buckboy, are you prepared to spend at least $1 per bang(up to $2), even with reloads?
If yes, then I would lean you towards the 300RUM and 210gr to 225gr match bullets. You should also consider barrel lengths at least 28" long. The longer the better. Bullets will run 40 to 65cents each. The lighter bullets just don't have the ballistics to be interesting beyond 1000yds.
I have a 35" 300RUM. Haven't played with it much due to shooting costs and bullet availability. I will this year though. Come on Hornady, make then 210's.
Burning anywhere from 100 to 120gr of powder in these cannons eats up lbs of powder in a hurry. Better be prepared to buy in kegs. Unless you work with surplus, your powder will run $25 on the cheap side to $40/lb at the local store. 55 to 70rds per lb. That's 35cents at the cheapest.
Brass life is also something to consider. Wby brass, if available, will be around $3 by the time it lands in your mail box (that can also be said for Lapua and Rigby based cartridges). I would expect 5 to 8 firings before it grows too much. That adds 38 to 60 cents to your per rd cost. RUM would be much cheaper (60cents each???) .
So unless your banker has a very liberal 'food' budget, that cannon is not going to make a lot of noise on an annual basis. There are alternatives (338 is not one of them as it costs even more to shoot).
If you are just plinking anyways, initial muzzle velocity really doesn't mean much at 1500yds. Even 'high BC' bullets will drift in the wind.
So if set on a 30cal, consider a smaller case like the 300Wby, win mag, even the 300WSM. All can launch 178 to 200gr bullets and for plinking, it will work.
There is a US club that was plinking to 3000m with 308's. I have shot mine to 1450yds with good effect (almost deal calm though). Crappy in the wind.
Better yet, consider the 7mm and 6.5mm. The LR ballistics of these are better then almost all 30 and 338 bullets. Shooting costs are 1/2 to 1/3. Barrel life is about the same. Recoil and noise is way down too.
For a plinker that has an effective range of a mile or more (I will be shooting mine this year to at least 1800m/1960yds), I would suggest the 7RM or 7WSM or 7 Mystic launching 162gr Amax in a 24" to 26" barrel (longer is better if building a custom rifle). The 280 and even the small 7-08 will work in longish barrels.
A 6.5-284, 6.5 Mystic would do the same thing but no off the rack rifles. A 260 or 6.5X55 will work but tend to come with short barrels.
Even the 243 can be made to work if you go with a custom twist barrel and launch 105/107/155gr bullets. There are many more exotic choices but you are forced to build custom rifles and shoot boutique bullets.
I have cannons in all calibres except for the 40's and 50's. I shoot LR so can say from experience that unless you want to do LR hunting, a big expensive bullet doesn't do much more then a quality high BC small cal VLD slug.
In fact, the smaller cases are easier to tune for more accuracy.
Better to get a platform that you and your banker can afford to use. You are planning on shooting this thing aren't you?????
Jerry
Some of my cannons past and present to wet your appetite
300RUM
300Wby but twist was too slow for my tastes.
My prototype 6.5 Mystic. Another coming this year.
My 6.5-06 now with another CGN member.
A Krieger barreled 7RM soon to be on its way to another happy owner.
My LR Savage 7RM w/muzzle brake
The 7 Mystic