Like most spherical rifle powders, W748 can be more difficult to ignite and burn well at low density than tubular powders (which achieve progressive burning primarily by granule geometry rather than deterrent coating). That said, 748 isn't as heavily coated as some of the slower numbers and generally does OK at medium loading density. Magnum primers may help.

I wouldn't expect great performance from any medium rifle powder at density below about 60%, but the QuickLoad estimates below show reasonable burn performance at densities from 62 to 79%, all at acceptable Krag pressures.

Cartridge          : .30-40 Krag
Bullet             : .308, 170, Hornady FP 3060
Cartridge O.A.L.: 3.089 inch or 78.46 mm
Barrel Length      : 30.0 inch or 762.0 mm
Powder             : Winchester 748

Predicted data by increasing and decreasing the given charge,
incremented in steps of 2.0% of nominal charge.

Step    Fill. Charge   Vel.  Energy   Pmax   Pmuz  Prop.Burnt B_Time
%       %    Grains   fps   ft.lbs       psi         psi      %          ms

-20.0    62    32.00   2083    1638   21558   4414     92.4    1.992
-18.0    64    32.80   2129    1712   22768   4543     93.3    1.948
-16.0    65    33.60   2175    1786   24043   4668     94.3    1.905
-14.0    67    34.40   2221    1862   25389   4788     95.1    1.864
-12.0    68    35.20   2266    1939   26806   4905     95.9    1.824
-10.0    70    36.00   2312    2017   28299   5016     96.6    1.785
-08.0    71    36.80   2357    2096   29873   5122     97.3    1.748
-06.0    73    37.60   2401    2177   31533   5223     97.9    1.711
-04.0    74    38.40   2446    2258   33283   5318     98.4    1.675
-02.0    76    39.20   2490    2340   35128   5407     98.8    1.640
+00.0   78    40.00   2533    2423   37075   5489     99.2    1.606
+02.0   79    40.80   2577    2506   39129   5565     99.5    1.569

Note how the "Prop.Burnt" percentage increases as charge and density are incremented.