For what it is worth, I have two double rifles with McGowan barrels and am very satisfied with their accuracy.
Both barrels on each rifle are sub minute of angle.
My standard load for the Simson Suhl..405 WCF double is 300 grain bullets and they all shoot to the same point of aim at 2225-2250 fps velocity. My 400 grain hand loads at 2050 fps perform the same way; this my DG load.

The other double is a Beretta .45-70 and is also quite accurate with 300 grain bullets and well regulated. I have not loaded 400 grainers for it yet but will being trying that come spring time.

McGowan did a good job with the barrels.

Have fun with your custom rifle.

PS I am also an 1895 Winnie Fan and shoot a .405 - a proven DG rifle with 400 grain Woodies at 2076 fps.

Last edited by crshelton; 02/15/19.
CRS, NRA Benefactor Life Member, Whittington Center, TSRA, DWWC, DRSS
Android Reloading Ballistics App at http://www.xplat.net/