There are a few other rules for biathlon rifles beyond the important sponsorship section. The main one (unrelated to the stock) is the weight of the trigger. That is checked before the start of every race. They check the rifle weight only at random or major events.
The crucial one for you to know, and I'm sure you already know, is the height of the stock, from the top of the rifle, near the chamber, down to the handhold/platform for standing. It was limited to 120mm
. One of the racers ahead of me had to take a hacksaw bc his hold was too long. A bit sad to see 30 minutes before the start of IBU Cup Championships. Second, the cheek piece thickness must not be more than 40mm. Otherwise, I'm sure Peter knows all the current regs
I was going to send you a PM; you can use the NdFeB magnets to hold in the magazines instead of the bulky holders. One of the US builders started using them 5 years ago. I never raced with his stock, but can tell you it is smooth and keeps clean lines for the stock. PM if you want more details or pictures. Also, I hope you guys do something neat for the 6 relay rounds. Also, you could make custom molded rear butt plates with top and bottom hooks. You could even get so advanced to mold it for prone position, high on the butt, and standing, low on the butt plate
For those that don't know the sport, as Berk said the main advantage to a carbon rifle is that you can put the weight in different places to balance the rifle. It can be difficult because you need to balance the rifles differently for the two positions: prone (heavy weight towards the end of the barrel) and standing (weight on the rear of the stock and weight, again, towards the end of the barrel). Most biathletes uses rifles well over weight because they are more stable to shoot when your heart-rate and breathing is at their max.
For the record, the IBU is much more reasonable than the UCI.