internal set-up: today's parts are way cheaper than the old days (hehehe old days - tiguwang na gyud hehehe)
for compression - if you got the funds, buy a bore-up kit...this kit is consist of the ff:
> bore-up cylinder
> cylinder head
> air seal nozzle (ASN)
> piston head with bearing
for spring - a 120 equivalent spring would give you the modest rate-of-fire (ROF) averaging at 400-420fps but would be stronger on other aeg's with good compression...a 130 equivalent spring would give you a higher fps but be careful other springs available in the market are rated at 130 but would give you a higher fps & a high maintenance aeg due to inconsistent quality
spring guide with bearing- since most of the ACM in the market is setting their aeg's with a piston head with bearing as a freebies, this internal part is a must, mura ni sila ug mag-asawa, pares gyud ni sila...this would minimize broken & bend springs
piston body- your choice of what to use but if your looking for a good PB, look for the one's with metal tooth...
kinds of piston body's:
>polycarbs - most of this comes with your stock aeg's
>aluminum - dali mahilis compared to metal tooth
>metal - although it would break but of the choices, i would go for this PB
>titanium - the strongest among all others but the price is right hehehehe
>stainless steel - it's not stainless it's just coated steel, chrome plated i think
gears - for a high ROF set-up, a 1:13 gear ratio is the best option for you but springs higher than a 130 would obliterate this gears...for aeg's with stronger springs- a helical set-up is a must
shim-set - this part goes together with your gears, 'mag-asawa pud ni sila'...a good shim would last your gears a long time but of course gears are one of the moving part in your internal set-up so it's of no guarantee
Metal bushings - now a days, aeg's are set-up with metal bushings, it's good thou but incase you'll damage this part, there are tons of choices to choose from...just make sure you'll buy the same diameter with regards to your gearbox
the least of your concerns:
anti-reverse latch (ARL)- although the ARL that comes with your stock set-up, would do...always advice & ask your airsmith to check it from time to time...
contact switch - if your using a battery higher than 9V & your into semi mode, check this one also for wear & tear but if you don't want to change it from time to time...you can always buy a mosfet, you can always ask mospat...
tappet plate - incase this part would be busted, GUARDER tappet plates are so far the best that i had used for years but there are other brands in the market
spring set - this are those tiny springs that would skip/shoot out from your gearbox if your not careful with the opening process...this are already available in the market, not like before were we utilized the spring in pens & would mold & pattern it from the original one
rubber bucking - or most commonly known as a rubber hop-up, although there are tons of bucking brands in the market today, for me if available i always go for the guarder bucking...sa tinagalog pa 'iba talaga yung nakagawian na' hehehe
pinion gear - this is the gear attached to your aeg motor, go for the Eagle Force brand
AEG motor - if available go for the most trusted brand - Tokyo Marui (TM) but of course ACM brands are of good quality this days...i'm using the Eagle Force brand & still using it from the day one i bought my primary aeg...but this brand is a bit hard to find and if your really looking for this brand, you can always find it in the worldwide net hehehe
Hop-up assembly - careful with this parts, this would depend on what brand your aeg is
gearbox - although there are reinforced gearbox sold in the market but be careful in buying one, other aeg's are not drop fit with other brands
this are just basis for your internal set-up and of my own experience & opinion...others may have other opinions & experiences...& of course open for your criticism & remarks!