Upon (long) reflection, I
will tackle the scope question. My views differ from most on this.
I feel that 16x to 18x is adequate for distances out to 1000yd. My reasons are simple; not everything in the field of view needs magnification. Higher magnification reduces context (surroundings, for size reference). This can make finding the aiming point difficult, and can drive one crazy while trying to get a steady aim.
I also believe that as distances decrease substantially,
higher magnification can help resolve a finer aiming solution for
smaller aiming points.
The distinction is one about a
sharp enough aiming resolution as opposed to a
popular one. I have six of them
( https://www.midwayusa.com/product/1009354695?pid=835407 ), one on each of my pair of Stag Model 6 Target rifles, one on my PSA 24" 308 AR, and one on each of my Savage Model 11 Long Range Hunters (one is .223, one is .308). My sixth is my 2002 Savage 10FP, rebarreled in .260 Rem (28in), an amazingly accurate LR rifle. The two Stag 6's are identical, intended for private one-on-one comps.
An accomplished marksman can hit anything worth hitting out to 1000yd with 16X. My own LR rifles have scopes with up to 32x, but they are seldom used at more than 20x, unless the target is in close, and this is because adjusting aim for the comparatively higher optical axis requires significantly more accuracy.
BTW, I started my serious distance shooting at Cherry Ridge, but now live 50mi N of Mexico in SE AZ; in a small older development named Sunsites.
FYI, the 6.5CM is totally adequate for 1000yd comp, and bigger cartridges are more hassle than you'd need for that kind of comp.
Your rimfire is not a bad practice chambering. I prefer .223/5.56 for that because handloading that chambering allows me better control over my ammunition quality. I have also fired the 223 in 600yd F Class at the National level (2018 SW National LR Championships at Ben Avery Range, Phoenix, AZ. I didn't win; I don't normally win. I compete with myself, so I always lose. If my health doesn't improve soon, I'll be done with competition.
Greg