Well I'm a little late but I only just obtained an ICS C-15 and just tore it down last night. Discovered a bunch of interesting things, I'll do a full review soon.
However.... feedback from my new ICS C-15 with Colt Trademarks:
1. Outer Barrel Screws cut into the Inner Barrel
The 3 hex grub screws that are used on the barrel extension cut too far into the outer barrel and actually protrude into the barrel cavity. This results in the outer barrel pressing against the inner barrel, making removal next to impossible.
The only way to remove is to loosen the 3 screws and then it slides nicely. Looks like the grooves cut into the threaded portion for the grub screws to rest against is too deep. Will do some experiments to see if this is true.
This causes the inner barrel to not float and hence the hop-up spring does not have any effect pushing against the body/barrel and does not seat 100% against the mech box like it is designed to do.
Not sure if this is a global issue with all ICS M4's or mine was just a bad Q/C issue.
2. Barrel handguard retainer was too close to the body making removal and insertion of the hand-guards next to impossible (especially being full-metal RIS/RAS units). Solved by loosening those 3 damn screws and turning the outer barrel one turn and then re-tightening the 3 screws.
3. Design and sell a Metal Hop-up of the same design. I love this ICS hop-up, it holds it's setting due to the "clicks" (haven't torn it apart yet to see how it does it) and is very easy to adjust. However it is very brittle and if you try to pull out the inner barrel by the hop-up, you might snap it.
4. Solid Stock Sling Loop Insert
Provide or sell a simple plastic or metal insert with a M5 threaded hole in the middle that allows one to remove that huge rear stock sling hook (which just gets caught on vests, webbing, gear, etc) and slide this insert into the hole in the stock, thereby covering the hole (which when left open, still catches in vests, webbing, gear, etc). It needs the M5 thread to allow the lower bolt to thread into it.
Currently I'm using a spare nut from a TM stock spring guide (it just happens to be M5 as well) but the slot/hole is still present in the stock.
Don't want to permanently fill it as I may want to use it one day... never know.
5. Leave the colour as is. I love it, its slightly darker than the stock TM colour and seems to match a real colt nicely. Maybe the early ICS M4's were much lighter, but this Colt version is perfect.
I'll see if there are more items to add.