Just got my roller project done and this is what it is...
RKS gain twist barrel,#5 action,Treebone wood, MVA 10x A scope
looking for some starting load data for the saeco #571 300 gr bullet, starline long brass
powder on hand
AA2495
AA5744
Unique
3fg OE
I would try BP first but I need to find some .030 wads

Agreed - I tried the smokeless route on a 40-65 Browning and was quite frustrated. Once I switched to black powder the rifle got happy and so did I. Really enjoy shooting it now

If I can find .030  wads local or on my side of the border...great I just bought a 95.00 USD item today and with exchange it came to $135.00 Canuck  so a bag of 1000 wads @ 20.00 plus shipping and exchange will run me prob 60.00

I used to have a Fred Cornell Wad punch in this caliber approx 20yrs ago and sold it with my last 38/55....lesson learned  now need to get another...

I get excellent results in my .38-55 using 4198 and a Hudson 315 bullet. My charge is a mild 23.0 grs. No fillers, wads, or anything else over powder.

Alliant Reloader 7 works very well in 38-55.
AND it meters very well compared to the LONG stick 4198
beltfed/arnie

I use my Harrell's Precision measure with 4198 and no problem.

I'd second reloader 7 or 4198.  Also, I shoot mid 20 grains with a 360 grain bullet, so Vall's load is a good place to start.

I wouldn't use any of the powders you listed that you have on hand.   There's a lot of disagreement about this statement I'm going to make.  I believe two of those powders are subject to detonation occasionally when used on loads like this; so I avoid their use.  Reloader 7 and 4198 have absolutely no history of detonations in this use that I've ever heard of, and have a lot less velocity variation in the .38-55.    

AA 2495 is to slow a powder in my opinion for a straight wall case with cast bullet, it will not burn clean until velocities and pressures are higher than you want.

My 38/55 was set up for Black Powder. Wanted a practice load shooting to the same sights. Used 17grs of 4759, when it was discontinued switched to 17 grs 4227, no wads. Charges thrown into a scale pan from a Harrell’s measure, check weighing, pass or dump & throw another. It’s possible  to  throw a light charge working the measure too fast. Want piece of mind all have the same point of impact, weighing does that. 

Rarely use BP today, main match does not require it .   If I could not get 4227 would use 4198 and a bit more powder, same methods. 

Boats

I have two 38 molds. Original mold when CPA built the rifle 1993 was a LBT two cavity 290 gr spritzer. Was waiting for a Barnett that never arrived Veral delivered the 290 quickly. Dozen years later switched to a Hoch round nose single cavity 305 gr. 

Offhand shooter monthly matches & couple practice sessions it’s high volume. I used to practice with 245 gr store bought bullets shoot the 305 Hoch in matches. There is no score difference offhand 200 yards 245 or 305. Winter cast a lot of 305 run out switch to the 245. Grandsons learning offhand with that rifle & it’s .22 barrel. # of CF rounds fired will go up considerably soon 

I use 17 grs 4227 all three bullets. Also my 1993 barrel is slower twist that what’s common today. Doubt 330 grs often used today would be successful. 

Your 270 ought to be fine, our club matches don’t, allow Gas Checks if they did I might use them . Often use a 1903 30/06 Arsenal built Sporter practice 208 gr gas check 18 grs 4227 it’s a very good combo 200 or 300 yards.

Boats