I can recommend IMR4320, AA2460 for 55 & 60's...These have worked well for me in the past with 55-60's. The 1-14" twist combined with the 250's velocity should shoot the 55's good...
Do you know how to do a ladder TEST,Craten/Audette? test)??? I used to spend hours upon hours at the range tweaking a load until I found the sweet spot...NOW I usually shorten my load developement time to at around 3 maybe four trips maX!
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A very good friend of mine who shoots circles around me taught me this system and beleive me I will never go at it any other way because this WORKS!
Quick ladder lesson,I am no expert-but this has saved me hours/days of frustration and I hope it helps you if I can explain it clearly enough):
First I would like to start off by saying all of my loads are developed at 300 yards and mostly off of a bi-pod/rear bag at least for my F-T/R. I would recommend at least 200 yards, VLD's and some BT's like 300 to strut their true potential. If you have ever seen a quaterback throw a PERFECT SPIRAL the ball usually leaves the hand with a slight wobble then it TRIMS out and spins like its on a shaft! YOUR bullet believe it or not does somethnig similar,though not as dramatic), and that is why I shoot 300yards,the bullet is good and stable spinning on its axis).
1) Pick the powder,in the burn rate you want), primer, bullet, case you want to shoot! I usually start .010" to .018" off the lands!
2)If you know that you will probably shoot somewhere between 34-37 grains___load 2 EACH 34/34, 34.5/34.5, 35/35, 35.5/35.5, etc...SO when you are done you have two rows,like climbing a ladder- 34-37,in 1/2 grain increments in large and medium sized cases- small cases like the 223 do it in tenths of a grain, like 22, 22.2, 22.4, etc...) low to high then a 2nd row same THING 34-37)
3)PUT up two spotters or targets! CLEAN YOUR GUN REALLY GOOD-THEN fire one fouler somewhere down range. THEN fire the first row at the 1st SPOTTER! SAME Point of aim! no scope adjustments you are looking for the vertical dispersion!
4)DO NOT clean!
Shoot the 2nd row at the 2nd spotter, I usually give the gun about 5-10 minutes to cool a little with a heavy barrel and around 15-20 minutes for a sporter wieght barrel.
5) You need to try and chart your bullet impacts as you go, so you know what did what. I usally just take a note pad and draw them in on it as I go...I then trasnfer the powder weight to the target I just shot that way I can use it as a reference.
EVALUATE___IF the first trip looks promising I leave things alone IF not I try the same thing with a second seating depth, i.e., started .010" off I mgiht go back and try .020" VLD and BERGERS usually like to be in or at the rifling so they are an exception as far as seating depth.
6)Collect your target,I usually use a cheap roll of brown wrapping pape from the dollar store to paster up my backer then simply put up two stickers,the stickers I use came off of a massive role I found on the side of the road/ The roll is getting smaller now though as I have had it a few years)!
7) Evaluate your shots, YOU are looking for vertical dispersion,MY orange dot I PULLED/CALLED the low 43.5g shot) BUT note that the four shots in the center measure approx 1 1/2" wide by .800" high,300 yards)___THAT is what you are looking for because anything in between those loads should shoot,if you want speed- load the upper edge,mine would have been 44.5 grains)! I settled on 44g because in the long haul I can sqeeze a few more rounds out of a KEG with 1/2 grain less in each round give or take a little every 90th bullet is a free-bee!
8) DAY TWO at the range load up what ever MIDDLE charge you want to shoot and load up groups of 5 or 6 with .010" in or out of what you just shot and one duplicate depth load AGAIN all of these will have all componants then same ONLY difference is the seating depths! THIS is your final sqeeze for accuracy! IF you are happy___go home and load away! **I am going back tomorrow for final testing__looking for a little more on the small side!
9) THIRD day_ go back and ZERO your RIG!
If I have not bored you yet!
I attached a ladder test I did yesterday with my 308 at 300 yards! shooting 175SMK, Horn match brass, Wolf, and Varget...Loaded to sets of 6 rounds, 43g, 43.5g, 35.7g, 44g, 44.5g, and 45g,shot this before before barrel set-back so I went back to try it again-he used a diffeent reamer this time/ longer throat) I threw in the 35.7 because before the barrel was set back the 35.5-44 really shot well...THE ORANGE DOT! SIX powder weights with 1.5 grains from the top to the bottom and it shot roughly a 2.458" outside group under ONE MOA! NOTICE 45grains went high and right believe or not this barrel shot 45gr high/right before the barrel was set back with the weight!
Then I went back today to try a group with 44g's...with TWO seating depths one the SAME and one .010" out. EVERY barrel has a sweet spot,sine wave harmony) there is a high and low portion of the wave and most will shoot really well in either the top or bottom velocity wise...Kind of what Browning tried to do with their BOSS system a few years back where a non-reloader could tune his or her barrel versus tuning the load to find the SWEET SPOT...
The actual target you see is from today with 44grains nothing else changed to see what the group would do! I called the ONE the dropped down and right. The group measures around 1.8" outside, and again the low shot was me, as far as the acutal group the wind was switching left/right with short bursts and could account for the horizontal spread/or my trigger finger? THIS load would probably be VERY small if it was shot in a test craddle in a tunnel. I still have not set a zero yet looking for group then I will dial in! I'm going back tomorrow for day three...going to try .005" either side of the previous depths. SHOULD be a keeper in there somewhere. I am hoping for more stable conditions.
GOOD LUCK with your rifle!