For factory A-bolt...mag length is approximately 3.387 and at that length the bolt closed no problem, so that is my max length if I want little to no mag length buffer.
Where would you start COAL for load dev? Nosler recommends 3.320.
I am using Nosler brass, Fed 210M, and H4831SC. This is my first batch of loads ever and doing my first load development.
I always start at magazine length less .010" for some wiggle room, then work backwards if it doesn't prove adequate for accuracy at max length. I work backwards in .010" steps, some proport .005" is best, but I've never found it so.
I had good luck with H4831 and 130 NAB's 56 gr and Fed 210M's. I started at 3.340 to see how it shot. Some 270 wins like the slower burning 4831 and others like the 4350's you will know as soon as you get started. You can fine tune length once you see of it shoots.
I found VV N 560 out shot everything I tried including 4831, H4350, RL 19, H100 and Ramshot Hunter.
If you have not shot it yet try H4831sc and H4350 if you have some and see which one shoots best. I read somewhere that rifles with short throats like the 4350 burn rate and long throats like the 4831 burn rate.
The 270 usually doesn't have that long of a throat compared to the mag. length so I'd try MagnumManiac"s suggestion of mag. length with a bit of wiggle room first. Rifles will vary a bit in throating so it would be best to measure your max. oal. to jamb to make sure you know it.
Another powder to work with for the 130 bt is rl17. Both my old a-bolt and my current tikka 270's have worked well with it. My brother's old x-bolt also shot well with rl17 and 130bt's; in fact I didn't bother to do a load work up as the ammo worked when I shot his stuff through my rifle. I'm using 54 rl17, 130bt, wlr, 3.34" oal., win or rem brass.