A little help reloading a 45-60

[Milano]

Hello
I am reloading for a Chapparal 1876 with a 22" barrel in 45-60.  I am shooting 300 grain Hornady JHP 0.458 bullets, using Federal 210 Large Rifle primers, IMR 4198 powder, with a COAL of 2.2350.  During my development I started at 26 grains increasing by half a grain until 28.5 grains was reached.  I shot 5 rounds of each through a chronograph and the rounds were not left in the sun. My results are such:
26 gr avg 1096 ft/s,
26.5 gr 1173,
27 gr 1230,
27.5 gr 1139,
28 gr 1186,
28.5 gr 1223. 
The groups were good about 3" at 100 yards with no sign of excessive pressure. But after shooting the 27 grain load the extreme spread and standard deviation became erratic and the velocity did not increase as expected.  I am wondering if I should try using magnum primers or if irregularities such as this are expected.  I am not trying to overload this cartridge but I would like to get some velocity and consistancy. 
Any guidance would be greatly appreciated.  Thank you.

[Joe Lansing]

    I had similar results using 4759. The charge did not give me the published results: the velocities were significantly lower. I tried using filler (Quaker Puffed Rice) and my velocities increased dramatically without signs of over pressurization. All traces of unburnt powder were gone: there was significant unburnt powder prior to using filler. The trick is to back down your charges before using filler. I didn't , and when all that unburnt powder burned, my m.v. jumped up to over 1600 f.p.s. I backed down to my lower starting charge and my m.v. matched the published data, about 1300+ f.p.s.
    You have got to burn all the powder or your results will be erratic, depending on the amount of unburnt powder ,shot to shot, and filler promotes burning all of the powder charge.

                                                                   J.L.