Re: Modified 8mm Mauser
You might also want to take it out of it's stock. There are proofs and stamps under the wood-line and also on the stock itself that might tell you something...if it's original.
The one pic of the stock doesn't show the steel crosspiece used for the recoil lug. So, I'll just assume it's not original.
In general when it says Mod 98 on the side of the action it's a 30's-WWII built rifle in Germany for Germany. If it was a WWI rifle it would have Gew 98 (Gew short for Gewher {man}) Czecchlosvakian made rifles were usually marked with the model if "appropriated" by Germany in WWII. They were mostly VZ-24's and some VZ-23's.
As noted the safety is aftermarket, but I can't tell what company made it, only that if it's a two position instead of three, it's Buehler style that allows for the use of a scope with no other modification than to replace the safety lever. IMO, these are the least safe, safeties I've ever seen. Due to the ergonomics of handling rifles, these seem to forever be getting knocked off safe.
Anyhow, the 8mm Mauser is an outstanding round. Very comparable to the 30-06. For long range shooting the best rounds are the 196-200 gr. Nosler makes a great 200 gr. bullet in their custom Competition and their Accubond. The 8mm started with a heavy round nose, then in 1905 Germany went with the 150 gr. Spitzer and prior to WWII, they went with the 198 gr. spitzer. It's notable the 198 gr. bullet was meant to be optimal in the Machine gun and was engineered to remain stable out to 2500m. As originally built, the M/K98 rifles handle this round wonderfully.
If you love Mauser rifles like I do, a great book to pick up is <span style="text-decoration: underline">"Mauser: Military Rifles Of The World"</span> by Robert W.D. Ball