A recent acquisition, a 98 Mauser with a mint barrel in 25-06 has me scratching my head.
I was fireforming brass with some 117 Hornady B/T and every shot keyholed.
As said the bore is excellent so maybe the culprit is barrel twist which I have yet to measure ??

Could be a 1-12". I'd check that first.

Check twist rate first, if it is 1:12" twist, you won't be able to run anything heavier than 100gr, and it may not shoot any TSX 100gr bullets either.
Don't sweat it, though, just use 100gr Partitions and shoot everything you would have with thise 117's.
I hace shot fallow deer with 100gr Partitions and 100gr BT's, both get the job done.

Cheers.

My brother used to have an Oberndorf Kurz Mauser in 250 Savage which was accurate with any 100gr bullet and the 117gr Hornady round nose but would not group some 120gr bullets we once got for it, the brand escapes me but they were a long sleek hollow point. They did show signs of instability on the target. \

The Hornady round nose was a good killer on red deer.

if your load was on the weak side - you might have to push them a bit faster

As I understand it, there's a confluence of 3 factors which must come together to prevent keyholing.

1) Length of Bullet (which often but not necessarily correlates with bullet weight).
2) Rate of twist.
3) Velocity.

If you can't push your bullet fast enough for the rotation imparted by the rifling to stabilize it, it will keyhole.

There are 3 possible solutions to keyholing:

1) Use a shorter bullet (which will usually be lighter, but not necessarily).
2) Increase the rate of twist.
3) Increase the velocity.

Any one of those, if changed enough, should do the trick. Or a combination of any two or all 3 together.

I've had bullets keyhole in my 6mm Rem. with its 1:12 twist. Long 95 grain bullets keyhole, shorter 95 gr. bullets do not.

Just how I've come to think of things.