Bearless,
I believe the factory barrel length for the TRG-S in 338 Lapua is only 25" to start with. In fact it may be less then that at 24 3/4" or something like that. I know that its shorter then the same rifle in the 7.62 Warbird or 30-378 because I have fitted muzzle brakes to alot of them and they are all very short compared to the other rounds.
As far as FPS loss per inch. Dan Lilja did a study on this with a larger 338 magnum, the 338-378 Wby and the 338-416 Improved.
In both cases, with the 300 gr SMK, velocity loss was around 20 to 22 fps per inch if I recall correctly cutting barrel length down to this class. Velocity loss was only around 18 fps with barrel lengths over 30" when loosing one inch of barrel.
The 338 bore is an amazingly effienct bore diameter. The reason is because you can get relatively heavy bullets in this bore diameter that are still relatively short in baring surface and the higher expansion ratio of this bore diameter allows the use of faster burning powder to be used.
It has been written that these larger bores would require a longer barrel to be effective, this is simply the opposite in fact. The larger the bore, the higher the expansion ratio of the round and as such they are less dependant on long barrels to produce top end velocity.
One glaring example of this is a couple of XP-100 pistols I made several years back. One was a 338 WSM wildcat, the other was a 300 WSM. Both had 15" 1-10 Lilja barrels with custom chambers built to similiar specs in relation to the rounds.
With the 180 gr Ballistic Tip loaded in both rounds, The 300 WSM topped out at around 2775 fps. In the 338 WSM however, it could drive the 180 gr Ballistic Tip to 2920 fps!!! Why?? Same powder capacity, same bullet weight, so why the large increase in velocity, easy!!
The baring surface on the 338 cal 180 is much less then the 30 cal 180. So there is less bore friction. Also, the powder that is appropriate for the two rounds is dramatically different.
The 300 WSM needed Rl-22 to get top velocity whereas the 338 WSM was very happy with the MUCH faster burning Rl-15.
Simply put, in my opinion, forget about whatever velocity lose you may have with the 1" shorter barrel, it is meaningless in every way. YOu are talking about 20 fps at the most in my opinion from a 25" barrel down to 24". Even from 26 to 24 it would be less then 50 fps.
There are so many factors that effect velocity potential that it is impossible to compare what is written in a loading manual and get any idea at all about what will happen in real world conditions.
I believe its the Hornday #5 manual that lists the 243 Win as producing the exact same velocity as a 6mm-284. Simply rubbish in the extreme and meaningless to be honest for comparision purposes.
Forget about your velocity loss if the rifle shoots well, you will never miss it.
Kirby Allen(50)