max variation isn't too bad, but shows you can turn all the brass to the smallest size, ie: get rid of the humps. Many cartridges can run upwards of .005".

Once trued (turned) in size, they will give you the extra .002" to .004" clearance needed to shoot proper sized grooved lubricated bullets.

Forester and Hornady both make outside neck turners that can be used to true your brass. Inside reaming does not true the brass wall thickness, only outside turning can do that. That way, you could shoot grooved lubed bullets of the proper size. Outside neck turning can be done with a mandrel and in a lathe, or with a proper outside neck turning tool. It should be much cheaper than buying a single PP mould, probably, then normal moulds (gas checked) will work just fine.

PP bullets do or rather can slug up some to fit, but need a verty fast powder to do that. Normally, smokeless powders are very poor at slugging up an undersized bullet.

Real black powder works perfectly in the slugging up or obturating of the bullets to fill or fit perfectly. Trouble is, it introduces the need for case cleaning, BP lube balls between the bullet and powder, special wads that allow burning powder access to the lube ball, special lubes that soften BP fouling, and much reduced performance compared to smokeless powder. It could/would turn your Ruger into a .450/400BPE, though - if that is attractive to you.

Flat base works just fine for PP,w ith the excess paper merely folded over the base to almost the centre of the bullet. No twisting and cutting off of 'tails' that actually hurts accuracy (did in my rifles). I switched to flat base from the cupped base and used proper length patches for each bullet weight for improved accuracy over the twisted tail, cupped base bullets. Too - I wanted to use am undersized bullet for easy chambering with BP fouling. The cup-based bullet I had was already bore size and too large for this. With black powder loads, my after patching bullet diameter was .451"- ie: bore size. According to Paul Mathews you should be able to push the patched bullet through the bore and leave rifling marks only on the paper. This was how I made mine - using proper thickness patching. Accuracy was wonderful right off the bat with the first load tried.