Me respondo XD
Hey Caustic-Echo,
I'm Robin, one of the developers of the XL Burst at Airsoft Innovations.
I want to address a couple of issues.
First I must emphasize that we do not condone modifying our products or using them in ways which are not described in the included instruction manual. There are dangers associated with operating the grenade in ways not described and it should not be done.
No to address the issues your modification has brought up.
For the two burst elements to pop instead of just bulging out, the ambient temperature (or really the temperature of the propane within the grenade) needs to be at least about 22degC or 72degF. Propane's pressure is very dependent on its temperature. At 16degC it's at 93psi, at 21degC it's at 110psi, and at 26degC it's at 128psi. Below 22degC, you should use 1 burst element. Unfortunately you will only get to 120dBa @5ft which has about 1/3rd the sound energy of 125dBa @5ft. This is prescribed in the manual

We use propane for a couple of different reasons. One reason is the expansivity of propane which is about 27x at liquification pressure (around 100-150psi at operating temperature). The volume of liquid injected into the grenade during refilling is about 4ml. The expansion chamber (between the shuttle and the burst element) is about 7x this, 28ml. At this ratio, the expansion chamber is still at full liquification pressure even after the propane has moved into the much larger chamber.
If you switch to 800psi CO2 in the shuttle (which will break the shuttle, as you've experienced), you go down to 1/7th of that pressure after expansion which is still only 114psi. Unless you fill the grenade with liquid CO2 which is highly dangerous. So you're actually no further ahead by using CO2.
Another issue is interaction of CO2 with the seals in the XL Burst. The CO2 will destroy the seals. We have unfortunately rather extensive practical experience with this issue. There are very few seal materials that can withstand CO2 at high pressure, and they're very expensive.
The high pressure of the CO2 will likely destroy your fill valve as well. The small seal on the stem (the small bit in the middle of the valve) will be extruded through the filling port, severely damaging the seal, and you'll need a replacement.
If you have any questions or need clarification on anything I've described, please let me know! If anyone needs help with any Airsoft Innovations product, best to go to our website and open up a support ticket. Julian will be pleased to help! (Edit removed link to comply with sub rules)
Robin