A few questions before I invest.
 

My M4 now looks the way I want her so I am going to start to upgrade her internally. I figure I'll start with a a 6.03 tightbore. Now this is my question, The gun started off as a g&g gr-16. Is it worth it to add a Modify high torque gear set to a g&g mechbox, or would it just be better to get a completely new mechbox?

My experience with G&G mechbox shells have been quite positive. I consider them to be well made, and not necessary to replace unless you're going to put in a ridiculous spring.

If you're going to upgrade internally, start with a better hopup sleeve, followed by better airseal parts, followed up better bearing parts, followed up a new hopup unit.

My G&G already comes with a one piece hop up. Would it not be better just to buy a one piece metal hop up unit than to buy each piece seperate? And please correct me if i'm wrong but doesn't the Modify gear set come with good bearings?

Buy what each piece separate?

When I say airseal parts, I mean nozzle, piston head, cylinder head and cylinder.

When I say bearing parts, I mean piston head with bearing and bearing spring guide.

Throwing in some Sorbothane is always nice, too.

I mean won't it be more cost effective to buy a whole new hop up unit rather than a new hop up sleeve, then a new hop up unit?

Depends if you can find a good hopup unit that includes hopup sleeve. Brands I'd recommend (KA and Prometheus) don't.

There's nothing wrong with the internals of the gun. The airseal in the G&G guns are fine. Nothing that needs to be upgraded there. Start with a hopup rubber and a tightbore. Right there, that will increase both your accuracy and range pretty significantly. Your hopup may be plastic, but it works. No need to change what isn't broken and functions properly.

If you're going to dig into the mechbox to upgrade it for more velocity, all you really need is a bearing spring guide and a new spring.

Unless you're going for some crazy-ass spring, torque-up gears aren't required. You can get the similar effect (reduce trigger lag) by installing a larger battery and / or a high torque motor.
Personally, I'm a big fan of not fucking with things when they work the way they should.

You won't need a new box if your just adding that gearset. By other box I assume you mean something reinforced which is problematic with the modular gearsets, it can be done but you need to talk to someone specifically on here about it. You can get Modify gears that are not modular that will work just fine in reinforced boxes but other than that, due to the extra beefiness of a reinforced box they won't work. Jugglez of airsoftparts.ca can probably answer that for you better than I. Just a note on the STOCK gears, they are pretty sturdy and if you aren't going to upgrade springs or the like, leave it for now they will be fine.

I would recommend replacing (at the very least) the stock piston that comes with your gun as I've taken a look now at two pistons from G&G GR16s (different suppliers on each.) One broke outright after 4000 rounds and the other is starting to show wear after 10k. That's weak in my opinion as I used to have an AK upgraded to 490 FPS with well over 25K rounds through it, a ton of full auto without any piston related problems. No problems actually. I personally upgraded both my G&G GR16s with Modify FTKs from airsoftparts.ca. There are pictures HERE at the end of the post, as well as a wee bit of a review on the internals and some pictures of the broken piston I'm speaking of.

Anyway, yeah, at least the piston and while your doing that you may want to consider just replacing the entire cylinder/nozzle section all together. Your call.

If you're using a metal body right now I would go to a metal hopup. I don't know if you have a metal body or not and who makes it, but in my case I have Dboys metal bodies and I find although the seal right now is OK the hop unit seems a bit more wiggly than it should be. I have metal units on the way so as of right now I can't say for sure if that will fix it but who knows. Someone please correct me if I'm wrong, but point being if your in a metal body you probably should replace your stock hopup with a metal one. Just my opinion.

Anyway, time to go check the mail...

Cheers and GL

.... this is as well VERY true. :)

Alright, so for now a tightbore, a new motor, and a new hop up rubber.

High torque motor for what? What kind of spring are you using in there?

Question is, why are you upgrading at all?

High torque motor isn't really required unless you're pushing a really heavy spring. If I'm not mistaken, you're keeping this gun under 350 fps so you can use it indoors. No need for high torque anything there. I'm sure that even wtih the stock spring, new hopup, tightbore, and a bearing spring guide, you could get the gun up to the 330-340 mark without having to replace anything else.

You can get it up to the 360ish mark with just adding a tighter bore barrel. It comes stock with a 6.04, by just going to a 6.03 awhile back on mine it brought it to the 360 mark. Spring guide is a bearing one but the tube on the guide itself is plastic.

The CAS G&G don't come with a tightbore.

But it's the same mechbox as the higher end models, so if that one has a bearing guide, then so does your gun Tom.

Yes it does have a spring guide. I do notice that there is lag on the trigger. It seems like in semi mode it " takes a moment to ramp up", thats the best way I can explain it. And it does come witha 6.04 barrel. IT says it has a high torque motor as well but to be honest it seems a little flimsy comapred to my other guns.