I purchased several machineguns over the past year from a widow.  I got while the getting was good.  Now that they've cleared and I actually have them all, I'm wanting to stock up on some spare parts.  I'm looking for help from people that are more familiar with these weapons to what kind of spare parts I should pick up.  I'm not sure what typically breaks in these.  I'd like enough parts for two lifetimes.  So what do I need for these?

Browning 1919A4
Uzi
HK MP5
M16
Thompson 1928

Just from having a M16, I would say stock up on the springs, every spring that goes in lower including the buffer spring.  Change them out after about every 4K-6K rounds.  I would do the same for other MG's get the springs.

UZI:
- at least one parts kit
- spare extractors
- spare top cover or two
- spare barrel nut or two
- several spare barrels
- spare bolt or two
- piles of mags
- recoil springs

MP5
- parts kit would be nice though spendy
- one to two barrels
- bolt head
- rollers in various sizes
- roller retaining plates
- recoil springs
- firing pins
- as many mags as you can afford

M16
- bolt groups
- firing pin retaining pins
- cam pins
- extractors
- extractor springs
- recoil springs
- hammer and trigger springs
- uppers of all flavors
- gas rings
- lots of mags

Thompson and 1919 I don't know.

Don't forget to buy ammo.

barrel ratchet spring
firing pin assy.
recoil captive spring assy.
trigger
extractor assy. or at least small parts for extractor assy.

a couple extractor springs.  a spare bolt will come with just the installed one.

Forgot extractors, extractor springs, and hammer springs for the mp5. The extractor springs are a weak link in the mp5. It is a good idea to have plenty, and buy factory HK German.

For the 1919a4, definitely get a spare complete Lock frame.  The ears on the lock frame sometimes break off.  Also time it in the gun this way you won't have to fiddle around timing it if you break one (Plug & play).  I would also get a complete top cover.  Why - if you ever have a OOB (rare but can happen), you can fix the old top cover with a vise and a hammer, but you may not have a vise where you are shooting.  
 

also,  get a second barrel and barrel extension headspace it with yoor Bolt in case you have a case head Sep and cant clear the chamber quickly

 

The M16 is usually  the easiest. If you own a AR15 your basically have 99% of the spare parts you will need, just need to pick up the full auto parts, basically the hammer, the selector, the sear & pin and a M16 bolt carrier if your ar15 doesnt already have one.