Fletchette rounds

[4thPointOfContact]

Or.... work up a couple of these...

 

[Shadow 7D]

But I doubt that they will stabilize or be anything close to accurate.

 

[Strahley]

If you need armor piercing, get an M1A and some full metal jacketed ammo

Or just drop a 12ga slug in the chamber

 

[Kliegl]

They used to make double ought buck for a 20. I killed my first deer with it, some 22 years ago. I hit it with 3 of the 5 pellets. Not bad for a 9 year old.