Originally Posted by antelope_sniper
Let's compare Hornady's data for the 6 ARC to the 6x45 loaded to 5.56 pressures. 18"

Code
Cartridge          : 6 x 45mm
Bullet             : .243, 90, Hornady ELD-X 2441
Useable Case Capaci: 24.350 grain H2O = 1.581 cm³
Cartridge O.A.L. L6: 2.260 inch = 57.40 mm
Barrel Length      : 18.0 inch = 457.2 mm

Predicted Data for Indicated Charges of the Following Powders.
Matching Maximum Pressure: 61000 psi, or 420 MPa
or a maximum loading ratio or filling of 110 %
These calculations refer to your specified settings in QuickLOAD 'Cartridge Dimensions' window.
C A U T I O N : any load listed can result in a powder charge that falls below minimum suggested
loads or exceeds maximum suggested loads as presented in current handloading manuals. Understand
that all of the listed powders can be unsuitable for the given combination of cartridge, bullet
and gun. Actual load order can vary, depending upon lot-to-lot powder and component variations.
USE ONLY FOR COMPARISON !

33 loads produced a Loading Ratio below user-defined minimum of 70%. These powders have been skipped.
Powder type          Filling/Loading Ratio  Charge    Charge   Vel. Prop.Burnt P max  P muzz  B_Time
                                      %     Grains    Gramm   fps     %       psi     psi    ms
---------------------------------  -----------------------------------------------------------------
Alliant AR-Comp *C *T              110.0     23.9     1.55    2722   100.0    60972    8206   0.888  ! Near Maximum !
Winchester 748                     105.6     25.5     1.65    2716    97.2    61000    9082   0.890  ! Near Maximum !
Hodgdon BL-C2                      108.3     26.5     1.72    2714    96.1    61000    9265   0.894  ! Near Maximum !
Accurate 2520                      109.0     25.8     1.67    2707    98.0    61000    8937   0.902  ! Near Maximum !
Hodgdon H4895                      110.0     24.7     1.60    2688    95.5    60792    9059   0.903  ! Near Maximum !
Hodgdon H335                       100.2     24.6     1.59    2683    98.6    61000    8703   0.898  ! Near Maximum !
Hodgdon H322                       106.4     23.2     1.50    2683    99.3    61000    8502   0.895  ! Near Maximum !
ADI AR 2219                        100.0     23.2     1.50    2683    99.3    61000    8502   0.895  ! Near Maximum !
Ramshot Wild Boar                  108.0     25.9     1.68    2683    94.5    61000    9011   0.896  ! Near Maximum !


Now scroll down to the 90gr bullets in the ARC.

https://www.hornady.com/assets/site/hornady/files/load-data/6mm-arc-gas-58-105gr-data.pdf

Since I know you're a velocity hound, here what the 22" barrel looks like at 5.56 pressures:

Code
Cartridge          : 6 x 45mm
Bullet             : .243, 90, Hornady ELD-X 2441
Useable Case Capaci: 24.350 grain H2O = 1.581 cm³
Cartridge O.A.L. L6: 2.260 inch = 57.40 mm
Barrel Length      : 22.0 inch = 558.8 mm

Predicted Data for Indicated Charges of the Following Powders.
Matching Maximum Pressure: 61000 psi, or 420 MPa
or a maximum loading ratio or filling of 110 %
These calculations refer to your specified settings in QuickLOAD 'Cartridge Dimensions' window.
C A U T I O N : any load listed can result in a powder charge that falls below minimum suggested
loads or exceeds maximum suggested loads as presented in current handloading manuals. Understand
that all of the listed powders can be unsuitable for the given combination of cartridge, bullet
and gun. Actual load order can vary, depending upon lot-to-lot powder and component variations.
USE ONLY FOR COMPARISON !

33 loads produced a Loading Ratio below user-defined minimum of 70%. These powders have been skipped.
Powder type          Filling/Loading Ratio  Charge    Charge   Vel. Prop.Burnt P max  P muzz  B_Time
                                      %     Grains    Gramm   fps     %       psi     psi    ms
---------------------------------  -----------------------------------------------------------------
Winchester 748                     105.6     25.5     1.65    2827    98.3    61000    7174   1.010  ! Near Maximum !
Hodgdon BL-C2                      108.3     26.5     1.72    2826    97.4    61000    7360   1.014  ! Near Maximum !
Alliant AR-Comp *C *T              110.0     23.9     1.55    2821   100.0    60977    6369   1.008  ! Near Maximum !
Accurate 2520                      109.0     25.8     1.67    2815    98.9    61000    7031   1.022  ! Near Maximum !
Hodgdon H4895                      110.0     24.7     1.60    2799    96.8    60794    7198   1.024  ! Near Maximum !
Ramshot Wild Boar                  108.0     25.9     1.68    2793    96.0    61000    7176   1.018  ! Near Maximum !
Hodgdon H335                       100.2     24.6     1.59    2790    99.4    61000    6861   1.020  ! Near Maximum !
Hodgdon H322                       106.4     23.2     1.50    2787    99.8    61000    6666   1.016  ! Near Maximum !
ADI AR 2219                        100.0     23.2     1.50    2787    99.8    61000    6666   1.016  ! Near Maximum !
ADI AR 2206H                       110.0     24.7     1.60    2784    96.1    60506    7157   1.028  ! Near Maximum !
Accurate 2495                      110.0     24.2     1.57    2784   100.0    57077    6832   1.067  ! Near Maximum !
Vihtavuori N530 *C                 105.7     23.9     1.55    2773    97.6    61000    6924   1.023  ! Near Maximum !
Shooters World Match Rifle         110.0     25.4     1.65    2773    98.6    57689    6978   1.046  ! Near Maximum !
Lovex D073.6                       110.0     25.4     1.65    2773    98.6    57689    6978   1.046  ! Near Maximum !
IMR 8208 XBR                       108.1     24.3     1.57    2764    97.5    61000    6842   1.025  ! Near Maximum !

OH TEH NOES! THE 6X45 LAGS BY 100FPS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Politics is War by Other Means