I noticed on H&N website that these are specified only for PCP rifles. Do you know why?
Because the pellet are light and hard they do not generate a lot of back pressure. H &N is worried that a spring/piston/gas piston gun will damage the piston seal because the piston will slam into the end of the air chamber. PCP airguns have no pistons. Is this a valid precaution? No one has ever complained that this has happened.
This alloy is harder than lead pellets. Will it damage my barrel over time? What about residue buildup?
There is no lasting damage. Any cleaning procedure will remove the residue.
Has anyone used these in a Crosman 1077?
I'm shooting indoors on paper and plinking targets. I'd like to avoid dust. Will these pellets shatter when shot into a standard metal pellet trap or will they deform but remain intact?
No detectable dust. They deform nicely and predictably. Being lead free, there is no lead to worry about in any form. I shoot these indoors into a modified Crosman 853 trap, a Gamo Rocker trap and an Air Venturi Stampede, all at 10m. They work very well for my indoor range as described.
They will deform.
Stick some rags inside the trap and then it won't matter. (and it will be a lot quieter)
What is the pellet material made of? Is it pure or an alloy?
mostly tin. USA Shooting team tested these and found them accurate.
Are these pellets allowed in sanctioned competitions?
sure, no competitions out there make restrictions on the material used in a pellet. You can shoot paper spitballs if you like as long as it's safe and gets you the gold medal go for it!
It would depend on the competition and its specific rules. You would want to ask the competition organizer.