If you pull off the tip and see that it is a pin. And your recliner is fabric. I think they could be upholstery pins.
Yup
It's also unlike any upholstery fastener I have ever seen.
They look more like double-headed nails. They are used in temporary structures so that the top head is exposed making it easier to pull out of the structure. Also used in electric fencing, where the electrified wire is wrapped around the nail in between the two nail heads.
The gray cap is to protect yourself from getting jabbed?