This is less an airline issue than it is a TSA issue. That is, the airline would (if you were carrying a firearm) want to make sure it's in a TSA-approved fashion. Since it's just a part, it's really up to the TSA to decide whether it goes through or not:
Firearm parts, including magazines, clips, bolts and firing pins, are prohibited in carry-on baggage, but may be transported in checked baggage.
Firearm magazines and ammunition clips, whether loaded or empty, must be securely boxed or included within a hard-sided case containing an unloaded firearm. Read the requirements governing the transport of ammunition in checked baggage as defined by 49 CFR 175.10 (a)(8).
Small arms ammunition (up to .75 caliber and shotgun shells of any gauge) must be packaged in a fiber (such as cardboard), wood, plastic, or metal box specifically designed to carry ammunition and declared to your airline.
However, they don't specifically say how to pack just parts.
You may want to Ask TSA:
The only problem is that TSA agent training is .... uneven, and you could follow the law and still have your bag opened and examined. Which is why having the lock on a hard suitcase with a firearm is a Good Thing®, since they have to call you over to unlock it and examine it in your presence. Theoretically.
Hard suitcases are pretty cheap, you could get one for $70 or so at your local discount retailer (TJ Max, Marshalls) or look in a thrift store for the old Samsonite clamshells.