Found an unusual wildcat brass case - what is it?

[RoyceP]

Case diameter is about 5/16" and 1.25" OAL. The bullet end is sized for a 6mm. Headstamp is illegible, looks like it was used a lot. I got it with a bag of 250 45 ACP cases that were from a range in central Texas.It has a very distinct shoulder. Tiniest center fire rifle round I have ever seen. Might be useful for squirrel or mice.

 

[SonOfGun]

It is not a wildcat.

It is a FN 5.7 X 28

 

[FrankenMauser]

Yep. It even says 5.7x28, right on the case head.

 

[RoyceP]

It's close to the 5.7 pistol round but with a shorter neck and it is 6mm inside the case mouth. Might have started out as one of those.

 

[higgite]

It's close to the 5.7 pistol round but with a shorter neck and it is 6mm inside the case mouth. Might have started out as one of those.

Do you think it might have expanded to fill the chamber when it was shot?

 

[TX Nimrod]

It is well known that when fired the 5.7x28 case shoulder is pushed forward, shortening the neck. Does a .244” bullet fit in the fired case neck?

One of hundreds of discussions:

https://fnforum.net/forums/5-7x28mm-ammunition/7996-5-7x28-neck-stretch.html


.

 

[RoyceP]

That makes sense - it is likely just a result of firing. From the discussion that you linked to it appears that reloads must be resized due to this being "normal".
Thanks for the great answer.