I am working on figuring out my next cartridge. I liked the 7 LRM but the brass selection is very limited and Gunwerks, the promoter, has abandoned it in favor of the 7 PRC which became my next choice.
Somewhere along the way someone said something about looking at heavy for caliber 6.5 (Or it might have been 6 mm) bullets and I had a look. The Berger 6.5 mm, 153.5 LR Hybrid has a BC of 0.365. The Berger 184 F-Open Hybrid, which I would likely use if I go with the 7 PRC, has a G7 BC of 0.365.
I ran trajectory calcs for both at 3,000 MV and the drop, windage, and time of flight were identical out to 2,500 yards, the furthest I would expect to ever shoot.
Am I missing something or would both bullets, if launched at the same MV, be affected equally by the wind and by gravity?
If you intend to shoot that far, you're well into fringe behavior and the simplifications that worked at 1000 yards are breaking down. They can only put so much information on a box of bullets and moving their customers from the G1 to G7 models was challenging enough for the bullet companies.
Those bullets are aggressive enough in design and at 2500 yards you're looking at shooting them out the bottom of the transonic so the G7 model starts to break down.
Using AB Analytics and the CDMs for those bullets, 2500 yards at sea level, 3000 fps:
153.5 Hybrid, 39.2 mils elevation, 5.3 mils 10 mph wind
184 Hybrid, 37.3 mils elevation, 5.0 mils 10 mph wind
You can do similar comparisons with 4DoF for Hornady bullets if you don't have AB or AB doesn't have a CDM for the Hornady bullet.
Given all the other factors that'll be affecting hits at that distance, that's not a huge distinction but little additional problems just keep piling on.
Within a given set of bullet design parameters, bullet weights scale by the cube of caliber. The 153.5 grain 6.5mm scales to a 191 grain 7mm. To drive that 6.5 bullet to 3000 fps will require a larger case than what scales to the 7mm 184 grain at the same speed. It'll take enough 7mm case to scale to driving a 190 to 3000 fps and then a bit more. Pulling a number not entirely out of space, figure maybe 1/3 the barrel life to go to the 6.5.
I was shooting a lot of the 195 EOLs to 2300 yards when they first came out. BC is great, but at that distance BC consistency is the dominant contributor to vertical dispersion. Out of the box, with an 8 twist, the 180s have it all over the 190 and 195s as far as vertical spread beyond 2000 yards. The 190 ATips are a step past that in vertical consistency, but you'll need to keep it in your pants with pressure and know when the throat of a barrel is just about done to use them. I'd expect the 153.5 hybrids to behave like the 190/195 hybrids at that distance.
Running the Sg up to the 1.8-2.1 range helps reduce BC spread. Because you've started at a fairly high performance level and done this comparison at the same speed, the extra twist required to match the 184 Sg with the longer for caliber 6.5 bullet will have enough additional RPM that it'll affect bullet performance towards the end barrel life. Compromise on the Sg to keep the 6.5 rpm in line and it'll show up in vertical at 2500 yards.
Another issue is it takes some rounds down range to develop the fine motor skills and strategies required for a mile and beyond. If you have a strong background reloading for 1K, that'll be a good start. As you head for a mile, recognize that there is enough intrinsic accuracy in the required equipment that endless testing at 100 yards isn't a good way to spend barrel life or select bullets. Bullets should be selected and barrels retired at a mile or more based on vertical. Dumping the heavier bullet for better wind strategy is another good idea. Vertical is the problem. Wind is on you. Vertical is velocity and BC spreads. Take every opportunity to trade 100 fps less for half your SD. Uniform bullet dimensions are only the start of managing BC spread. The condition of the barrel, the pressure you subject the bullets to, and your reloading practices all contribute. Excessively aggressive bullet designs will also bite you.
I think your first plan with the 7LRM and 190s from a 30" barrel in a magnum action was solid. Taking a couple little steps back to the 7PRC and 180/190s will make for better practice time at 1K and a mile. There are a lot more hours in the year when shooting at 1 mile is feasible than 2500 yards.
