Perhaps you’d feel more comfortable with the gun if I tell you about the background. This isn’t some single rifle (pun there) that someone decided to unload because it wasn’t right. It is part of a very extensive collection of big and very big/dangerous game hunting guns and ammo from the estate of an older man who used to take all this stuff on safaris. From the clues I’ve seen in dealing with all this, I’d say most of it has not been used much, if at all, since about the late 1980s. This rifle is just one of some 60 or so big game hunting rifles and shotguns, as well as about 50 handguns, that I acquired from this estate. It isn’t one gun that’s being dumped due to a problem. These guns all came with over a dozen 1.5 cu ft U-Haul boxes packed full of boxes of ammo. I had hundreds of boxes of all this ammo. Most of it has all sold by now. I’m down to just a few boxes of the “small” stuff – 375 H&H Magnum and .30-06, and that’ll all be gone by next week.
I’d actually planned to try shooting this rifle myself before I sold it (I wanted to shoot at one of our pesky squirrels with it), but then the ammo all flew out the door in a huge flurry, along with other calibers at the same time. I’m not used to such high volume sales, and it was a struggle just to keep up with it and not make any mistakes. Then, the next thing I knew, the ammo was all gone. So I guess I won’t get to shoot it. Unless I contact one of my buyers and beg them to sell me back a couple rounds. ;-)
Anyway, I’ve been so busy with selling the handguns and ammo that I haven’t even looked at selling the long guns that came with it. I’ll probably be trickling them out for the rest of the year. The only reason this Heym (the star of the collection), is up for sale now is because one of my buyers of 458 Win Mag and 416 Rigby ammo was in a conversation with me and it came out that I had this Heym and a couple Dumoulin safari rifles. I sent him some pics and he made an offer on all 3. I thought the offer was OK on the Dumoulins, but I just wasn’t sure on the Heym. They seem to be worth between $5K and $15K, but that’s an awful big range. So I told him I’d put it up on GB and if no one topped his offer in one 2-week run, he could have it (note the reference to that in the item description). The Dumoulins are still pending. So that’s how the Heym kind of jumped to the head of the line in selling the long guns.
I’m confident that this rifle was part of a large collection of working guns that were used on multiple safaris and that were in use up until the man apparently stopped going on safari. It definitely is not a one-off that someone is trying to unload because there’s a problem with it. As far as ammo, I can tell you that all of the 458 Win Mag ammo was of these types:
https://www.gunbroker.com/item/891641621
https://www.gunbroker.com/item/892945461
https://www.gunbroker.com/item/891642036
The owner had many, many boxes of these 3 types of ammo, so I assume that the gun worked well with them. The collection also includes a Rem 700 in 458 Win Mag, so presumably some of that ammo would have been used in the Remington. So I don’t know which ammo was used with the Heym, but it would have been one of those 3 types.
Not to mention typing "beltless" when I meant "rimless." Sigh, perils of typing on an I-Phone in a great Tex Mex restaurant (full - an hour wait - with no masks - may God bless Texas!) over a second hand-crafted Patron Margarita. 
Get J.J. to work up a load, wipe down the barrels, but some oil on the stock, and go shoot a buffalo. Looks like a pretty good deal to me.
