Open up your stock 1500.
Take out everything and get the turret. Pry the cap off with a crowbar or something to create leverage. Careful, shit may fly everywhere.
Pull out the stock barrel assembly by unscrewing a few small screws on the bottom of the top half of the turret. The stock barrels and the black piece should fall out.
Take the back plate of the turret and widen the hole where the air from the tank comes through.
Drill 9/16" holes through the front half of the turret. Use a vise to clamp it down so you don't fuck up the turret. You might have to dremel a bit to get the PETG to fit nicely. Test to make sure it fits.
Now you're going to put in your barrels. Use approximately 13 wraps of e-tape around your PETG. Make sure it fits in snugly. Fill the voids between the turret and the barrels with epoxy putty.
This next picture has a couple steps.
While the epoxy putty is curing, take out the other half of your turret. You'll need one 1/16" thick 2 1/4" x 5/8" rubber washer. 2" in outside diameter will also work. Glue that on the back half of the turret. Drill your hole for rear-loading at the top of the turret with a 9/16" drill bit. You also need to cut out the rubber that is over where the air comes in.
After you've done all that, it's time to bust out the palm sander. Sand the front half of the turret where you applied the epoxy putty down. You'll want to sand it to the point where the rotation mechanism from the front and the back, both meet, while making a good seal with the barrels. It takes some time, and it's not fun, but it turn out great.
Assemble your turret back together. Use a piece of something longer than the barrel that will fit on the pin for the turret. Hammer the cap down and it should go together. It you pry the piece off too many times it will no longer hold the turret together.
Make sure you buy some barrel spacers from me, and put the black piece back on. You can just glue it on, or use the barrel spacer to keep it from moving.
Now you'l need one of two materials to do the airflow. This time, I used 5/8" ID 3/4" OD Polyester tubing. It fits directly over the stub for the back of the turret. You'll have to modify the shell a bit to get the polyester to fit correctly. I've also used 21/32" brass which has the relatively the same ID as the polyester. It has a thinner wall so it doesn't take as much dremeling of the shell. Either works.
Super glue it to the tank and the turret.
You can also replace the pump. I chose to in a different way. I used the pump from here:
I had some broken Hornet pumps I wanted to fix. A bunch of them that I have broken where the pump head, and the pump shaft meet like the one on the bottom. I used 1/4" PVC rod which nests in the back of the pump head with a little electrical tape. Over that is a 1/4'x1/2" Polycarbonate tube. Solvent weld that all together and it's rock solid.
I modified it to fit the 1500 grip by putting two bolts through it.
Next I had to modify the shell to fit the pump.
The hornet pump fits in nicely in the stock 1500 pump tube.
You'll also have to cut the rear-loading slot and make a dart slot if you fancy. Make sure you also glue the spring on the firing pin where it connects to the trigger. This will help the air release faster.
And you're done.
Final thoughts and comments:
These are probably my favorite blasters ever made. They are quire simple, easy to pump, and not too big. The range coupled with the rate of fire makes this an awesome primary at any war. When properly modified, and with good dart/barrel fit, you should get around 120-130' with slingshot weighted darts, and probably around 100-110' w/ Slug darts. Yay, rear-loading!