OK here goes....
On a TM MK23 is the hop up rubber or metal?
The Hop-up rubber, is rubber... the part that presses the rubbed down onto the BB is metal...
I bought the tm mk23 for a good deal and it shoots dead accurate.
The TM MK23 (an the Y&P Mk23) shoot quite accurate as a stock platform.. the BB's drop off quickly when completely stock, but once an extended barrel is installed that problem can quickly be eliminated. The fact the weapon is forcing the gas in one direction makes the replica very consistent, and once you dial in the Hop-up, that makes shooting consistent on the guns part. you learn where it shoots and you can consistently put the BB in the same spot time and time again.
However when I look at the hop up it looks like it's chipped on the right side looking at it from the back
I don't know what part you are talking about here... more information is needed to address this.
So like are there any replacements?
So like, not really. Your best bet is to find some one with a broken one and salvage the part off of it... Some stores may sell some parts, but I haven't found a good place to get parts from.
Also do I need to oil this gun in the internals?
Yes and no... the replica doesn't really need to be oiled no, most of the parts are plastic and since there is no blow back the oil will really just sit on the internals and catch dust and debit from normal usage. HOWEVER the manual does suggest oiling it (don't do it the way the manual says and oil the top BB of your mag, you will have to fire between 100 to 300 rounds to clean off the hop-up rubber before your gun shoots strait again)
It slightly bothers me but the fact that it shoots 0.20g bbs dead accurate does not worry me. It's got a modified extended barrel.
Like I said before the guns accuracy can be related to it's constancy of gas flow... the extended barrel is really only going to help make the BB go farther.
Also I want to use heavier bbs with it for the heck of it. Should I use 0.23 or 0.25? Would I need to put the hop up higher or lower?
I have seen them fire .43 .36 .28 .25 .23 and .20 bb's
I my personal preference is the .28's The accurate range is around that of an AEG going higher may gain you a few feet more, but the cost of the BB's tends to be way higher...
Correct me if I'm wrong here but generally the heaver the BB is the more hop-up you will need to get it to spin...
With 0.20s the hop up is more than halfway turned.
Because of the way the hop-up works with the MK23 that statement means very little...
The hop-up rubber on the MK23 comes with a rubber nub inside. The metal piece labeled hop-up nub in the photo is pulled down onto the rubber by the adjustment knob. when you turn the hop-up up the piece is pulled down more.
SO, if you get to the point where the nub inside the rubber piece is no longer working, you have get a new MK23 hop up rubber (the only piece that you may be able to find sold individually)
However, if the rubber is still in good working order and the hop up is not working correctly, the metal piece can be slightly (SLIGHTLY I SAY!) bent to compensate for more or less hop-up as needed.
A photo for your enjoyment and to help clarify my above statements...
Hope this helps... if you need more info or photos let me know...