Here's a firing assembly modification I read about in the pbnation Shocker forum. It's a homebrew method of expanding the marker's air volume by drilling a new hole in the bolt guide. ULP is a common acronym for Ultra Low Pressure in this case.
1. Drill a new hole all the way down the rear end of the bolt guide, stopping just before the first of the two front 12/70 o-ring grooves. This new hole will be used to store air pressure from within the fire chamber.
2. Drill a pair of angled air ports to connect the fire chamber section to the new ULP hole drilled from the back.
3. The backside of the bolt guide needs to be sealed up with a set screw. For this, drill a slightly wider hole from the backside of the bolt guide, diameter depending on the set screw thread size being used. A pair of 3/8"dia set screws are used in my instructions, but the mod can also be done using a similar size metric screw, such as 9mm. Use a tap drill chart to determine the drill size.
4. Thread the larger rear hole for the appropriate thread matching your set screws. The threads must be deep enough that the first set screw will go in past the vent ports (around the base of the bolt guide shaft).
5. Loctite one of the set screws and install it deep enough that it doesn't cover the vent ports.
6. Drill a hole in the other set screw, then loctite it in place at the back of the bolt guide. The hole drilled on the inside will allow venting.
In my experience, the hardest part of this mod is drilling the deep hole all the way down the guide and keeping it straight to ensure the bolt guide doesn't become weakened. For this you will need to use a lathe; I don't think this hole could be drilled by hand with the required accuracy. See my pictures below.
On a similar vein, the new set screw threading needs to be deep enough that the set screws don't cover up the bolt ports. Again, the bolt guide is weakened so the threading cannot be larger than 3/8-16 or 3/8-24. I used 3/8-16 thread size in my expample, but in my opinion a 3/8-24 would be more appropriate. (fine threads will seal with loctite easier).
I've also performed this modification with a non-HE bolt guide, which required a new air groove to be created inside the fire chamber area...
