It looks like an aluminum head. markings;
SIG-PROT 19 VB85 F6.81
head diameter .761"
primer diameter .300"
diameter where it goes into case?.704
material: aluminum
it has an extracter groove
total length is .650"
is this some kind of priming system for bagged powder in a large gun, or something like that?

Hard for me to visualize this from your description, but I've had a variety of aluminum subcaliber chambers (sold off now) of this general size & configuration (*IF* I'm reading your description correctly!) which allowed the use of shotshell primers or small European rimfires in larger weapons.

It is not a barrel sleeve. it looks like you cut the head off the cartridge, then turned the case portion down so you could put it back into the forward part. it has the primer, extracter groove, just like a regular case, but has no body. I will try to get a picture of it to post.

Sounds like what you have is a 19MM signal flare. I believe these were used in an H&K made projector. I have a few in my stash.

Tom- thanks for the lead- I searched flares and found it. it is a 19mm launched from a hk efl.