I was wondering if anyone knew the correct COL for military 45-70-500 loads. I have always loaded them to the SAMMI spec and I am wondering if loading them longer might help but I don't know the correct military length.

Here is an example of what I'm talking about.

Also I usually use 52gr by volume of FFg 777 and at the SAMMI length its a compressed load but I will need to use a filler for the longer loading. Can I get away with cream of wheat or fine walnut media? Do I need a wad under the filler or can I put it directly on the powder?

I know I found it online once.  My Sharps would not group until I loaded the 405 grain bullets I used seating them out to 500 grain length.  You could always just seat them to fit your own rifle.

I'll have to just do that. I will be curious to find out how long that will be since I am casting the military style 500gr bullet with a lyman mold and the shank forward of the driving bands is quite long. Right now the M1888 ramrod bayonet rifle I have will shoot about 3 - 4 moa but I can hit a 8" gong at 200y about half the time. I am currently loading to the SAAMI max COL.

With lead bullets, one is frequently better off if the bullet sticks far enough out of the brass for the lead to engrave on rifling.

Otherwise, one is limited by what a magazine will allow.  You are not limited by magazine lengths, so seat the bullet farther out.

I wouldn't chase some arbitrary cartridge length for the sheer sake of having the old mil spec length.

EDIT:  If it chambers OK, it's OK to shoot.

What house hold items can I use as filler in the case? I am shooting 777 and it needs to be compressed. I am thinking cream of wheat.

I've used C.O.W. as a case filler in rifles and pistols.  Works OK.  I try not to need it, but sometimes I do.  I suppose you know that leaving an empty void in blackpowder loadings is considered a mistake?

Yes I do and I am probably overly paranoid about it too.

Google Culver Shooting Pages. There is a Trapdoor thread topic. Ask your question there, many knowledgeable Trapdoor guys. Primary Mr. Dick Hosmer.

or...cerro-cast your chamber and seat your bullet to that.

The .45-70-500 as loaded by the Frankfort Arsenal had an COL of 2.73", a bit longer than the SAAMI spec of 2.55".

My 1874 Sharps repro is chambered in .45-70 but has a long throat that allows the bullets to be seated shallow in the case.  I suspect the chamber could be reamed to the .295'' longer .45-90 and not get past the current throat, and I'm tempted to do that, except it shoots exceptionally well in it's current condition.

The 480 grain pointed bullets shown on the right below taper fairly quickly and can be seated deeper into the rifling than the traditional 500 grain government profile bullet.    They are seated on top of 82 grains of FFg and a card wad.   To get that much black powder in the case requires a 36" drop tube and a 1/4" of compression in a compression die, in addition to the shallow seated bullet.    The end result was a very accurate load in my particular rifle, with very consistent single digit SDs in velocity, although the actual increase in velocity over a more normal load was minimal.  It kept shooting better with heavier charges (I assume because it liked the increasing load density) so I kept at it until I reached the practical maximum for the case, bullet and chamber in this rifle while staying with still sane amounts of powder compression.  

Modern .45-70 brass is much thicker than the old balloon head cases and holds less powder.  The only way you'll get 70 grains of FFg in a .45-70 case now is with a shorter, lighter bullet that can be seated shallower, and/or by using a compression die.

Since I no longer live out west and I'm no longer shooting at very long distances, most of my shooting with the Sharps is done with commercial cast bullets (shown toward the center below) from a 405 grain flat nosed mold designed for lever action rifles.   I'm using Unique with a piece of 5/8" square 1/4" thick polyester batting as filler to take up the extra space in the case and producing black powder velocities.  Even with the inexpensive commercial cast bullets, and with the traditional seating depth, this load will produce 1 MOA accuracy and it's great fun to shoot on plates out to 300 yards.  

Corn meal and cream of wheat are often used as fillers, as are stacked card wads and felt wads with reduced black powder loads, or with more normal black powder compressed loads where the resulting extra space needs to be taken up.  For faster burning smokeless powder loads, a polyester wad is more common and some shooters feel it melts on firing and helps prevent leading in the bore.