I recently aquired a Ruger #1 in 38/72 Win. 
I'm looking for best accuracy at this stage before it becomes an off hand 100/200 yd Gong rifle. 
I've always achieved best accuracy in my other rifles by Breech seating, but have never used anything but a full case of BP. 
What suggestions are there for fillers, as I'd like to start at 65 grns, which is what I'm getting with my seated 325grn bullet. There is enough room for at least another, uncompressed 10 grns, but not sure it's needed. Cheers Ian.

Dusty

Don't use fillers they are more trouble than they are worth. Drop your Smokeless powder and seat a wad to between .050" and .100" off the powder this will do 3 things first it will prevent ringing the chamber, second it will keep the powder from migrating all up the cartridge, third it will act to prevent accidentally double chargeing the case.

40 Rod

I would try both felt wads, and a stack of 0.060" veg wads and see which one works best.   I'm not sure what powder granulation you are using, but you might want to consider Fg.   

Chris.

Thanks guys,
Yes I fireformed 25 cases with Smokeless, and have got some good accuracy using the method 40-Rod suggested. 
But I want to get the best accuracy I can with BP. I will try the stack of wads ( I've got .060 LDPE) and the Florists Foam (I used that years ago). Possibly also Semolina. 
Here in Oz we have two choices for BP. Swiss and Wano P grades. I'll try to get it to work with Wano P (the cleaner equivalent to F) But do have Swiss 1F and 1 1/2 F in the cuoboard as well.
Powder is pricey here, Wano @ $80 kg. Swiss @ $100 kg. Swiss is pretty much sold out Australia wide, next shipment due in August with no idea as to the cost at this stage. 👎
Cheers boys, the Range re opens tomorrow so I might get a go at it. Weather report for the first day of Winter, Cold and Wet. 🤣😂👍🇦🇺

Dusty,

Air is a pretty good filler.  Very uniform, and cheap too.  And no, it won't blow up the rifle.  I use it all the time in a 577/450 military martini.  I do fill the case minimum 75% with powder though.

20 years ago worked on breech seating Black most of the winter  38/55  for offhand matches. Breech only way I ever shot 32/40 & smokeless. Plan was to work it out then drive west & shoot some BP only Schuetzen summertime. Practicalities of the match set the methods. No sense developing something that’s not useful in matches. 

Did not like the powder drops I was getting at the bench while shooting. Works fine smokeless not as well with black.  Ended up charging the case at home. Drop tube Powder nearly to the mouth. Thick cork wad On top. Made a hand push wad seater with a stop, used to even up wad positions before firing the shot. Some wads would move a bit while transported . This loses the advantage of one case shooting, however found one case needed to add a case cleaning step.

Breech seated the bullet rifle on  shooting box muzzle pointed down range. Wad seated with a quick push, case in the chamber mount and shoot. Back to the shooting box wipe with damp patch then dry patch.

Very good results, key was consistent powder charge and most important wiping between shots. Air space between the wad and bullet made no difference.

Gave it up and went back to fixed BP loads. Breech seating better groups but not enough to make a score difference offhand.  Also prefer a fast reload at times working wind changes. Did keep the wipe between shot routine & carried it to all BP loads. Fixed had been blow tubing. Wiping far better .

Boats 

Still experimenting with the semi-breechseating thing in my 32-20.  Basically because it is pretty hard to breechseat in a martini, but I've gotten some pretty good scores with it.  Not always though.  And yes, wiping is critical, I could tell when seating the bullet with the plugged case if the shot was out of the group or not by the resistance felt.

Funny thing is - when I seated the bullet too far forward (but with the base still in the case), I got a pretty nasty crud ring in front of the case mouth, that required quite a few patches to remove.
With the bullet base deeper in the case, no issues with it.  But I haven't yet done the full breechseating though.

Dusty
If I were you I would go with what boats said, fixed ammunition, and save the all the hassle of breech seating, especially if the comps you are shooting in have a time limit.
If you know Charles Toohey or Alan Vaisham pass on my regards if you see them.
Mike.