I had GA Precision do my last one for me using a Bartlein barrel, with the chamber cut specifically for feeding from mags of course, but shooting bullets like the 139gr Scenar.
I ended up liking the 130gr VLD more, and mine feeds them reliably for some reason, whereas everyone else I know can't get VLD's to work in a gasser because they hit the breech of the barrel through the locking lug notches between the barrel extension teeth where the feed ramps are.
Anyway, there are several companies that have drop-in barrels for .260 Rem., but I'm not sure what reamer they are using. I get .4" - .6" groups for 5rds repeatably, so I'm happy with how GAP did me, as usual.
One of the biggest considerations to look at is your bolt, namely the firing pin aperture ID. Make sure it isn't larger than .060". Most are closer to .080", which will lead to cratered and pierced primers in many cases. BTDT
I would start at your BCG, then your barrel, and work outwards to the gas system, then recoil system while looking at the receiver set.
I personally think ArmaLite Inc. has the best BCG since it has balanced loading on the lugs, sprung firing pin, and an actual engineering department with two decades of RDT&E and CS with AR10's.
There are a lot of lightweight carriers coming onto the market, following JP's trend with the LMOS, which is meant to be used in conjunction with adjustable gas.
If I were doing a .260 Rem build right now, I would want to do it on DPMS GII receivers and BCG/Extension.
I would also look at the JP Silent Capture Spring system for the AR10.
Geissele triggers are very popular as well. I have an SSA in mine. SSA-E with the skeletonized hammer will reduce PIV.
Krieger, Lilja, JP, Precision Firearms, and BHW have drop-in options last time I checked.
This is basically a reloader's rifle, at least mine is.