with the vacuum seal thingamajigs, do you run the risk of having the plastic not necessarily melt but become more malleable and stick to the guns?
Not in my experience. I had a box of spare mag springs, assorted screws, firing pins, a spare bolt etc.. It kept getting in my way everytime I turned around so I bagged all the stuff in a vacuum bag and put it and some other, not used leftover stuff in a box in the attic to get it out of the way.
That was 8 years ago. Every once in a great moon if I need a replacement screw because I bugger one up, I pop up to the junk box in the attic and cut the bag open, then reseal it.
I gave the stuff inside a shot of corrosion X when I first stored it, the parts still look like they were just put in the bag and the corrosion X doesn't eat the bags or turn to a sticky mess.
I have made my own battle packs of 22lr when I got a bunch on sale a few years back. Drop a couple of bricks in a seal it. It has taken several years but I finally shot the rest of those bricks up about 6 months ago.
The only caveat I am aware of when using the bags, is to make sure you don't get a puncture from a sharp edge of anything.
Otherwise the vacuum sealer is a hell of a deal for storing just about anything. I use it to store carb parts, food, matches or anything else I want to keep dry.
They work like a champ for me, and while the bags aren't the cheapest resource on this planet, the protection and convenience they offer is worth it. I buy a box of 6 rolls from Walmart for about 30 bucks. Each roll is 22 ft long and 11 inches wide. Each roll lasts a long time for me.