1gewehr

Marine Infantry were issued M1 Garand from mid-1943-onwards. But the M1903 Springfield served them well during heavy fighting earlier. If you are going to stick with a bolt-action rifle, I'd go with a No4 Lee-Enfield. I have one that will reliably do 2" groups with new PPU ammo. If you can shoot that well, you're doing great! And the Lee-Enfield is still a decent price as far as guns go these days. As far as other out-of-the box ideas: an M1 Garand is still a good option. Ammo is readily available, clips are still reasonable ($1-2 each), and you can get one directly to your door at a decent price from CMP.

My primary rifle is a M4gery with iron sights in 6.5G. I also have one with a 20" barrel that I use with whatever scope I want to use at the time. I also have an upper in 5.56mm that I can plink with using a .22lr conversion kit. If you only have funds for one rifle, get it in 5.56mm with a .22 kit. That'll take care of all varmints from mice and moles up to coyotes. I also have an FN-FAL in 7.62NATO that is good for larger varmints. There are chamber inserts in .32acp that are VERY useful for small critters (rabbits, skunk, etc) and are pretty quiet. But the chamber inserts make the rifle a single shot.

I know that you specified shotgun. But, you also said it was for around the property and for varmints (two and four legs). In my part of the woods, 'varmints' usually means coyotes. Coyotes are usually at the max range (or further) for shotgun use. My recommendation would be an AR15. Pick your caliber, but I use 6.5 Grendel. It's instant death on coyotes, and an inexpensive IR sight will allow use in the dark. And, around the house, it's handier and lighter than an 18" barrel shotgun. (Unless you hang all sorts of crap on it). Just my humble opinion!

I had one many years ago. It was only a tiny bit larger than my Star PD .45 carry gun, significantly heavier, and more ammo sensitive, So, I sold it.