Varget with 69 gr Matchkings or their younger brother the 69 gr Tipped matchking.

Longer barrels = more velocity... Always.

With the same load... Going from a 16" barrel to a 26" barrel you'll gain at least 200 fps on average. Sometimes 300 fps.

I got best accuracy with 25 gr of Varget under the 69 gr matchking, Winchester brass, 205m primer, 2.300" COAL.
But you should work up from 23 (ish) grains because you never know.