In Arena Breakout there is only 1 type of 12.7x99mm ammo.
Find this bullet base damage, penetration power, and compatible weapons inside this article here.
12.7x99mm Ammo Chart
Inside this chart, you can find a detail of 12.7x99mm ammo base damage, penetration level, and bullet velocity.
This ammo is exclusively compatible with AX50 Bolt-Action rifle.
12.7x99mm Ammo Graph
Here you can find this ammo base damage and it's penetration power.
12.7x99mm M2
The 12.7x99mm M2 is a tier 9 penetration ammo that deal 246 base damage plus 61 penetration power and 34.2 armor damage.
The ammo is insanely powerful and it's has extremely fast bullet velocity of 928m/s which is great for medium to long range gunfight.
Price wise, the 12.7x99mm m2 ammo cost you around 3024 Koen per unit. It's probably the most expensive ammo in Arena Breakout.
Conclusion
In Conclusion, If you want to use the AX50 bolt action rifle, the 12.7x99mm m2 is the only bullet that you can use.