hikaru

I understand where you're coming from re: exploding guns, so I'll add my contribution and hope it helps you in your research.

With no easy access to AR-15 parts or BBBs in Australia, I've had success with extra O-rings and a gen 3 NF spring (I've also been using a sharpshooter 1 spring in another longshot, harder to cock than the NF though). I haven't needed to reinforce the bolt sled, and from what I can see there are no visible signs of stress.
You may have already found that the NF spring isn't long enough and does rattle a bit.
The sharpshooter 1 spring however is just the right length, and sits neatly underneath the stock LS spring.

I've been using the sharpshooter one for about 2 years now, no explosions yet.
All of my shooting has been indoors.

Nice!

But doesn't that interfere with the charging indicator arm thing? (if you choosed to use it)

Melbourne, Vic

Longshot, Vulcan, Recon, 2 x Maverick
I had to get each one of those imported to Australia (even though vulcans are recons are available here now)

Dart Attack's Longshot sidearm integration
EDIT: It's finished!
The integrated gun doesn't have a trigger plate, so to fire, you just pull back and let go.
(one day I'll get around to rigging the firing mechanism to the main trigger)