Is the hard to open bolt a sign of high pressure?
Probably. This ammunition was surplused because it was no longer safe to store or safe to issue. One problem with old ammunition is that as gun powder ages, combustion pressure rise.
Is this ammo going to damage my rifle?
If the over pressure indications are genuinely from the ammunition, than it could. Rifles and pistols have been blown up with old ammunition. See the pictures of the shattered Garand in post 37 of bought ammunition.
I have been telling the shooting community about the problems of old gunpowder for years. I am going to tell you, it goes in one ear and out the other. Shooters really, really, really, don't want to believe that their ammunition is not immortal. I think it comes down to denial. They believe they are going to live forever, and therefore their stash of ammunition also has to be there forever. You can understand the logic. "
I am immortal and so is all of my stuff." Anything that contradicts this basic desire is erased from their memories.
Recent threads where I have commented about the problems of old gunpowder:
Ammo storage
https://www.thehighroad.org/index.php?threads/ammo-storage.826681/#post-10655071
Bought ammo
https://www.thehighroad.org/index.php?threads/bought-ammo.827113/page-2#post-10656970
Now if the ammunition is in fact causing over pressure conditions the best thing to do is pull the bullets and dump the powder. Inspect the cases, look for signs of corrosion. If the cases look OK, load with nice new gunpowder and shoot a few. If you do not experience case cracks or case separations, then it is probably OK. The release of fuming nitric acid gas from gunpowder embrittles brass. If the fired brass has an abnormal number of cracked case necks, pull the bullets from the reloaded stuff, save the powder, and dump all the cases. If you get hurt firing cheap old ammunition, whatever ever savings you might have had will be offset by your medical deductible. Offset by orders of magnitude, it will be an exponential difference.