With a 4" difference in barrel length?
Go back and read the question again. Most 6.5prc and 6.5cm barrels are roughly 150-200fps when apples to apples, hack 4" off the prc, and it's a coin toss on which of those two options are faster. So.... what am I doing wrong?
150gr+ projectiles and RL26, VV570, or H1000. The difference isn't 200fps, it is the difference between burn rate categories and 150gr class bullets. Here is QL using the same bullet; 20" 6.5PRC vs 24" 6.5CM. I can run it with different bullets if you want but it doesn't ever get closer. CM doesn't work with the slower powders used in the PRC.
Cartridge : 6.5 PRC
Bullet : .264, 150, Sierra HPBT MatchK 1755
Useable Case Capaci: 60.467 grain H2O = 3.926 cm³
Cartridge O.A.L. L6: 2.966 inch = 75.34 mm
Barrel Length : 20.0 inch = 508.0 mm
Powder : Alliant Reloder-26 *C
Predicted data by increasing and decreasing the given charge,
incremented in steps of 1.0% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !
Step Fill. Charge Vel. Energy Pmax Pmuz Prop.Burnt B_Time
% % Grains fps ft.lbs psi psi % ms
-10.0 86 51.75 2567 2195 45591 15098 99.8 1.312
-09.0 87 52.33 2597 2246 47241 15207 99.9 1.290
-08.0 88 52.90 2627 2298 48945 15306 100.0 1.269
-07.0 89 53.48 2657 2351 50708 15395 100.0 1.248
-06.0 90 54.05 2686 2403 52534 15477 100.0 1.227
-05.0 91 54.63 2716 2456 54423 15557 100.0 1.207
-04.0 92 55.20 2745 2509 56379 15637 100.0 1.187 ! Near Maximum !
-03.0 93 55.78 2774 2563 58405 15715 100.0 1.167 ! Near Maximum !
-02.0 94 56.35 2803 2617 60503 15792 100.0 1.148 ! Near Maximum !
-01.0 95 56.93 2832 2671 62677 15868 100.0 1.129 ! Near Maximum !
+00.0 96 57.50 2861 2725 64928 15942 100.0 1.111 !DANGEROUS LOAD-DO NOT USE!
Cartridge : 6.5 Creedmoor Hornady
Bullet : .264, 150, Sierra HPBT MatchK 1755
Useable Case Capaci: 43.090 grain H2O = 2.798 cm³
Cartridge O.A.L. L6: 2.800 inch = 71.12 mm
Barrel Length : 24.0 inch = 609.6 mm
Powder : Hodgdon H4350 *T
Predicted data by increasing and decreasing the given charge,
incremented in steps of 1.0% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !
Step Fill. Charge Vel. Energy Pmax Pmuz Prop.Burnt B_Time
% % Grains fps ft.lbs psi psi % ms
-10.0 93 36.00 2395 1910 44559 8116 96.4 1.428
-09.0 94 36.40 2420 1950 45959 8202 96.8 1.408
-08.0 95 36.80 2445 1990 47402 8286 97.1 1.388
-07.0 96 37.20 2469 2031 48891 8367 97.4 1.369
-06.0 97 37.60 2494 2072 50427 8446 97.7 1.350
-05.0 98 38.00 2519 2113 52012 8521 98.0 1.331
-04.0 99 38.40 2544 2155 53647 8595 98.3 1.313 ! Near Maximum !
-03.0 100 38.80 2568 2197 55335 8665 98.5 1.295 ! Near Maximum !
-02.0 101 39.20 2593 2239 57077 8732 98.7 1.277 ! Near Maximum !
-01.0 102 39.60 2617 2282 58876 8797 99.0 1.260 ! Near Maximum !
+00.0 103 40.00 2642 2324 60733 8858 99.1 1.243 ! Near Maximum !
+01.0 104 40.40 2666 2368 62651 8917 99.3 1.226 !DANGEROUS LOAD-DO NOT USE!