If the next 1/2 grain step is too high, then Speer uses the lower 'even' step. 3143 fps etc.) which is shot and recorded from an actual rifle, not a long pressure barrel as an additional check for real world performance and pressure. 27.5 - 28.0 etc.) and gives an odd number velocity ( i.e. velocity step powder charge is too high only the 100 fps lower velocity is shown in the book - which can be somewhat lower than a real maximum working load which would fall somewhere in between in velocity, if you get my drift. In addition Hornady uses odd powder charges to 'fit' an even velocity ( i.e. Perhaps a newer lot of 748 has a slightly different burning rate? Here are some examples of previous Hornady data: 223 data to be low - at least in my rifles, which is reflected in much lower velocities than Hornady shows, but I haven't tried W-748.

223/5.56 powders I've found I prefer H335 for 55 grain bullets. My opinion of the Speer data at 28 grains is that it is dangerous and should never be used when substituting another manufacturers bullet and I've not loaded Speer 55 fmj's and don't know if it is ok with them. Some 55 fmj's such as Win and military bullets have a long sharply pointed tip and overall length would be about 2.250' while some have a blunter rounded tip and overall length would be from 2.200' to 2.230'. I'd try the length recommended by the bullet manufacturer. As far as overall length it depends on the bullet you're using. I wouldn't go above the 26.4 grain level. I've had no problems with high outdoor temperatures loading at this range.

For a good load that will reliably cycle your AR, is easy on brass, shows no pressure signs for me in my rifles, try 25.9 -26.1 grains of Win 748 powder. I've used a lot of 748 with Win, Hornady, military surplus, RP and a couple other 55 fmj bullets.

Hornady 55 gr fmjbt load data