I developed loads for pointed 200.20X bullets (estimated G7 BC = 0.340) with both H4895 and Varget in a 30" barreled F-TR rifle. The measured average muzzle velocities obtained with the optimized loads were 2674 fps (43.1 gr H4895) , and 2641 fps (43.7 gr Varget). After calibrating Quickload by adjusting the powder burn rate (Ba) such that predicted velocities exactly match my measured velocities, I then changed barrel length in the program to 28" and 26", and recorded the predicted velocities. The table below summarizes the predicted effect of barrel length on velocity with the two powders at 600 yd and 1000 yd using pointed 200.20X bullets. Predicted velocities are shown in
blue, measured average velocities are shown in
black.
Predicted windage values were generated for a 10 mph full value wind at the indicated distances using the online JBM Ballistics calculator. The inputs were the estimated G7 BC for pointed 200-20X bullets (0.340), generic atmospheric inputs (70 degrees F, 60% humidity, 29.90" Hg, 1000 ft elevation), and the measured 30" barrel velocities, or the velocities predicted by QuickLoad for 28" and 26" barrels. The JBM outputs are summarized in the following table:
What these estimates indicate is that when loaded to equal pressure, the 30" barrel will give somewhere in excess of 60 fps greater velocity than the 26" barrel. The caveat to these predictions is that there is no way to know without going through the actual load development process exactly where a specific setup will tune in. Once the program has been "calibrated" to a specific setup, I have found the QL estimates to be generally very good. However, it is possible a 26" barrel could tune in slightly faster (or slower) than values given above. Ultimately, the velocity where the load actually tunes in with a given powder is the final answer. In other words, only the velocity window where the load shoots optimally is really of any interest. Obtaining higher velocity with a load that groups poorly is of little use.
I used predicted values for the 200.20X because I have solid data for that bullet out of a 30" barrel, and because that is the bullet you indicated you wish to use. However, I have done more testing than I want to think about with other .308 bullets and I can tell you that these predictions are going to be pretty close to what you would determine if you actually tested using the 200.20Xs. As an example, Bryan Litz and the Michigan F-TR team have been shooting the 200.20Xs out of 28" heavy contour barrels. I couldn't find the specific post that gave the exact velocity value they were achieving in that setup, but it was in the low 2600 fps range, which matches very well with the predictions above. If you do enough testing with various bullet weights and barrel lengths, what you will find is that as the barrel becomes shorter, you will reach a point where you can no longer hit a given accuracy node without going to very high pressure.
A perfect example of this can be seen with the .284 Win cartridge and 180 Hybrids. Most F-Open shooters using this setup are running 32" (or longer) barrels and are obtaining velocities in the 2800-2825 fps range. Why? Because if you try to hit the same node with a 30" barrel, the pressure required can reach a level where brass life might be very poor. So barrel length is not solely about the maximum velocity that can be achieved in a given barrel length. It is also about where the accuracy nodes are located, and what operating pressure is required to safely hit those nodes. Again, velocity means very little if the precision is poor, or if hitting a particular accuracy node kills your primer pockets in only a few firings.
What these estimates also suggest is that the difference in windage for loads developed to equal pressure in 26" versus 30" barrels will range from 0.1 to 0.2 MOA at 600 yd, and 0.2-0.3 MOA at 1000 yd. It is up to the individual to decide how much predicted windage differences really constitute a significant value. However, it's important to point out the possibility that you might find the velocity difference to be even larger after actually tuning in a load with a given barrel length. There is a limit to how exact predictions of this sort will be. IMO - 0.2 MOA at 600 yd and 0.3 MOA at 100 yd represent a significant difference. When you correlate those values to the distance between scoring rings on the MR and LR targets, it's pretty easy to see that the decrease in windage with the higher velocity could amount to saving a few points in a match if there was much wind.
In my hands, velocity differences in the 25-30 fps range amount to such small differences in estimated windage that it is questionable whether I could actually shoot the difference. If all else is equal (i.e. precision, brass life, etc.), I will take the extra 25-30 fps every time; however, whether it is really of significant benefit is difficult to prove. When velocity differences start getting into the 50+ fps range, IMO, you're now reaching a point where the difference in windage
does become significant, and it can mean the difference between winning or not winning a match. By today's standards, even one or two points difference during the course of a 3 x 20 match can decide the outcome. But again, the importance of that is still up to the individual to decide. If you do not think the velocity and windage differences shown above are significant, then by all means you should go with the 26" barrel.
In reference to the controversial posts earlier in this thread, all I can say is that this is the internet. Everyone has an opinion, and everyone wants to put their opinion out there. I get that. However, when someone is requesting information that will have a direct bearing on whether they spend their hard-earned money on a particular new piece of equipment, and someone posts an opinion that is misinformed, I do not think it is incorrect to point it out. The same is true when data such as velocities obtained for commercial ammunition that have little bearing on handloads are used to try and support that argument. As I pointed out previously, commercial ammunition is simply not optimized for the long barrels we use in F-Class and the velocity values obtained using it is misleading at best when compared to what you can get with optimized handloads. If that seems harsh, it was not meant that way, but so be it. It's not nearly as harsh as the situation where someone takes misinformed advice, spends their hard-earned money on something, and only then finds out that the item they bought won't do what they want it to do. As I stated earlier, this is the internet. Everyone has an opinion and wants to share it. Unfortunately, determining the difference between uninformed and informed opinions must also be left up to the reader.
FWIW - I have also included the QuickLoad outputs for the optimized loads with the 200.20Xs, as well as QL outputs for the same two powders loaded to equal
pressure in 28" and 26" barrels. You may find something in the information that will be of use for your 200.20X reloading endeavors, once your new barrel has been installed.
