Earlier this year, I started playing around with a handload for a DTA SRS Covert, using an 18" 10-twist barrel I had made some time ago by Mark Gordon at Short Action Customs. My main intent was to come up with something fun to shoot with "reasonable" precision that didn't consume the same reloading components I use for my F-TR match .308s. With that consideration and the fact that the barrel was not specifically chambered for the longer .30 cal bullets we typically use in F-TR (i.e. 185s, 200s, etc.), I decided to try 168 Hybrids, as I have quite a few on hand. I also decided on IMR4895 powder, as I also have a sufficient amount of that powder on hand that wasn't getting used for anything else. Finally, I had several hundred pieces of one-fired Lapua Standard (LRP) .308 brass that had been sitting in a box unused for a few years.
My initial testing suggested that somewhere close to 41.8-42.0 gr IMR4895 would be very close to a predicted OBT node for an 18" pipe (Node 6, 0.9886 ms). That load gave me approximately 2570 fps with the 168 Hybrids, at predicted fill ratio and pressure of 91.3% and 56,400 psi, respectively. Note: I wasn't trying come up with a load just under MAX at a predicted pressure of around 60-61K, just develop a fun load that shot halfway decently so the rifle didn't end up spending all its time in the safe. In that context, OBT Node 6 was readily achievable with the chosen components, whereas any load faster than ~2585 fps and greater than ~57K psi pressure would have been an in-between node. Not that I couldn't have pushed it a little harder, I simply chose not to.
My point for posting this is that some comparisons can be made, even though they are a bit loose. I have shot load with the 168 Hybrids at ~2905 fps over H4895 for years using an F-TR rifle with a 30" barrel throated short (~.085" freebore). A crude velocity comparison using the formula for kinetic energy (1/2MV*2) based on relative bullet weights and velocity suggests that loaded to equal pressure as the 168s, the 200s would have a predicted velocity in the 2650-2660 fps range, or very close to where they would actually tune in with H4895 from a 30" pipe (also an "in-between node, but one commonly used by F-TR shooters with 200s). Using the same crude method for estimating velocity between two different weight bullets with the 2570 fps IMR4895/168 Hybrid load I developed for the 18" 10-twist DTA rifle, the 200s would likely have a velocity somewhere in the 2350 fps range.
Of course there are numerous caveats to making these estimates. First off, I would probably not choose IMR4895 as a powder for the 200.20Xs in a short-barreled rifle. H4895 would be a better choice, IMO. Also, the DTA rifle wasn't throated for a long bullet like the 200.20X. Third, I wasn't trying to create a load with predicted pressure closer to MAX (i.e. 60-62K psi). Finally, .308 Palma brass (SRP) would withstand slightly higher pressure much better than the Standard .308 Win brass I am using. What that all suggests is you could likely boost that predicted velocity a bit with an 18" barrel that was throated appropriately for the 200s by using Palma brass and a better powder selection. I would't be surprised if you could safely get the 200s into the 2450-ish fps velocity range from an 18" barrel by making those kinds of changes and without generating grossly excessive pressure.
As Wade mentioned above, it is not uncommon to hear that velocity loss/gain per inch of barrel will average around 20 fps. In my hands using rifles chambered in .308 Win with barrels from 16" to 30", it seems as though with commercial ammunition preps, that is a pretty fair estimate for barrel lengths between about 16" and 26" length. After about 26" barrel length, factory ammunition doesn't seem to increase velocity much with a longer barrel. By analogy, the decrease when going from a 30" barrel to 26" would also be somewhat less than the 20 fps value. So by choosing an optimal throat length for the 200s and using a powder with an appropriate burn rate, you're probably looking at somewhere in the neighborhood of a 200-250 fps decrease in velocity by going with an 18" barrel instead of 30". Part of that will depend on exactly where the load actually tunes in. Using my QuickLoad files for the 200.20Xs with either Varget or H4895 and simply shortening the barrel length to 18", the predictions are that loads would likely tune in somewhere in the 2350-2400 fps range. Like the typical 2650 fps load with Varget or N150 from a 30" barrel that a lot of F-TR shooters are currently using, it would likely be an "in-between" node, falling somewhere between OBT Node 6 and 7 in terms of barrel time. I doubt it would be wise to hit it hard enough to actually reach node 6, and Node 7 would be much slower than necessary.
