from what ive seen of them unless you'r takeing on the m4 massive then there's no need to upgrade them performance wise,as for batterys go to the component shop website,again unless you'r useing it for walk on days then the original voltage but highest capacity that will fit as most ww2 games have an ammo limit so high rof isnt a great thing,component shop give size measurements on their batterys.these guns are usually in need of a spring downgrade as they come out the box at about 400fps with 0.2g bb's 
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if you really cant leave it alone then a tight bore barrel is a good option but dont get a cheep one ,a mad bull barrel is a good not too dear option.
armoury
m1a1 Thompson,sten mk2,mp40,stg44,sterling,mk2 bren gun,lee Enfield no4 mk1,Mauser Kar98, Walther ppk,smith and Weston m10 and Mauser m712
Give me a big enough hammer and a place to stand and I could fix the world.
i'll kill a man in a fair fight or if i think he's going to start a fair fight or over a woman or.......
a problem shared is a problem halved ,but an advantage shared is no advantage at all
if a job's not worth doing then its certainly not worth doing well
Don`t bother
My MP44 used last Saturday at CIA game with original battery lasted all day no problem and that was at 600 rds per life!
Best advice get a good battery charger that discharges and then goes onto trickle so the batteries don`t cook and it will be fine.
Tight bore barrel not sure if it is worth it leave that to the rifles, spend the money on extra nice kit uuummmm 
