I'm not familiar with the Buckhorn but if it is a "magnum" capable rifle it should be OK to shoot heavier bullets. Before doing so you need to contact CVA customer support to get their recomendations.

I contacted them with the same question and this is what they told me, not a direct quote but essentially the same. The magnum rated guns have a limit of 400 grains for conicals when using 150 grain pellet loads. When shooting bullets over 400 grains the max powder load should be limited to 100 grains or less. They indicated that this applied to bullets up to and including the largest Powerbelt which I think is 530 grains.

I shoot the 460 No Excuse in my Mag Bolt using 90gr of powder and typically get 1.5" or smaller groups at 100 yards and 4" or less at 200 yards. They make a nice 150 yard load using a 6" point blank range sight in. Here is a chart for my load.

Name: 50 Cal, NoExcuses SWC, 460 grn
Ballistic Coeff: 0.294
Bullet Weight: 460
Velocity: 1470
Target Distance: 126
Scope Height: 1.500
Temperature: 70
Altitude: 500
Ballistic Data
~~~~~~~~~~~~~~~
Range Elevation Velocity Energy ETA Drop Max Y 10mph Wind Deflect
0 yds -1.50 in 1470 fps 2207 fpe 0.000 sec 0.00 in -1.50 in 0.00 in
25 yds 1.12 in 1422 fps 2065 fpe 0.052 sec 0.51 in -0.47 in 0.13 in
50 yds 2.66 in 1377 fps 1935 fpe 0.106 sec 2.11 in -0.06 in 0.63 in
75 yds 3.04 in 1333 fps 1816 fpe 0.161 sec 4.86 in 0.66 in 1.44 in
100 yds 2.20 in 1292 fps 1705 fpe 0.218 sec 8.83 in 1.72 in 2.53 in
125 yds 0.11 in 1252 fps 1601 fpe 0.277 sec 14.05 in 3.13 in 3.86 in
150 yds -3.41 in 1216 fps 1510 fpe 0.338 sec 20.71 in 4.95 in 5.60 in
175 yds -8.41 in 1182 fps 1427 fpe 0.401 sec 28.84 in 7.20 in 7.65 in
200 yds -14.94 in 1151 fps 1352 fpe 0.465 sec 38.50 in 9.90 in 9.97 in

This is a stout load and will cause a flinch to develop if you are recoil sensitive.

I would suggest you also consider the heavier PowerBelts such as the 348 or even the 405 if you are happy with the PB's. 150 yard shots would be flatter shooting than the NE's and you wouldn't have to worry as much about heavy recoil, etc. The 348HP is the most accurate bullet I've ever shot in my gun, often shooting into less than an inch at 100 yards using 100gr of powder. I just don't like paying $1 each for them and I like to try different loads to see what works and what doesn't.

Again, check with CVA before using the larger bullets so you have verification for yourself from them. Good luck.